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Lung resistance (RL) is determined by airway and parenchymal tissue resistance, as well as the degree of heterogeneity in airway constriction. Deep inspirations (DIs) are known to reverse experimentally induced increase in RL, but the mechanism is not entirely clear. The first step toward understanding the effect of DI is to determine how each of the resistance components is affected by DI. In the present study, we measured RL and apparent airway resistance (RAW, which combines the effects of airway resistance and airway heterogeneity) simultaneously before and after a DI in acetylcholine (ACh)-challenged ex vivo sheep lungs. We found that at normal breathing frequency (0.25 Hz) ACh-challenge led to a doubling of RL, 80.3% of that increase was caused by an increase in RAW; the increase in apparent tissue resistance (RT) was insignificant. 57.7% of the increase in RAW was abolished by a single DI. After subtracting RAW from RL, the remaining RT was mostly independent of ACh-challenge and its reduction after a DI came mostly from the change in the mechanical properties of lung parenchyma. We conclude that at normal breathing frequency, RL in an unchallenged lung is mostly composed of RT, and the increase in RL due to ACh-challenge stems mostly from the increase in RAW and that both RAW and RT can be greatly reduced by a DI, likely due to a reduction in true airway resistance and heterogeneity, as well as parenchymal tissue hysteresis post DI. [ABSTRACT FROM AUTHOR]

Lung resistance (RL) and elastance (EL) can be measured during positive or negative pressure ventilation. Whether the different modes of ventilation produce different RL and EL is still being debated. Although negative pressure ventilation (NPV) is more physiological, positive pressure ventilation (PPV) is more commonly used for treating respiratory failure. In the present study, we measured lung volume, airway diameter, and airway volume, as well as RL and EL with PPV and NPV in explanted sheep lungs. We found that lung volume under a static pressure, either positive or negative, was not different. However, RL and EL were significantly higher in NPV at high inflation pressures. Interestingly, diameters of smaller airways (diameters <3.5 mm) and total airway volume were significantly greater at high negative inflation pressures compared with those at high positive inflation pressures. This suggests that NPV is more effective in distending the peripheral airways, likely due to the fact that negative pressure is applied through the pleural membrane and reaches the central airways via the peripheral airways, whereas positive pressure is applied in the opposite direction. More distension of lung periphery could explain why RL is higher in NPV (vs. PPV), because the peripheral parenchyma is a major source of tissue resistance, which is a part of the RL that increases with pressure. This explanation is consistent with the finding that during high frequency ventilation (>1 Hz, where RL reflects airway resistance more than tissue resistance), the difference in RL between NPV and PPV disappeared. [ABSTRACT FROM AUTHOR]

Deep inspiration (DI)-induced bronchodilation is the first line of defense against bronchoconstriction in healthy subjects. A hallmark of asthma is the lack of this beneficial effect of DI. The mechanism underlying the bronchodilatory effect of DI is not clear. Understanding the mechanism will help us unravel the mystery of asthma pathophysiology. It has been postulated that straining airway smooth muscle (ASM) during a DI could lead to bronchodilation and bronchoprotection. The hypothesis is currently under debate, and a central question is whether ASM is sufficiently stretched during a DI for its contractility to be compromised. Besides bronchoconstriction, another contributor to lung resistance is airway heterogeneity. The present study examines changes in airway diameter and heterogeneity at different lung volumes. Freshly explanted sheep lungs were used in plethysmographic measurements of lung resistance and elastance at different lung volumes, whereas the airway dimensions were measured by computed tomography (CT). The change in airway diameter informed by CT measurements was applied to isolated airway ring preparations to determine the strain-induced loss of ASM contractility. We found that changing the transpulmonary pressure from 5 to 30 cmH2O led to a 51% increase in lung volume, accompanied by a 46% increase in the airway diameter with no change in airway heterogeneity. When comparable airway strains measured in the whole lung were applied to isolated airway rings in either relaxed or contracted state, a significant loss of ASM contractility was observed, suggesting that DI-induced bronchodilation and bronchoprotection can result from strain-induced loss of ASM contractility. [ABSTRACT FROM AUTHOR]

