Search

Full muscarinic stimulation in bovine tracheal smooth muscle caused a sustained contraction and increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) that was largely resistant to inhibition by nifedipine. Depletion of internal Ca(2+) stores with cyclopiazonic acid resulted in an increased efficacy of nifedipine to inhibit this contraction and the associated increase in [Ca(2+)](i). Thus internal Ca(2+) store depletion promoted electromechanical coupling between full muscarinic stimulation and muscle contraction to the detriment of pharmacomechanical coupling. A similar change in coupling mode was induced by ryanodine even when it did not significantly modify the initial transient increase in [Ca(2+)](i) induced by this stimulation, indicating that depletion of internal stores was not necessary to induce the change in excitation-contraction coupling mode. Blockade of the Ca(2+)-activated K(+) channel by tetraethylammonium, charybdotoxin, and iberiotoxin all induced the change in excitation-contraction coupling mode. These results suggest that in this preparation, Ca(2+) released from the ryanodine-sensitive Ca(2+) store, by activating Ca(2+)-activated K(+) channels, plays a central role in determining the expression of the pharmacomechanical coupling mode between muscarinic excitation and the Ca(2+) influx necessary for the maintenance of tone.

Aim: To assess the cytotoxic effects of cobra cardiotoxin (CTX) on rat aorta., Methods: Measure of contractility of aortic rings with or without endothelium., Results: In endothelium-intact rings, CTX 10 mumol.L-1 induced a transient relaxation followed by a sustained contraction. Removal of the endothelium or pre-incubation of the rings with NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) abolished the transient relaxation but did not affect the magnitude of the contractile response induced by CTX. CTX itself induced contraction of vascular smooth muscle but also reduced contractions induced by phenylephrine (PhE) or KCl stimulation in a concentration-dependent manner. Contraction induced by CTX was dependent on the external Ca2+ concentration. Maximal contractile response to CTX was obtained in medium containing Ca2+ 1 mmol.L-1. This response decreased with higher Ca2+ concentration and disappeared when Ca2+ 7 mmol.L-1, organic and inorganic calcium channel blockers were present in the external solution before CTX addition. In preparations with the endothelium intact and incubated with CTX, relaxation by acetylcholine (ACh) stimulation of the tension induced by PhE was decreased. Endothelium-dependent relaxation to ACh was preserved when Ca2+ 5 mmol.L-1 was added to the medium prior to CTX., Conclusion: CTX first triggers the release of NO from the endothelium which results in muscle relaxation, and then causes smooth muscle contraction, Ca2+ and Ca2+ channel blockers prevented the effect of CTX.

1. Muscarinic stimulation-induced tonic contraction of airway smooth muscle is independent of membrane potential. This contraction is not sensitive to inhibition by voltage-operated Ca2+ channel blockers or by K+ channel openers. 2. Cyclopiazonic acid (CPA) inhibits Ca2+ loading of internal stores but does not affect maximal tonic contraction induced by acetylcholine (ACh) in steady state conditions. 3. After depletion of internal Ca2+ stores with CPA, ACh-induced tonic contraction becomes dependent upon values of membrane potential. The contraction is then sensitive to voltage-operated Ca2+ channel blockers and to K+ channel openers. 4. Treatment of trachealis muscle with CPA potentiates the M2-mediated component of ACh stimulation, but this potentiation is not entirely responsible for the switch in excitation-contraction (E-C) coupling. 5. It is proposed that depletion of internal Ca2+ stores with CPA and promotion of M2-stimulation can lead to a switch in E-C coupling in trachealis smooth muscle from pharmaco- to electromechanical mode, perhaps by targeting a plasma membrane K+ channel.

