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The locus ceruleus (LC) contains neurons that increase their firing rate (FR) in vitro when exposed to elevated CO2/H+ and have been proposed to influence the respiratory network to make compensatory adjustments in ventilation. Prp57 transgenic mice express green fluorescent protein (GFP) in the LC and were used to isolate, culture, and target LC neurons for electrophysiological recording. We hypothesized that GFP-LC neurons would exhibit CO2/H+ chemosensitivity under primary culture conditions, evidenced as a change in FR. This is the first study to quantify CO2/H+ responses in LC neuron FR in cell culture. Neurons were continuously bathed with solutions containing antagonists of glutamate and GABA receptors, and the acid-base status was changed from control (5% CO2; pH ∼7.4) to hypercapnic acidosis (9% CO2; pH ∼7.2) and hypocapnic alkalosis (3% CO2; pH ∼7.6). FR was quantified during perforated patch current clamp recordings. Approximately 86% of GFP-LC neurons were stimulated, and ∼14% were insensitive to changes in CO2/H+. The magnitude of the response of these neurons depended on the baseline FR, ranging from 155.9 ± 6% when FR started at 2.95 ± 0.49 Hz to 381 ± 55.6% when FR started at 1.32 ± 0.31 Hz. These results demonstrate that cultured LC neurons from Prp57 transgenic mice retain functional sensing molecules necessary for CO2/H+ responses. Prp57 transgenic mice will serve as a valuable model to delineate mechanisms involved in CO2/H+ responsiveness in catecholaminergic neurons.

Previously we reported that oxytocin (OT)-containing neurons of the hypothalamic paraventricular nucleus (PVN) project to the pre-Bötzinger complex (pre-BötC) region and phrenic motoneurons innervating the diaphragm (D). The aim of these studies was to determine pathways involved in PVN stimulation-induced changes in upper airway and chest wall pumping muscle activity. In addition, we determined the role of OT-containing neurons in the PVN in mediating increased respiratory output elicited by PVN stimulation. Neuroanatomical experiments, using pseudorabies virus (PRV) as a transneuronal tracer in C8 spinalectomized animals showed that PVN neurons project to hypoglossal motoneurons innervating the genioglossus (GG) muscle. Furthermore, microinjection of the PVN with bicuculline, a GABAA receptor antagonist, significantly increased ( P < 0.05) peak electromyographic activity of GG (GGEMG) and of DEMG, frequency discharge, and arterial blood pressure (BP) and heart rate. Prior injection of OT antagonist [d-(CH2)5,Tyr(Me)2,Orn8]-vasotocin intracisternally or blockade of OT receptors in the pre-BötC region with OT antagonist l-368,899, diminished GGEMG and DEMG responses and blunted the increase in BP and heart rate to PVN stimulation. These data show that PVN stimulation affects central regulatory mechanisms via the pre-BötC region controlling both respiratory and cardiovascular functions. The parallel changes induced by PVN stimulation were mediated mainly through an OT-OT receptor signaling pathway.

Adenosine is a known inhibitor of respiratory output during early life. In this study we investigated the developmental changes in adenosine A2A-receptor activation on respiratory timing, as well as the relationship between adenosine and GABA. The specific adenosine A2A-receptor agonist CGS-21680 (CGS) or vehicle control was injected into the fourth ventricle of 14-day ( n = 9), 21-day ( n = 9), and adult ( n = 5) urethane-anesthetized rats while diaphragm electromyogram was monitored as an index of respiratory neural output. CGS injection resulted in a decrease in frequency and/or apnea in all 14-day-old rats and in 66% of 21-day-old rats. There was no effect of CGS injection on respiratory timing in adult rats. Prior injection of the GABAA-receptor blocker bicuculline at 14 and 21 days eliminated the CGS-induced decrease in frequency and apnea. We conclude from these studies that the inhibitory effect of A2A-receptor activation on respiratory drive is age dependent and is mediated via GABAergic inputs to the inspiratory timing neural circuitry. These findings demonstrate an important mechanism by which xanthine therapy alleviates apnea of prematurity.

This review summarizes recent work on two basic processes of central nervous system (CNS) control of cholinergic outflow to the airways: 1) transmission of bronchoconstrictive signals from the airways to the airway-related vagal preganglionic neurons (AVPNs) and 2) regulation of AVPN responses to excitatory inputs by central GABAergic inhibitory pathways. In addition, the autocrine-paracrine modulation of AVPNs is briefly discussed. CNS influences on the tracheobronchopulmonary system are transmitted via AVPNs, whose discharge depends on the balance between excitatory and inhibitory impulses that they receive. Alterations in this equilibrium may lead to dramatic functional changes. Recent findings indicate that excitatory signals arising from bronchopulmonary afferents and/or the peripheral chemosensory system activate second-order neurons within the nucleus of the solitary tract (NTS), via a glutamate-AMPA signaling pathway. These neurons, using the same neurotransmitter-receptor unit, transmit information to the AVPNs, which in turn convey the central command to airway effector organs: smooth muscle, submucosal secretory glands, and the vasculature, through intramural ganglionic neurons. The strength and duration of reflex-induced bronchoconstriction is modulated by GABAergic-inhibitory inputs and autocrine-paracrine controlling mechanisms. Downregulation of GABAergic inhibitory influences may result in a shift from inhibitory to excitatory drive that may lead to increased excitability of AVPNs, heightened airway responsiveness, and sustained narrowing of the airways. Hence a better understanding of these normal and altered central neural circuits and mechanisms could potentially improve the design of therapeutic interventions and the treatment of airway obstructive diseases.

A central neuronal network that regulates respiration may include hypothalamic neurons that produce orexin, a peptide that influences sleep and arousal. In these experiments, we investigated 1) projections of orexin-containing neurons to the pre-Bötzinger region of the rostral ventrolateral medulla that regulates rhythmic breathing and to phrenic motoneurons that innervate the diaphragm; 2) the presence of orexin A receptors in the pre-Bötzinger region and in phrenic motoneurons; and 3) physiological effects of orexin administered into the pre-Bötzinger region and phrenic nuclei at the C3–C4 levels. We found orexin-containing fibers within the pre-Bötzinger complex. However, only 0.5% of orexin-containing neurons projected to the pre-Bötzinger region, whereas 2.9% of orexin-containing neurons innervated the phrenic nucleus. Neurons of the pre-Bötzinger region and phrenic nucleus stained for orexin receptors, and activation of orexin receptors by microperfusion of orexin in either site produced a dose-dependent, significant ( P < 0.05) increase in diaphragm electromyographic activity. These data indicate that orexin regulates respiratory activity and may have a role in the pathophysiology of sleep-related respiratory disorders.