The cyclic interaction between myosin crossbridges and actin filaments underlies smooth muscle contraction. Phosphorylation of the 20-kDa myosin light chain (MLC20) is a crucial step in activating the crossbridge cycle. Our current understanding of smooth muscle contraction is based on observed correlations among MLC20 phosphorylation, maximal shortening velocity (Vmax), and isometric force over the time course of contraction. However, during contraction there are changes in the extent of phosphorylation of many additional proteins as well as changes in activation of enzymes associated with the signaling pathways. As a consequence, the mechanical manifestation of muscle contraction is likely to change with time. To simplify the study of these relationships, we measured the mechanical properties of airway smooth muscle at different levels of MLC20 phosphorylation at a fixed time during contraction. A simple correlation emerged when time-dependent variables were fixed. MLC20 phosphorylation was found to be directly and linearly correlated with the active stress, stiffness, and power of the muscle; the observed weak dependence of Vmax on MLC20 phosphorylation could be explained by the presence of an internal load in the muscle preparation. These results can be entirely explained by the Huxley crossbridge model. We conclude that when the influence of time-dependent events during contraction is held constant, the basic crossbridge mechanism in smooth muscle is the same as that in striated muscle. [ABSTRACT FROM AUTHOR]

Taking a big breath is known to reverse bronchoconstriction induced by bronchochallenge in healthy subjects; this bronchodilatory effect of deep inspiration (DI) is diminished in asthmatics. The mechanism underlying the DI effect is not clear. Observations from experiments using isolated airway smooth muscle (ASM) preparations and airway segments suggest that straining of ASM due to DI could lead to bronchodilation, possibly due to strain-induced reduction in ASM contractility. However, factors external to the lung cannot be excluded as potential causes for the DI effect. Neural reflex initiated by stretch receptors in the lung are known to inhibit the broncho-motor tone and enhance vasodilatation; the former directly reduces airway resistance, and the latter facilitates removal of contractile agonists through the bronchial circulation. If the DI effect is solely mediated by factors extrinsic to the lung, the DI effect would be absent in isolated, nonperfused lungs. Here we examined the DI effect in freshly isolated, nonperfused sheep lungs. We found that imposition of DI on isolated lungs resulted in significant bronchodilation, that this DI effect was present only after the lungs were challenged with a contractile agonist (acetylcholine or histamine), and that the effect was independent of the difference in lung volume observed pre- and post-DI. We conclude that a significant portion of the bronchodilatory DI effect stems from factors internal to the lung related to the activation of ASM. [ABSTRACT FROM AUTHOR]

Airway smooth muscle (ASM) in vivo is constantly subjected to oscillatory strain due to tidal breathing and deep inspirations. ASM contractility is known to be adversely affected by strains, especially those of large amplitudes. Based on the cross-bridge model of contraction, it is likely that strain impairs force generation by disrupting actomyosin cross-bridge interaction. There is also evidence that strain modulates muscle stiffness and force through induction of cytoskeletal remodeling. However, the molecular mechanism by which strain alters smooth muscle function is not entirely clear. Here, we examine the response of ASM to iso-velocity stretches to probe the components within the muscle preparation that give rise to different features in the force response. We found in ASM that force response to a ramp stretch showed a biphasic feature, with the initial phase associated with greater muscle stiffness compared with that in the later phase, and that the transition between the phases occurred at a critical strain of ~3.3%. Only strains with amplitudes greater than the critical strain could lead to reduction in force and stiffness of the muscle in the subsequent stretches. The initial-phase stiffness was found to be linearly related to the degree of muscle activation, suggesting that the stiffness stems mainly from attached cross bridges. Both phases were affected by the degree of muscle activation and by inhibitors of myosin light-chain kinase, PKC, and Rho-kinase. Different responses due to different interventions suggest that cross-bridge and cytoskeletal stiffness is regulated differently by the kinases. [ABSTRACT FROM AUTHOR]