The aim of this study was to assess the nature of vascular hyporeactivity to vasopressor agents in rats with endotoxemia. Endotoxemia was induced in rats by bacterial endotoxin (E. Coli lipopolysaccaharide, LPS). In LPS-treated rats, the reactivity of endothelium-denuded aortic rings to phenylephrine (PE) and potassium chloride (KCl) was characterized by a decreased magnitude of contraction, a slower onset of contraction and a faster rate of relaxation when compared to the control aortic rings. Addition of L-arginine (L-arg), the substrate of nitric oxide synthase (NOS), but not D-arginine (D-arg), reduced further PE-induced contraction in rings from LPS-treated rats. Inhibition of contraction in rings of LPS-treated rats was partially antagonized by the inhibitor of NOS, N omega-nitro-L-arginine methyl ester (L-NAME). Thus, production of non-endothelial nitric oxide (NO) was in part responsible for the hyporesponsiveness to PE. Rings from LPS-treated rats also displayed hyporeactivity and decreased sensitivity to Ca2+ in depolarizing medium (60 mM K+). Hyporeactivity and hyposensitivity to Ca2+ could only be partially reversed by L-NAME. The inhibitory effects of LPS-treatment on both PE-and KCl-induced aortic responses and the reversal effects of L-NAME confirm the contention that NO formation is involved in vascular hyporesponsiveness in endotoxic shock. The partial reversal by L-NAME of the hyporesponsiveness to KCl- and PE-induced contraction, and hyposensitivity to Ca2+ in depolarized aorta suggest that factors other than the action of nonendothelial source of NO formation in vitro from L-arg also contribute to endotoxin-induced vascular hyporesponsiveness to vasopressor agents.

In smooth muscle, contractions under pharmacomechanical and electromechanical coupling mechanism control rely on mobilization of intracellular calcium and on calcium influx from the extracellular space, respectively. In airway smooth muscle, pharmacomechanical coupling seems to predominate. Even if extracellular calcium enters the cell during agonist stimulation, and contributes to the maintained plateau phase of a prolonged stimulation-induced contraction, it does not result from membrane depolarization and is not part of the electromechanical coupling mechanism. Pharmacomechanical and electromechanical coupling are most likely not independent processes: we present here an example of cross talk between plasma membrane and sarcoplasmic reticulum, which results in an almost complete switch in the E-C coupling process from pharmaco- to electromechanical mode in tracheal smooth muscle.

Damage to endocardial endothelium (denudation of the superficial tissue) by brief exposure to a 100-microL bolus of detergent (Triton X-100, 1% by volume stock) decreased the twitch force of papillary muscle (and trabeculae) by approximately 30% to a new but steady level without changes in resting tension. The decline in twitch force was evident immediately after the addition of Triton. Modification of the action potential measured from the contracting tissue appeared only later, when the change in contraction was already well established (i.e., after approximately 2 min). Action potential shortened in duration at 50% repolarization by approximately 100 ms and increased in plateau amplitude, although the latter increase was not always observed. A similar treatment procedure applied to strips of ventricular wall with the endocardium exposed to the superfusion solution resulted in a substantial decrease in action potential duration (approximately 110 ms). In contrast, treatment of strips of epicardial layers of ventricular walls (with epicardial side facing the superfusion solution) did not produce a similar result. In beta-stimulated (1 microM isoproterenol) and partially depolarized preparations (with 20 mM KCl), with intact endocardium, electrically evoked contractions were followed by aftercontractions, which were suppressed following Triton treatment. Action potentials in a depolarizing medium also shortened in duration (approximately 50 ms), although following a delay (2-3 min). The decay to steady state of postextrasystolic potentiated beat was slower after endocardial damage than under control conditions. This suggests an increased Ca2+ recirculation through the sarcoplasmic reticulum between two consecutive beats (35% before Triton vs. 45% after Triton).(ABSTRACT TRUNCATED AT 250 WORDS)

Norepinephrine (NE) overflow from field-stimulated rat was deferens preparations was quantified directly by electrochemical detection using high performance liquid chromatography. The effect of agonist (BHT 920) and antagonist (rauwolscine) of prejunctional alpha 2-adrenoceptors on NE overflow was assessed and compared with their effect on the smooth muscle mechanical response to field stimulation. Increasing the stimulation frequency from 2 to 30 Hz resulted in an increase in muscle tension together with an increase in NE overflow. Addition of 1 microM rauwolscine to the medium resulted in a significant increase in muscle contraction to field stimulation which reached a maximum at 5 Hz. On the other hand, NE overflow increased linearly with the frequency of stimulation within the range studied. Addition of 0.1 microM BHT 920 to the medium significantly decreased the amplitude of contractions at lower stimulation frequencies (2 to 10 Hz) but elicited no significant changes at high frequencies. BHT 920 did not significantly affect NE overflow for all range of stimulation frequency. The simultaneous recording of field-stimulation induced contractions and NE overflow indicates that in the rat vas deferens, rauwolscine acts like a pure alpha 2 adrenoceptor antagonist at a prejunctional level. BHT 920 did not appear to affect selectively prejunctional alpha 2 adrenoceptors but also may activate postjunctional alpha 1 adrenoceptors.