Hyperoxic exposure enhances airway reactivity in newborn animals, possibly due to altered relaxation. We sought to define the role of prostaglandinand nitric oxide-mediated mechanisms in impaired airway relaxation induced by hyperoxic stress. We exposed 7-day-old rat pups to either room air or hyperoxia (>95% O2) for 7 days to assess airway relaxation and cAMP and cGMP production after electrical field stimulation (EFS). EFS-induced relaxation of preconstricted trachea was diminished in hyperoxic vs. normoxic animals ( P < 0.05). Indomethacin (a cyclooxygenase inhibitor) reduced EFS-induced airway relaxation in tracheae from normoxic ( P < 0.05), but not hyperoxic, rat pups; however, in the presence of NG-nitro-l-arginine methyl ester (a nitric oxide synthase inhibitor) EFS-induced airway relaxation was similarly decreased in tracheae from both normoxic and hyperoxic animals. After EFS, the increase from baseline in the production of cAMP was significantly higher in tracheae from normoxic than hyperoxic rat pups, and this was accompanied by greater prostaglandin E2release only in the normoxic group. cGMP production after EFS stimulation did not differ between normoxic and hyperoxic groups. We conclude that hyperoxia impairs airway relaxation in immature animals via a mechanism primarily involving the prostaglandin-cAMP signaling pathway with an impairment of prostaglandin E2release and cAMP accumulation.

GABA is the main inhibitory neurotransmitter that participates in the regulation of cholinergic outflow to the airways. We have tested the hypothesis that a monosynaptic GABAergic circuit modulates the output of airway-related vagal preganglionic neurons (AVPNs) in the rostral nucleus ambiguus by using a dual-labeling electron microscopic method combining immunocytochemistry for glutamic acid decarboxylase (GAD) with retrograde tracing from the trachea. We also determined the effects of blockade of GABAA receptors on airway smooth muscle tone. The results showed that retrogradely labeled AVPNs received a significant GAD-immunoreactive (GAD-IR) terminal input. Out of a pooled total of 3,161 synaptic contacts with retrogradely labeled somatic and dendritic profiles, 20.2% were GAD-IR. GAD-IR terminals formed significantly more axosomatic synapses than axodendritic synapses ( P < 0.02). A dense population of GABAergic synaptic contacts on AVPNs provides a morphological basis for potent physiological effects of GABA on the excitability of AVPNs. GAD-IR terminals formed exclusively symmetric synaptic specializations. GAD-IR terminals were significantly larger ( P < 0.05) in both length and width than unlabeled terminals synapsing on AVPNs. Therefore, the structural characteristics of certain nerve terminals may be closely correlated with their function. Pharmacological blockade of GABAA receptors within the rostral nucleus ambiguus increased activity of putative AVPNs and airway smooth muscle tone. We conclude that a tonically active monosynaptic GABAergic circuit utilizing symmetric synapses regulates the discharge of AVPNs.

In this study, we have investigated the ultrastructure and function of the catecholaminergic circuitry modulating the output of airway-related vagal preganglionic neurons (AVPNs) in ferrets. Immunoelectron microscopy was employed to characterize the nature of catecholaminergic innervation of AVPN at the ultrastructural level. In addition, immunofluorescence was used to examine the expression of the α2A-adrenergic receptor (α2A-AR) on AVPNs, and norepinephrine release within the rostral nucleus ambiguous (rNA) was measured by using microdialysis. Physiological experiments were performed to determine the effects of stimulation of the noradrenergic locus coeruleus (LC) cell group on airway smooth muscle tone. The results showed that 1) catecholaminergic nerve endings terminate in the vicinity of identified AVPNs but very rarely form axosomatic or axodendritic synapses with the AVPNs that innervate the extrathoracic trachea; 2) AVPNs express the α2A-AR; 3) LC stimulation-induced norepinephrine release within the rNA region was associated with airway smooth muscle relaxation; and 4) blockade of α2A-AR on AVPNs diminished the inhibitory effects of LC stimulation on airway smooth muscle tone. It is concluded that a noradrenergic circuit originating within the LC is involved in the regulation of AVPN activity within the rNA, and stimulation of the LC dilates the airways by the release of norepinephrine and activation of α2A-AR expressed by AVPNs, mainly via volume transmission.

In this study, we examined effects of chemical stimulation of the ventrolateral region of the midbrain periaqueductal gray (vl PAG) on airway smooth muscle tone. We observed that in anesthetized, paralyzed, and artificially ventilated ferrets, vl PAG stimulation elicited airway smooth muscle relaxation. To clarify the mechanisms underlying this observation, we examined the GABA-GABAA receptor signaling pathway by 1) examining the expression of GABAA receptors on airway-related vagal preganglionic neurons (AVPNs) located in the rostral nucleus ambiguus region (rNA), by use of receptor immunochemistry and confocal microscopy; 2) measuring GABA release within the rNA by using microdialysis; and 3) performing physiological experiments to determine the effects of selective blockade of GABAA receptors expressed by AVPNs in the rNA region on vl PAG-induced airway relaxation, thereby defining the role of the GABAA receptor subtype in this process. We observed that AVPNs located in the rNA region do express the GABAA receptor β-subtype. In addition, we demonstrated that activation of vl PAG induced GABA release within the rNA region, and this release was associated with airway smooth muscle relaxation. Blockade of the GABAA receptor subtype expressed by AVPNs in the rNA by bicuculline diminished the inhibitory effects of vl PAG stimulation on airway smooth muscle tone. These data indicate, for the first time, that activation of vl PAG dilates the airways by a release of GABA and activation of GABAA receptors expressed by AVPNs.

In this study, we determined the projections of oxytocin-containing neurons of the paraventricular nucleus (PVN) to phrenic nuclei and to the rostral ventrolateral medullary (RVLM) region, which is known to be involved in respiratory rhythm generation. Studies were also designed to determine oxytocin-receptor expression within the RVLM and the physiological effects of their activation on respiratory drive and arterial blood pressure. Oxytocin immunohistochemistry combined with cholera toxin B, a retrograde tracer, showed that a subpopulation of oxytocin-containing parvocellular neurons in the dorsal and medial ventral regions of the PVN projects to phrenic nuclei. Similarly, a subpopulation of pseudorabies virus-labeled neurons in the PVN coexpressed oxytocin after injection of pseudorabies virus, a transynaptic retrograde marker, into the costal region of the diaphragm. A subpopulation of oxytocin expressing neurons was also found to project to the RVLM. Activation of this site by microinjection of oxytocin into the RVLM (0.2 nmol/200 nl) significantly increased diaphragm electromyographic activity and frequency discharge ( P < 0.05). In addition, oxytocin increased blood pressure and heart rate ( P < 0.05). These data indicate that oxytocin participates in the regulation of respiratory and cardiovascular activity, partly via projections to the RVLM and phrenic nuclei.