Smooth muscle contraction can be divided into two phases: the initial contraction determines the amount of developed force and the second phase determines how well the force is maintained. The initial phase is primarily due to activation of actomyosin interaction and is relatively well understood, whereas the second phase remains poorly understood. Force maintenance in the sustained phase can be disrupted by strains applied to the muscle; the strain causes actomyosin cross-bridges to detach and also the cytoskeletal structure to disassemble in a process known as fluidization, for which the underlying mechanism is largely unknown. In the present study we investigated the ability of airway smooth muscle to maintain force after the initial phase of contraction. Specifically, we examined the roles of Rho-kinase and protein kinase C (PKC) in force maintenance. We found that for the same degree of initial force inhibition, Rho-kinase substantially reduced the muscle's ability to sustain force under static conditions, whereas inhibition of PKC had a minimal effect on sustaining force. Under oscillatory strain, Rho-kinase inhibition caused further decline in force, but again, PKC inhibition had a minimal effect. We also found that Rho-kinase inhibition led to a decrease in the myosin filament mass in the muscle cells, suggesting that one of the functions of Rho-kinase is to stabilize myosin filaments. The results also suggest that dissolution of myosin filaments may be one of the mechanisms underlying the phenomenon of fluidization. These findings can shed light on the mechanism underlying deep inspiration induced bronchodilation. [ABSTRACT FROM AUTHOR]

Airway smooth muscle (ASM) from infant guinea pigs has less spontaneous relaxation during stimulation than ASM from adults. Inhibition of cyclooxygenase (COX), which catalyzes the production of prostanoids, increases this relaxation in infant ASM and abolishes age differences, thus suggesting that prostanoids reduce relaxation in infant ASM. In this study, we investigated whether leukotrienes are also involved in reducing spontaneous relaxation; whether the two COX isoforms, COX-1 and COX-2, differentially regulate spontaneous relaxation; and whether prostanoid release is developmentally regulated in guinea pig ASM. In different age groups, we measured relaxation during and after electrical stimulation in tracheal strips as well as prostanoid release from tracheal segments. Relaxation was studied in the absence and in the presence of a lipoxygenase inhibitor, a cysteinyl leukotriene receptor-i antagonist, a COX-1 inhibitor, or a COX-2 inhibitor. We found that inhibition of lipoxygenase or cysteinyl leukotriene receptor-1 antagonism did not increase spontaneous relaxation at any age, thus excluding a role for leukotrienes in this phenomenon. Inhibition of COX-2, but not COX-1, promoted spontaneous relaxation. The basal release of prostanoids was more abundant in tissue from infant animals and decreased significantly with age. Thromboxane B2 was the most abundant metaboiite released at all ages. Electrical stimulation and epithelium removal did not affect the age difference in prostanoid release. We conclude that increased basal prostanoid release contributes to the reduced spontaneous relaxation in immature guinea pig ASM compared with older animals. By regulating ASM relaxation, prostanoids may play a role in the airway hyperresponsiveness at a young age. [ABSTRACT FROM AUTHOR]