This review examines the ionic mechanisms underlying acetylcholine (Ach) depolarization of airway smooth muscle and suggests that multiple mechanisms are involved. Increased chloride and nonspecific cation conductance, and decreased or rapidly inactivating potassium conductances seem to be involved. Chloride ions also seem to play an important role in determining whether Ca2+ remains inside or is replenished in the sarcoplasmic reticulum (SR). The physiological role of Ach-induced depolarization is analysed and is suggested to be the promotion of the refilling of Ca2+ stores, partly through a direct refilling of SR-Ca2+ stores by way of an L-type Ca2+ channel. This refilling is promoted by Ca2+ channel agonists and is independent of the transmembrane potential. Ca(2+)-release by a variety of agonists leads to depolarization and stable membrane oscillations which depend on the action of the Ca(2+)-store uptake mechanisms in order to function. These oscillations may play a role in prolonged bronchoconstriction. Better knowledge of the control mechanisms of Cai2+ is likely to reveal new targets for the therapy of asthma and provide a better understanding of the function of airway smooth muscle.

The prejunctional activities of S-3341 and clonidine have been studied in the transmural field-stimulated epididymal part of the rat vas deferens. Both S-3341 and clonidine inhibited these neuronally induced contractions. At high concentrations, these agonists induced spontaneous contractions in the preparation, which were abolished by 10(-7) M prazosin. The inhibitory effects of S-3341 and clonidine were antagonized in a competitive manner by rauwolscine and were found not to be modified in a statistically significant manner by 10(-7) M prazosin. The pA2 values of rauwolscine against S-3341 and clonidine were the same, indicating the receptors influenced by these drugs were the same also. However, the efficacy of S-3341 is lower than that of clonidine (by a factor of approximately 100). This lower efficacy of S-3341 at the prejunctional level could be linked to the lack of a sedative effect of this compound at therapeutic concentrations.

The aim of this study was to evaluate the alpha 1-adrenoceptor properties of S.3341, a new alpha-adrenoceptor agonist. Experiments were performed by recording isometric tension of the rat portal vein. pD2 values and apparent intrinsic efficacy of alpha-adrenoceptor agonists (S.3341, norepinephrine, phenylephrine and clonidine) were assessed from cumulative concentration-response curves. Particular attention was paid to the variations in the amplitude of spontaneous contractions and the increase in the tonus of the vein induced by these agonists. The rank order of activity (pD2 values) of these agonists on the amplitude of spontaneous contractions and on the tonus was as follows: norepinephrine greater than or equal to phenylephrine greater than clonidine greater than S.3341. The activity of S.3341 on spontaneous contractions of the vein was found to be of the alpha 1-type only at concentrations higher than 10(-6)M. This concentration-dependent effect was antagonized by prazosin but not by rauwolscine. For concentrations lower than 10(-6)M, S.3341 decreased the amplitude of spontaneous contractions. This effect was not modified by prazosin or rauwolscine and was affected (decreased) after treatment of the vein with 6-hydroxydopamine (6-OHDA). S.3341 was barely able to increase the tonus of the vein, whatever the concentration used. Moreover, S.3341 was able to antagonize the increase in amplitude of spontaneous contraction and tonus of the vein induced by clonidine and phenylephrine. The effect of S.3341 on the slow inward calcium current was also tested and no effect was found, even at high concentrations. The results suggest that in the rat portal vein, S.3341 is a weak partial alpha 1-adrenoceptor agonist which exhibits a quite novel property not related to an adrenergic stimulation or a calcium entry blocking property. The mechanism of this novel effect remains to be elucidated.