We sought to define the effects of maturation and hyperoxic stress on nitric oxide (NO)-induced modulation of bronchopulmonary responses to stimulation of vagal preganglionic nerve fibers. Experiments were performed on decerebrate, paralyzed, and ventilated rat pups at 6–7 days ( n = 21) and 13–15 days of age ( n= 23) breathing room air and on rat pups 13–15 days of age ( n = 19) after exposure to hyperoxia (≥95% inspired O2 fraction for 4–6 days). Total lung resistance (Rl) and lung elastance (El) were measured by body plethysmograph. Vagal stimulation and release of acetylcholine caused a frequency-dependent increase in Rl and El in all animals. The Rl response was significantly potentiated in normoxic animals by prior blockade of nitric oxide synthase (NOS) ( P < 0.05). Hyperoxic exposure increased responses of Rl to vagal stimulation ( P < 0.05); however, after hyperoxic exposure, the potentiation of contractile responses by NOS blockade was abolished. The response of El was potentiated by NOS blockade in the 13- to 15-day-old animals after both normoxic and hyperoxic exposure ( P < 0.01). Morphometry revealed no effect of hyperoxic exposure on airway smooth muscle thickness. We conclude that NO released by stimulation of vagal preganglionic fibers modulates bronchopulmonary contractile responses to endogenously released acetylcholine in rat pups. Loss of this modulatory effect of NO could contribute to airway hyperreactivity after prolonged hyperoxic exposure, as may occur in bronchopulmonary dysplasia.

We studied the respiratory and blood pressure responses to chemical stimulation of two regions of the ventral brainstem in mice: the rostral and caudal ventrolateral medulla (RVLM and CVLM, respectively). Stimulation of the RVLM by microinjections of the excitatory amino acid l-glutamate induced increases in diaphragm activity and breathing frequency, elevation of blood pressure (BP), and a slight increase in heart rate (HR). However, activation of the CVLM induced a decrease in breathing frequency, mainly due to prolongation of expiratory time (Te), and hypotension associated with a slight slowing of HR. Because adrenergic mechanisms are known to participate in the control of respiratory timing, we examined the role of α2-adrenergic receptors in the RVLM region in mediating these inhibitory effects. The findings demonstrated that blockade of the α2-adrenergic receptors within the RVLM by prior microinjection of SKF-86466 (an α2-adrenergic receptor blocker) significantly reduced changes in Te induced by CVLM stimulation but had little effect on BP responses. These results indicate that, in mice, activation of the RVLM increases respiratory drive associated with an elevation of BP, but stimulation of CVLM induces prolongation of Te via an α2-adrenergic signal transduction pathway.

Premature infants respond to hypercapnia by an attenuated ventilatory response that is characterized by a decrease in respiratory frequency. We hypothesized that this impaired hypercapnic ventilatory response is of central origin and is mediated via γ-aminobutyric acid-ergic (GABAergic) pathways. We therefore studied two groups of maturing Sprague-Dawley rats: unrestrained rats in a whole body plethysmograph at four postnatal ages (5, 16–17, 22–23, and 41–42 days); and ventilated, decerebrate, vagotomized, paralyzed rats in which phrenic nerve responses to hypercapnia were measured at 4–6 and 37–39 days of age. In the unrestrained group, the increase in minute ventilation induced by hypercapnia was significantly lower at 5 days vs. beyond 16 days. Although there was an increase in tidal volume at all ages, frequency decreased significantly from baseline at 5 days, whereas it increased significantly at 16–17, 22–23, and 41–42 days. The decrease in frequency at 5 days of age was mainly due to a significant prolongation in expiratory duration (Te). In the ventilated group, hypercapnia also caused prolongation in Te at 4–6 days but not at 37–39 days of age. Intravenous administration of bicuculline (GABAA-receptor blocker) abolished the prolongation of Te in response to hypercapnia in the newborn rats. We conclude that newborn rat pups exhibit a characteristic ventilatory response to CO2expressed as a centrally mediated prolongation of Te that appears to be mediated by GABAergic mechanisms.

Sahin, Mesut, Musa A. Haxhiu, Dominique M. Durand, and Ismail A. Dreshaj. Spiral nerve cuff electrode for recordings of respiratory output. J. Appl. Physiol.83(1): 317–322, 1997.—The feasibility of using the spiral nerve cuff electrode design for recordings of respiratory output from the hypoglossal (HG) and phrenic nerves is demonstrated in anesthetized, paralyzed, and artificially ventilated cats. Raw neural discharges of the HG nerve were analyzed in terms of signal-to-noise ratios and frequency spectra. The rectified and integrated moving average activity of the HG nerve had a peak value of 1.74 ± 0.21 μV and a baseline value of 0.72 ± 0.11 μV at elevated respiratory drive induced by increases in CO2 or oxygen deprivation when recorded with 10-mm-long cuffs. The frequency content of the HG electroneurogram extended from several hundred hertz to 6 kHz. Spiral nerve cuff recordings without desheathing of the nerve provided large enough signal-to-noise ratios that allowed them to be used as a measure of respiratory output and had much wider frequency bandwidths than the hook electrode preparations. A major advantage of the cuff electrode over the hook electrode was its mechanical stability, which significantly improved the reproducibility of the recordings both in terms of signal amplitudes and frequency contents.

LaManna, J. C., M. A. Haxhiu, K. L. Kutina-Nelson, S. Pundik, B. Erokwu, E. R. Yeh, W. D. Lust, and N. S. Cherniack.Decreased energy metabolism in brain stem during central respiratory depression in response to hypoxia. J. Appl. Physiol. 81(4): 1772–1777, 1996.—Metabolic changes in the brain stem were measured at the time when oxygen deprivation-induced respiratory depression occurred. Eucapnic ventilation with 8% oxygen in vagotomized urethan-anesthetized rats resulted in cessation of respiratory drive, monitored by recording diaphragm electromyographic activity, on average within 11 min (range 5–27 min), presumably via central depressant mechanisms. At that time, the brain stems were frozen in situ for metabolic analyses. By using 20-μm lyophilized sections from frozen-fixed brain stem, microregional analyses of ATP, phosphocreatine, lactate, and intracellular pH were made from 1) the ventral portion of the nucleus gigantocellularis and the parapyramidal nucleus; 2) the compact and ventral portions of the nucleus ambiguus; 3) midline neurons; 4) nucleus tractus solitarii; and 5) the spinal trigeminal nucleus. At the time of respiratory depression, lactate was elevated threefold in all regions. Both ATP and phosphocreatine were decreased to 50 and 25% of control, respectively. Intracellular pH was more acidic by 0.2–0.4 unit in these regions but was relatively preserved in the chemosensitive regions near the ventral and dorsal medullary surfaces. These results show that hypoxia-induced respiratory depression was accompanied by metabolic changes within brain stem regions involved in respiratory and cardiovascular control. Thus it appears that there was significant energy deficiency in the brain stem after hypoxia-induced respiratory depression had occurred.