NAD(P)H oxidase is one of the critical enzymes mediating cellular production of reactive oxygen species and has a central role in airway smooth muscle (ASM) cell proliferation. Since reactive oxygen species also affect ASM contractile response, we hypothesized a regulatory role of NAD(P)H oxidase in ASM contractility. We therefore studied ASM function in wild-type mice (C57BL/6J) and mice deficient in a component (p47phox) of NAD(P)H oxidase. In histological sections of the trachea, we found that the area occupied by ASM was 17% more in p47phox than in wild-type mice. After correcting for the difference in ASM content, we found that force generation did not vary between the two genotypes. Similarly, their ASM shortening velocity, maximal power, and sensitivity to acetylcholine, as well as airway responsiveness to methacholine in vivo, were not significantly different. The main finding of this study was a significantly reduced ASM relaxation in p47phox compared with wild-type mice both during the stimulus and after the end of stimulation. The tension relaxation attained at the 20th second of electric field stimulation was, respectively, 17.6 ± 2.4 and 9.2 ± 2.3% in null and wild-type mice (P <0.01 by t-test). Similar significant differences were found in the rate of tension relaxation and the time required to reduce tension by one-half. Our data suggest that NAD(P)H oxidase may have a role in the structural arrangement and mechanical properties of the airway tissue. Most importantly, we report the first evidence that the p47phox subunit of NAD(P)H oxidase plays a role in ASM relaxation. [ABSTRACT FROM AUTHOR]

Deep inspiration counteracts bronchospasm in normal subjects but triggers further bronchoconstriction in hyperresponsive airways. Although the exact mechanisms for this contrary response by normal and hyperresponsive airways are unclear, it has been suggested that the phenomenon is related to changes in force-generating ability of airway smooth muscle after mechanical oscillation. It is known that healthy immature airways of both humans and animals exhibit hyperresponsiveness. We hypothesize that the profile of active force generation after mechanical oscillation changes with maturation and that this change contributes to the expression of airway hyperresponsiveness in juveniles. We examined the effect of an acute sinusoidal length oscillation on the force-generating ability of tracheal smooth muscle from 1 wk, 3 wk, and 2- to 3-mo-old guinea pigs. We found that the length oscillation produced 15-20% initial reduction in active force equally in all age groups. This was followed by a force recovery profile that displayed striking maturation-specific features. Unique to tracheal strips from 1-wk-old animals, active force potentiated beyond the maximal force generated before oscillation. We also found that actin polymerization was required in force recovery and that prostanoids contributed to the maturation-specific force potentiation in immature airway smooth muscle. Our results suggest a potentiated mechanosensitive contractile property of hyperresponsive airway smooth muscle. This can account for further bronchoconstriction triggered by deep inspiration in hyperresponsive airways. [ABSTRACT FROM AUTHOR]

Previously, we showed the shortening velocity of guinea pig tracheal strips was the greatest in juvenile (3-wk-old) compared with infant (1-wk-old) and adult animals (3-mo-old). The greatest shortening velocity was associated with the least resistance to shortening calculated from force-velocity curves among the three age groups. It remained to be verified if the stiffness of tracheal tissue, a measure of tissue response to geometrical deformations, is different among the three age groups. We hypothesized that stiffness of intact tracheal strips is lowest in the juvenile group and that this can explain the ontogeny of airway smooth muscle resistance to shortening and shortening velocity. Static stiffness measured through stepwise deformations showed no age-related differences. Evaluation of tissue response to oscillatory deformations showed that the dynamic stiffness of unstimulated tracheal strips was 8.35 ± 0.88, 4.15 ± 1.09, and 8.21 ± 1.57 kPa, and the phase angle was 10.3 ± 2.93, 2.46 ± 0.67, and 7.87 ± 1.77° in infant, juvenile, and adult, respectively. Unstimulated juvenile strips were significantly lower in dynamic stiffness and phase angle compared with unstimulated infant or adult strips. This maturational profile was independent of muscle strip preset length or oscillation mode/amplitude but was abolished at peak of contraction to either carbachol or electric field stimulation. These results suggest that the noncontractile components of tracheal strips are less stiff and contain fewer viscous/frictional elements in juvenile than in other age groups. This may provide a functional basis for reduced resistance to length changes in juvenile airway smooth muscle. [ABSTRACT FROM AUTHOR]

Assesses airway hyperresponsiveness in ovalbumin-sensitized mice. Correlation of airway hyperresponsiveness with neutrophils transmigration; Tissue infiltration of inflammatory and immune cells; Physiological indicators of airway obstruction.