The influence of viscoelastic gels as lining fluids on the pressure-velocity relationships in an airway tube model (Gaver et al. J. Appl. Physiol. 69: 74-85, 1990) was examined. A flow instability was observed due to the occurrence of a sol-gel transition in the viscoelastic properties under flow conditions. We further report measurements of the viscoelastic properties of airway secretions. Airway secretions are gels under small strains and have a yield stress of 4-7 dyn/cm2. Secretions from the pharyngeal airway show lower elasticity than secretions from the trachea. The airway reopening process is simulated using a Weibel lung geometry by incorporating the constitutive equations from the model gel studies and utilizing the rheological data on airway secretions. In these simulations, a "popping-open" phenomenon arises from a flow instability in airway generations 8-14 when the rheological properties of the lining fluids are assumed to be similar to those of pharyngeal secretions. On the basis of these studies, the elasticity of airway secretions plays an important role in airway reopening.

Lung resistance may be influenced by chemoreceptor activity and modulated by inspiratory neural output; however, it is unknown whether the contractile elements of lung tissue participate in these changes during early development. In anesthetized paralyzed open-chest piglets, we measured phrenic electroneurogram, lung resistance (RL), and tissue resistance utilizing alveolar capsules to partition the hypercapnic and hypoxic responses of RL into tissue (Rti) and airway resistance (Raw) components. Inhalation of 7% CO2 significantly increased RL (7.4 +/- 0.5 to 11.3 +/- 0.6 cmH2O.l-1.s), Rti (5.2 +/- 0.5 to 6.9 +/- 0.5 cmH2O.l-1.s), and Raw (2.2 +/- 0.2 to 4.4 +/- 0.4 cmH2O.l-1.s). Inhalation of 12% O2 caused more modest increases in RL, Rti, and Raw. Oscillations in tracheal and alveolar pressures appeared in synchrony with phrenic activity in response to both chemoreceptor stimuli. Cholinergic blockade eliminated these oscillations and significantly reduced the hypercapnic and hypoxic responses of RL, Rti, and Raw. These data demonstrate for the first time that hypercapnia and hypoxia elicit a cholinergically mediated increase in Rti which, just like the airway component of RL, is modulated by inspiratory neural output and is present during early development. Such coordination in neural function throughout the respiratory system may serve to optimize gas exchange during early postnatal life.

The tachykinin peptides substance P (SP) and neurokinin A (NKA) have been shown to induce tracheal smooth muscle contraction in piglets, and the enzyme neutral endopeptidase has been shown to modulate this effect. In these studies, we compared the SP and NKA responsiveness of piglet airways and lung parenchymal tissues in anesthetized paralyzed open-chest piglets 2–3 wk old, partitioning total lung resistance (RL) into airway resistance (Raw) and tissue resistance (Rti). During tidal breathing, pressure was measured at the trachea and in two alveolar regions by means of alveolar capsules. Intravenous administration of SP caused concentration-dependent increases in Rti and Raw and a decrease in dynamic lung compliance. Under baseline conditions, Rti contributed 74.6 +/- 1.9% (SE) of RL, and at any level of constriction, Rti accounted for > 50% of RL. The responses of Rti and Raw to NKA were negligible and were always significantly weaker than those to SP. These results indicate that both central airways and tissue contractile elements respond vigorously to SP, but not to NKA, in maturing piglets.

Stimulation of laryngeal afferent fibers triggers reflex apnea, which can be very long in the neonatal period. The purpose of this study was twofold: 1) to determine the effect of superior laryngeal nerve (SLN) stimulation on expiratory as well as inspiratory muscle activity and 2) to compare the respiratory response to SLN stimulation under conditions that would appear to alter the level of central chemosensitivity in anesthetized spontaneously breathing piglets. In protocol 1, we measured electromyogram (EMG) responses from the diaphragm (DIA), triangularis sterni, and transversus abdominis muscles to graded SLN stimulation; in protocol 2, we compared the DIA response to different levels of SLN stimulation during normocapnia, hypercapnia, and cooling of the ventrolateral medullary surface (VMS). SLN stimulation performed during normocapnia caused significantly greater inhibition of both expiratory muscles (triangularis sterni and transversus abdominis) than of the DIA. During hypercapnia, laryngeal stimulation-induced inhibition of all three muscles was significantly diminished, and the degree of inhibition for the three muscles became equivalent. Inhibition of the DIA EMG was significantly greater with SLN stimulation during VMS cooling than with a VMS temperature of 38 degrees C. We conclude that the magnitude of respiratory inhibition induced by laryngeal stimulation during early postnatal life is inversely related to the level of central chemical input and speculate that functional deficiency of structures located in the VMS may contribute to potentially life-threatening apnea in infants.

The effects of somatostatin and atrial natriuretic peptide applied topically to the ventral surface of the medulla (VMS) on tracheal tone and phrenic nerve activity (Phr) were studied in chloralose-anesthetized and paralyzed cats artificially ventilated with 7% CO2 in O2. Surface application of drugs to the chemosensitive areas of the VMS significantly decreased tracheal tension measured by changes in pressure in a balloon placed in a bypassed segment of the trachea (Ptseg). Application of somatostatin (9 cats) caused a mean decrease in Ptseg from 17.3 +/- 1.8 (SE) to 4.3 +/- 1.4 cmH2O (P < 0.01) and a reduction in Phr from 24.9 +/- 3.4 to 10.3 +/- 3.4 units (P < 0.05). Like somatostatin, application of atrial natriuretic peptide to the VMS (5 cats) produced tracheal relaxation (Ptseg decreased from 19.3 +/- 2.6 to 9.9 +/- 1.3 cmH2O, P < 0.01), but in contrast there was an insignificant reduction in Phr (from 18.5 +/- 3.6 to 16.1 +/- 3.8 units, P > 0.05). When parasympathetic activity was abolished by atropine methylnitrate and tracheal tone was restored with 5-hydroxytryptamine, somatostatin and atrial natriuretic peptide applied on the VMS had no effect on tracheal pressure, suggesting that observed changes were not caused by direct action of peptides on tracheal smooth muscle via the bloodstream or by facilitation of inhibitory pathways. Both somatostatin and atrial natriuretic peptide applications were associated with a slight but significant decrease in arterial blood pressure. These data suggest that somatostatin and atrial natriuretic peptide acting on the chemosensitive structure of the VMS may play significant roles in modulating para-sympathetic outflow to airway smooth muscle.

We compared maturation of the responses of the rib cage [triangularis sterni (TS)] and abdominal [transversus abdominis (TA)] expiratory muscles with each other and with the responses of the diaphragm (DIA) during hypercarbic and hypoxic stimulation. Studies were performed in anesthetized (urethan and chloralose) piglets of two age groups (< 6 days, n = 10; 14–21 days, n = 11) before and after bilateral cervical vagotomy. Hypercarbia (7% CO2–93% O2) was associated with comparable sustained increases in the minute electromyograms (EMGs) of both TS and TA, which were closely coupled to the DIA responses in both age groups. Hypoxia (12% O2–88% N2) caused a biphasic response of the minute EMG of both expiratory muscles and DIA; these biphasic responses were less prominent at 14-21 days than at < 6 days. Vagotomy caused an increase in the amplitude of both TS and TA (38 +/- 30 and 27 +/- 21%, respectively) as well as the DIA (45 +/- 16%) but did not affect their relative responses to chemostimulation. We conclude that during postnatal development 1) the rib cage and abdominal expiratory muscle responses to chemostimulation are coupled to each other and parallel those of the DIA and 2) the presence of vagal afferents attenuates the drive to both inspiratory and expiratory motoneurons under the current experimental conditions but does not influence the relative responses of expiratory muscles and DIA to hypercarbia or hypoxia. We speculate that comparable activation of inspiratory and expiratory pumping muscles serves to stabilize respiratory control in the face of altered chemosensory or vagal inputs during early postnatal life.

In animals with intact peripheral chemosensory afferents, hypoxia differentially affects upper airway (UA) and chest wall muscles. To determine the contribution of brain stem (BS) hypoxia to the response of UA and chest wall muscles during early life, we perfused the BS through a vertebral artery intermittently with blood from an extracorporeal circuit in nine newborn piglets (age 1–5 days). BS perfusions were performed with hypoxemic blood (arterial PO2 32 +/- 6 to 38 +/- 8 Torr) with different levels of BS PCO2 (28 +/- 2, 37 +/- 4, and 56 +/- 5 Torr) while systemic normocapnic hyperoxia was maintained (arterial PCO2 36 +/- 3 to 40 +/- 6 Torr, arterial PO2 345 +/- 73 to 392 +/- 37 Torr). Electromyograms (EMGs) of alae nasi (AN), external intercostal (EI), and diaphragm (DIA) were recorded. Normocapnic hypoxia of the BS induced a sustained increase in AN EMG (P < 0.01, analysis of variance) and depression of EI and DIA EMGs without a transient increase. These contrasting responses were also observed during hypocapnic and hypercapnic hypoxia of the BS and were not affected by inputs from the peripheral chemoreceptors or rostral cerebral structures that were not exposed to hypoxia. We conclude that, despite eliciting the known central respiratory depression, BS hypoxia causes an increase in the respiratory drive to an UA airway muscle. Thus, BS hypoxia elicits a selective rather than a generalized respiratory muscle depression. The respiratory muscles with high energy expenditure (DIA and EI) are depressed while UA muscles are stimulated or disinhibited. This response is independent of the level of BS arterial PCO2.

The present study evaluated the reflex response of the expiratory muscles to pulmonary congestion and edema. The electromyograms of two thoracic and four abdominal expiratory muscles were recorded in 12 anesthetized dogs. Pulmonary edema was induced by rapid saline infusion in six dogs and injection of oleic acid into the pulmonary circulation in the remaining six dogs. Both forms of pulmonary edema reduced pulmonary compliance, interfered with gas exchange, and induced a rapid and shallow breathing pattern. The electrical activity of all abdominal muscles was suppressed during both forms of pulmonary edema. In contrast, the electromyogram activity of the thoracic expiratory muscles was not significantly affected by pulmonary edema. Acute pulmonary arterial hypertension produced in two dogs by inflating a balloon in the left atrium had no effect on ventilation or expiratory muscle electrical activity. In two vagotomized dogs, pulmonary edema did not inhibit the expiratory muscles. We conclude that pulmonary edema suppresses abdominal but not thoracic expiratory muscle activity by vagal reflex pathway(s). Extravasation of fluid into the lung appears to be more important than an increase in pulmonary vascular pressure in eliciting this response.

The tachykinins substance P (SP) and neurokinin A (NKA) have been shown to induce airway smooth muscle contraction in mature animals, and the enzyme neutral endopeptidase (NEP) modulates this effect. We evaluated maturation of SP- and NKA-induced tracheal smooth muscle contraction and modulation of their effects by NEP in anesthetized, paralyzed, and artificially ventilated piglets less than 4 days, 2–3 wk, and 10 wk of age. Tracheal smooth muscle tension was measured in vivo from an open tracheal segment by use of a force transducer. Intravenous SP caused a dose-dependent increase in tracheal tension in all three age groups; however, the response in less than 4-day-old piglets was significantly weaker than in 2- to 3- and 10-wk-old piglets. NKA caused a dose-dependent increase in tracheal tension only in 2- to 3- and 10-wk-old piglets. The response of tracheal tension to NKA was weaker than the response to SP in all age groups. Atropine (2 mg/kg) significantly diminished the responses of tracheal tension to SP and NKA, indicating a cholinergic contribution to these responses at all ages. Intravenous thiorphan, a known NEP inhibitor, potentiated the effects of SP only in 2- to 3- and 10-wk-old piglets and did not affect the response of tracheal tension to NKA at any age. Biochemical analyses demonstrated a significant increase in tracheal NEP activity in comparably aged piglets over the first 10 wk of life.(ABSTRACT TRUNCATED AT 250 WORDS)

In dogs tracheal secretion is enhanced reflexly and by locally acting mediators such as substance P (SP). To evaluate the role of these mechanisms on submucosal gland secretion in the larynx (L) and pharynx (Ph), we compared the effects of mechanical stimulation of intrapulmonary irritant receptors and stimulation of pulmonary C-fiber receptors by capsaicin (20 micrograms/kg iv) with the response produced by intravenous SP. In six alpha-chloralose-anesthetized, paralyzed, and artificially ventilated dogs, submucosal gland secretion was monitored by analyzing the areas covered by hillocks of liquid and calculating the volume of secreted liquid (microliter) in the L and Ph. Mechanical stimulation of the carina increased both the number of hillocks and the volume of secreted liquid in the L. Excitation of pulmonary C-fiber receptors also increased the number of hillocks, and total volume of secreted liquid was elevated from 1.9 +/- 0.5 to 8.3 +/- 1.4 microliters (P less than 0.01). These responses were significantly reduced by prior cervical vagotomy and intravenous administration of atropine. Neither stimulation of irritant receptors nor stimulation of pulmonary C-fiber receptors caused discernible effects on Ph submucosal gland secretion. However, intravenous SP increased the number of Ph hillocks and elevated the volume of secreted Ph liquid from 1.0 +/- 0.6 to 10.2 +/- 1 microliters (P less than 0.01); similar responses to intravenous SP were observed in the L. Prior intravenous administration of atropine methylnitrate or bilateral vagotomy did not alter Ph or L secretory responses to intravenous SP.(ABSTRACT TRUNCATED AT 250 WORDS)

Airway secretion can be modified reflexly as well as locally. Previous studies indicate that neurons in a circumscribed region near the ventral surface of the medulla (VMS) can substantially modify airway tone and reflex responses to vagal inputs. In the present studies we assessed the importance of these neurons on tracheal gland secretion. We examined the changes in the number of hillocks of secretion appearing from submucosal glands in an exposed field of tracheal epithelium (1.2 cm2) coated with tantalum dust before and after interventions on the VMS. Experiments were performed in alpha-chloralose-anesthetized dogs paralyzed and ventilated with 40% O2. Stimulation of nicotinergic receptors by application of a pledget containing nicotine in 11 dogs caused a significant elevation in tracheal gland secretion in the subsequent 60 s, compared with a control period in which buffered saline was applied. Prior application of lidocaine or hexamethonium bromide to the VMS blocked the effect of topically applied nicotine. The central effects of nicotine were diminished by atropine methylnitrate given intravenously. In addition, lidocaine application to the VMS or focal cooling of intermediate areas to between 20 and 15 degrees C significantly decreased secretion rates reflexly produced by capsaicin-induced stimulation of pulmonary C-fiber receptors and by mechanical stimulation of the carina and larynx. These findings suggest that the ventral medulla contains cells near its surface that influence tracheal fluid secretion and modulate reflex responses of airway submucosal glands, probably by altering the level of general excitation within the central respiratory integrating circuits.

Stimulation of chemo-, irritant, and pulmonary C-fiber receptors reflexly constricts airway smooth muscle and alters ventilation in mature animals. These reflex responses of airway smooth muscle have, however, not been clearly characterized during early development. In this study we compared the maturation of reflex pathways regulating airway smooth muscle tone and ventilation in anesthetized, paralyzed, and artificially ventilated 2- to 3- and 10-wk-old piglets. Tracheal smooth muscle tension was measured from an open tracheal segment by use of a force transducer, and phrenic nerve activity was measured from a proximal cut end of the phrenic nerve. Inhalation of 7% CO2 caused a transient increase in tracheal tension in both age groups, whereas hypoxia caused no airway smooth muscle response in either group. The phrenic responses to 7% CO2 and 12% O2 were comparable in both age groups. Lung deflation and capsaicin (20 micrograms/kg iv) administration did not alter tracheal tension in the younger piglets but caused tracheal tension to increase by 87 +/- 28 and 31 +/- 10%, respectively, in the older animals (both P less than 0.05). In contrast, phrenic response to both stimuli was comparable between ages: deflation increased phrenic activity while capsaicin induced neural apnea. Laryngeal stimulation did not increase tracheal tension but induced neural apnea in both age groups. These data demonstrate that between 2 and 10 wk of life, piglets exhibit developmental changes in the reflex responses of airway smooth muscle situated in the larger airways in response to irritant and C-fiber but not chemoreceptor stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

The central effects of tachykinins (substance P, neurokinin A, and neurokinin B) on the distribution of the motor activity to rib cage and abdominal expiratory muscles were studied in anesthetized tracheotomized spontaneously breathing dogs and cats. Intracisternal application of substance P (11 dogs) in doses of 10(-5) to 10(-4) M caused diaphragm electrical activity to change insignificantly from 19.3 +/- 1.9 to 24.8 +/- 3.2 units (P greater than 0.05), produced a moderate increase of triangularis sterni activity from 12.6 +/- 2.2 to 19.2 +/- 2.2 units (P less than 0.05), and stimulated a large increase of transversus abdominis activity from 9.4 +/- 2.7 to 28.5 +/- 2.6 units (P less than 0.01). Comparable effects were seen with similar doses of neurokinin A (8 dogs) and neurokinin B (3 dogs) administered intracisternally. Local application of substance P to the ventral medullary surface (5 dogs and 4 cats) also caused expiratory muscle activity to increase more than diaphragm activity, and in addition transversus abdominis activity increased to a larger extent than triangularis sterni activity. Furthermore, administration of the substance P antagonist [D-Pro2,D-Trp7,9]-SP to the ventral medullary surface decreased respiratory motor output, with expiratory muscles activity being attenuated to a greater extent than diaphragm activity. Application of neurotensin and N-methyl-D-asparate to the ventral surface of the medulla produced responses similar to those observed as a result of central administration of tachykinin peptides. The results suggest that 1) mammalian tachykinins are involved in the regulation of thoracic and abdominal expiratory muscle activity, 2) these muscles manifest substantial differences in their electrical responses to excitatory neuropeptides acting centrally, and 3) inputs from modulatory neurons located in this vicinity of the ventral medullary surface seem to be distributed unevenly to different expiratory premotor and/or motoneurons.

The neonatal ventilatory response to hypoxia is characterized by initial transient stimulation and subsequent respiratory depression. It is unknown, however, whether this response is also exhibited by the upper airway muscles that regulate nasal, laryngeal, and pharyngeal patency. We therefore compared electromyogram (EMG) amplitudes and minute EMGs for the diaphragm (DIA), alae nasi (AN), posterior cricoarytenoid (PCA), and genioglossus (GG) muscles in 12 anesthetized spontaneously breathing piglets during inhalation of 12% O2 over 10 min. Minute EMG for the DIA responded to hypoxia with an initial transient increase and subsequent return to prehypoxia levels by 10 min. Hypoxia also stimulated all three upper airway muscles. In contrast to the DIA EMG, however, AN, PCA, and GG EMGs all remained significantly above prehypoxia levels after 10 min of hypoxia. We have thus demonstrated that the initial stimulation and subsequent depression of the DIA EMG after 12% O2 inhalation contrast with the sustained increase in AN, PCA, and GG EMGs during hypoxia. We speculate that 1) central inhibition during neonatal hypoxia is primarily distributed to the motoneuron pools regulating DIA activation and 2) peripheral chemoreceptor stimulation and/or central disinhibition induced by hypoxia preferentially influence those motoneuron pools that regulate upper airway muscle activation, causing the different hypoxic responses of these muscle groups in the young piglet.

The inspiratory phase of coughs often consists of large inspired volumes and increased motor discharge to the costal diaphragm. Furthermore, diaphragm electrical activity may persist into the early expiratory portion of coughs. To examine the role of other inspiratory muscles during coughing, electromyograms (EMG) recorded from the crural diaphragm (Dcr) and parasternal intercostal (PSIC) muscles were compared to EMG of the costal diaphragm (Dco) in anesthetized cats. Tracheal or laryngeal stimulation typically produced a series of coughs, with variable increases in peak inspiratory EMGs of all three muscles. On average, peak inspiratory EMG of Dco increased to 346 +/- 60% of control (P less than 0.001), Dcr to 514 +/- 82% of control (P less than 0.0002), and PSIC to 574 +/- 61% of control (P less than 0.0005). Augmentations of Dcr and PSIC EMG were both significantly greater than of Dco EMG (P less than 0.05 and P less than 0.002, respectively). In most animals, EMG of Dco correlated significantly with EMG of Dcr and of PSIC during different size coughs. Electrical activity of all three muscles persisted into the expiratory portions of many (but not all) coughs. The duration of expiratory activity lasted on average 0.17 +/- 0.03 s for Dco, 0.25 +/- 0.06 s for Dcr, and 0.31 +/- 0.09 s for PSIC. These results suggest that multiple respiratory muscles are recruited during inspiration of coughs, and that the persistence of electrical activity into expiration of coughs is not unique to the costal diaphragm.

The purpose of the present study was to assess the effects of bronchoconstriction on respiratory changes in length of the costal diaphragm and the parasternal intercostal muscles. Ten dogs were anesthetized with pentobarbital sodium and tracheostomized. Respiratory changes in muscle length were measured using sonomicrometry, and electromyograms were recorded with bipolar fine-wire electrodes. Administration of histamine aerosols increased pulmonary resistance from 6.4 to 14.5 cmH2O X l–1 X s, caused reductions in inspiratory and expiratory times, and decreased tidal volume. The peak and rate of rise of respiratory muscle electromyogram (EMG) activity increased significantly after histamine administration. Despite these increases, bronchoconstriction reduced diaphragm inspiratory shortening in 9 of 10 dogs and reduced intercostal muscle inspiratory shortening in 7 of 10 animals. The decreases in respiratory muscle tidal shortening were less than the reductions in tidal volume. The mean velocity of diaphragm and intercostal muscle inspiratory shortening increased after histamine administration but to a smaller extent than the rate of rise of EMG activity. This resulted in significant reductions in the ratio of respiratory muscle velocity of shortening to the rate of rise of EMG activity after bronchoconstriction for both the costal diaphragm and the parasternal intercostal muscles. Bronchoconstriction changed muscle end-expiratory length in most animals, but for the group of animals this was statistically significant only for the diaphragm. These results suggest that impairments of diaphragm and parasternal intercostal inspiratory shortening occur after bronchoconstriction; the mechanisms involved include an increased load, a shortening of inspiratory time, and for the diaphragm possibly a reduction in resting length.

These studies investigated the role of the intermediate area of the ventral surface of the medulla (VMS) in the tracheal constriction produced by hypercapnia. Experiments were performed in chloralose-anesthetized, paralyzed, and artificially ventilated cats. Airway responses were assessed from pressure changes in a bypassed segment of the rostral cervical trachea. Hyperoxic hypercapnia increased tracheal pressure and phrenic nerve activity. Intravenous atropine pretreatment or vagotomy abolished the changes in tracheal pressure without affecting phrenic nerve discharge. Rapid cooling of the intermediate area reversed the tracheal constriction produced by hypercapnia. Graded cooling produced a progressive reduction in the changes in maximal tracheal pressure and phrenic nerve discharge responses caused by hypercapnia. Cooling the intermediate area to 20 degrees C significantly elevated the CO2 thresholds of both responses. These findings demonstrate that structures near the intermediate area of the VMS play a role in the neural cholinergic responses of the tracheal segment to CO2. It is possible that neurons or fibers in intermediate area influence the motor nuclei innervating the trachea. Alternatively, airway tone may be linked to respiratory motor activity so that medullary interventions that influence respiratory motor activity also alter bronchomotor tone.

The abdominal muscles accelerate airflow during expiration and may also influence the end-expiratory volume and configuration of the thorax. Although much is known about their electrical activity, the degree to which they change length during the respiratory cycle has not been previously assessed. In the present study we measured respiratory changes in transverse abdominis length using sonomicrometry in 14 pentobarbital sodium-anesthetized supine dogs and compared length changes to simultaneously recorded tidal volume and transverse abdominis electromyograms (EMG). To determine muscle resting length at passive functional residual capacity (LFRC), the animals were hyperventilated to apnea. The transverse abdominis was electrically active in all animals during resting O2 breathing (eupnea). During inspiration the transverse abdominis lengthened above resting length in all 14 dogs by a mean of 3.7 +/- 1.1% LFRC; during expiration the transverse abdominis shortened below resting length in 13 of 14 dogs by a mean of 4.2 +/- 0.9% LFRC. Increasing hyperoxic hypercapnia (produced in 9 animals) progressively heightened transverse abdominis EMG and progressively increased the extent of muscle shortening below resting length (to 12.6 +/- 3.2% LFRC at a PCO2 of 90 Torr). During single-breath airway occlusion substantial inspiratory lengthening of the transverse abdominis occurred, both during O2 breathing and during CO2 rebreathing.(ABSTRACT TRUNCATED AT 250 WORDS)

To assess the role of structures located superficially near the ventrolateral surface of the medulla on the reflex constriction of tracheal smooth muscle that occurs when airway and pulmonary receptors are stimulated mechanically or chemically, experiments were conducted in alpha-chloralose-anesthetized, paralyzed, and artificially ventilated cats. Pressure changes within a bypassed segment of the trachea were used as an index of alterations smooth muscle tone. The effects of focal cooling of the intermediate areas or topically applied lidocaine on the ventral surface of the medulla on the response of the trachea to mechanical and chemical stimulation of airway receptors were examined. Atropine abolished tracheal constriction induced by mechanical stimulation of the carina or aerosolized histamine, showing that the responses were mediated over vagal pathways. Moderate cooling of the intermediate area (20 degrees C) or local application of lidocaine significantly decreased the tracheal constrictive response to mechanical activation of airway receptors. Furthermore, when the trachea was constricted by histamine, cooling of the intermediate area significantly diminished the increased tracheal tone, whereas rewarming restored tracheal tone to the previous level. These findings suggest that under the conditions of the experiments the ventral surface of the medulla plays an important role in constriction of the trachea by inputs from intrapulmonary receptors and in the modulation of parasympathetic outflow to airway smooth muscle.

We assessed the mechanical behavior of the geniohyoid and sternohyoid muscles during spontaneous breathing using sonomicrometry in anesthetized cats. When the animals breathed O2, the hyoid muscles either became longer or did not change length (but never shortened) during inspiration. During progressive hyperoxic hypercapnia, transient increases in geniohyoid muscle inspiratory lengthening occurred in many animals; however, at high PCO2 the geniohyoid invariably shortened during inspiration (mean 4.9% of resting length at the end of CO2 rebreathing; P less than 0.001). The PCO2 at which geniohyoid inspiratory lengthening changed to inspiratory shortening was significantly higher than the CO2 threshold for the onset of geniohyoid electrical activity (P less than 0.01). For the sternohyoid muscle, hypercapnia caused inspiratory lengthening in 13 of 17 cats and inspiratory shortening in 4 of 17 cats; on average the sternohyoid lengthened by 1.6% of resting length at the end of CO2 rebreathing (P less than 0.01). Sternohyoid lengthening occurred in spite of this muscle being electrically active. These results suggest that the relationship between hyoid muscle electrical activity and respiratory changes in length is very complex, so that the presence of hyoid muscle electrical activity does not necessarily indicate muscle shortening, and among the geniohyoid and sternohyoid muscles, the geniohyoid has a primary role as a hypopharyngeal dilator in the spontaneously breathing cat, with the sternohyoid muscle acting in an accessory capacity.

Studies in mammals have found that during breathing the triangularis sterni (TS) muscle regulates expiratory airflow and the end-expiratory position of the rib cage and furthermore that the respiratory activity of this muscle is influenced by a variety of chemical and mechanical stimuli. To assess the role of the TS during coughing and sneezing, electromyograms (EMGs) recorded from the TS were compared with EMGs of the transversus abdominis (TA) in eight pentobarbital-anesthetized dogs. During coughing induced by mechanically stimulating the trachea or larynx (n = 7 dogs), peak EMGs increased from 23 +/- 2 to 74 +/- 5 U (P less than 0.00002) for the TS and from 21 +/- 6 to 66 +/- 4 U (P less than 0.0002) for the TA. During sneezing induced by mechanically stimulating the nasal mucosa (n = 3 dogs), peak EMG of the TS increased from 10 +/- 3 to 66 +/- 7 U (P less than 0.005) and peak EMG of the TA increased from 10 +/- 2 to 73 +/- 7 U (P less than 0.02). For both muscles the shape of the EMG changed to an early peaking form during coughs and sneezes. Peak expiratory airflow during coughs of different intensity correlated more closely with peak TS EMG in three dogs and with peak TA EMG in four dogs; peak expiratory airflow during sneezes of different intensity correlated more closely with peak TS than TA EMG in all three animals. These results suggest that the TS is actively recruited during coughing and sneezing and that different neuromuscular strategies may be utilized to augment expiratory airflow.

The electromyographic (EMG) activities of the costal and crural diaphragm were recorded from bipolar fine-wire electrodes placed in the costal fibers adjacent to the central tendon and in the anterior portions of the crural fibers in 12 anesthetized cats. The EMG activities of costal and crural recordings were compared during posture changes from supine to head up and during progressive hyperoxic hypercapnia in both positions. The activity of both portions of the diaphragm was greater in the head up compared with supine posture at all levels of CO2; and increases in crural activity were greater than those in costal activity both as a result of changes in posture and with increasing CO2 stimuli. These results are consistent with the concept that diaphragm activation is modulated in response to changes in resting muscle length, and further, that neural control mechanisms allow separate regulation of costal and crural diaphragm activation.

Structures located near the ventral surface of the medulla (VMS) affect both cardiovascular tone and respiratory activity. In addition cooling the intermediate area of the VMS blocks the increases in parasympathetic activity and tracheal tone resulting from ventilation with hypercapnic or hypoxic gas mixtures, or due to stimulation of mechanoreceptors within the lung. Since cooling the surface of the VMS may affect fibers of passage as well as cell bodies, we performed studies in which pledgets containing N-methyl-D-aspartic acid (NMDA), a synthetic excitatory amino acid, were applied to intermediate area of the VMS. The studies were performed in chloralose-anesthetized, artificially ventilated cats. Application of pledgets containing NMDA (10(-7) mol at 10(-3) M) caused increases in tracheal pressure and the onset of phasic phrenic activity, but application of 10(-8) mol at 10(-4) M of NMDA could produce tracheal constriction without the appearance of phasic phrenic activity. Applying to the entire VMS either 2-amino-5-phosphonovalerate (2-APV, 10(-6) M), a specific antagonist to NMDA, or lidocaine (2%), a local anesthetic, 60 s before the application of pledgets containing NMDA, prevented the increase in tracheal tone and phasic phrenic activity. Intravenous administration of atropine methyl nitrate 0.5 mg/kg, a cholinergic antagonist, blocked tracheal responses to local application of pledgets containing NMDA but did not affect the increase in phasic phrenic nerve activity. These findings suggest that when stimulated, neurons near the surface of the VMS in the vicinity of the intermediate area increase the activity of parasympathetic fibers to the airway.

The steady-state and transient effects of hyperoxic hypercapnia on the electromyographic activities of the genioglossus (GG), posterior cricoarytenoid (PCA), and diaphragm (D) were studied in conscious unsedated cats with chronically implanted electrodes. Hypercapnia (inhalation of 3.4 and 7.4% CO2 in O2) increased the phasic electrical activity occurring during inspiration in all three muscles and also increased tonic activity of the GG. The GG responded to steady-state CO2 inhalation alinearly and with larger increases in activity than the PCA and D. Phasic GG activity was present in only 4 of 10 cats breathing 100% O2, whereas phasic PCA and D activity could be observed in all animals studied. When gas mixtures containing CO2 were given, the GG reached its new steady-state level more slowly than the D or PCA, and when CO2 was rapidly removed from the inspired gas mixture, the GG attained its steady state sooner than either the PCA or D. These results suggest that in awake unsedated animals, chemical stimuli do not affect either transient or steady-state responses of the GG in the same way as the D. These differences seem to be explained mainly by different threshold characteristics of hypoglossal and phrenic neurons but also in part by dissimilarities in their steady-state responses.