The relationship of ventricular dynamics and mitochondrial nitric oxide synthase activity in septic shock models.

The aim of the present study was to investigate the role of mitochondrial nitric oxide synthase (mtNOS) in the septic shock and analyze its relationship to ventricular contractility. Two models of septic shock [lipopolysaccharide (LPS)-induced and cecal ligation and puncture (CLP)-induced] were used. There was a significant depression of ventricular contractile parameters recorded in the late stage of the septic shock. After measurement of ventricular-dynamic parameters, mitochondrial and cytoplasmic fractions were isolated and their nitric oxide synthase (NOS) activity was assessed using a NOS activity assay kit. Both models showed a larger increase in mitochondrial NOS activity than that in cytosol. However, the increase in mtNOS activity in the LPS-induced shock model was less pronounced than in the CLP-induced model. Regression analysis shows that mitochondrial nitric oxide synthase (mtNOS) activity is negatively correlative to the left ventricular developed pressure in CLP model. The results suggest that mitochondrial NOS may mainly contribute to the ventricular depression in the septic shock.

The relaxant effect of urocortin in rat pulmonary arteries.

Urocortin is a potent vasodilator, which plays physiological or pathophysiological roles in systemic circulation. However, little is known about its action on pulmonary circulation. The present study was aimed to characterize some cellular mechanisms underlying the relaxant effect of urocortin in isolated rat pulmonary arteries. Changes in isometric tension were measured on small vessel myographs. Urocortin inhibited U46619-induced contraction with reduction of the maximal response. Urocortin-induced relaxation was independent of the presence of endothelium. Inhibitors of nitric oxide (NO)-dependent dilator, NG-nitro-L-arginine methyl ester or 1H-[1,2,4]oxadizolo[4,3-a]quinoxalin-1-one, did not affect the relaxation. Astressin (100-500 nM), a corticotropin-releasing factor (CRF) receptor antagonist and KT5720, a protein kinase A (PKA) inhibitor reduced urocortin-induced relaxation. Urocortin produced less relaxant effect in 30 mM K+- than U46619-contracted arterial rings. Urocortin did not reduce CaCl2-induced contraction in 60 mM K+-containing solution. Ba2+ (100-500 microM) but not other K+ channel blockers reduced the relaxant responses to urocortin. Urocortin also relaxed the rings preconstricted by phorbol 12,13-diacetae in normal Krebs solution while this relaxation was less in a Ca2+-free solution. Our results show that urocortin relaxed rat pulmonary arteries via CRF receptor-mediated and PKA-dependent but endothelium/NO or voltage-gated Ca2+ channel-independent mechanisms. Stimulation of Ba2+-sensitive K+ channel may contribute to urocortin-induced relaxation. Finally, urocortin relaxed pulmonary arteries partly via inhibition of a PKC-dependent contractile mechanism.

Intracellular sodium in mammalian muscle fibers after eccentric contractions.

The effect of eccentric contractions on intracellular Na(+) concentration ([Na(+)](i)) and its distribution were examined in isolated rat and mouse muscle fiber bundles. [Na(+)](i) was measured with either Na(+)-binding benzofuran isophthalate or sodium green. Ten isometric contractions had no significant effect on force (measured after 5 min of recovery) and caused no significant change in the resting [Na(+)](i) (7.2 +/- 0.5 mM). In contrast 10 eccentric contractions (40% stretch at 4 muscle lengths/s) reduced developed force at 100 Hz to 45 +/- 3% of control and increased [Na(+)](i) to 16.3 +/- 1.6 mM (n = 6; P < 0.001). The rise of [Na(+)](i) occurred over 1-2 min and showed only minimal recovery after 30 min. Confocal images of the distribution of [Na(+)](i) showed a spatially uniform distribution both at rest and after eccentric contractions. Gd(3+) (20 microM) had no effect on resting [Na(+)](i) or control tetanic force but prevented the rise of [Na(+)](i) and reduced the force deficit after eccentric damage. These data suggest that Na(+) entry after eccentric contractions may occur principally through stretch-sensitive channels.

Development of T-tubular vacuoles in eccentrically damaged mouse muscle fibres.

Single fibres were dissected from mouse flexor digitorum brevis muscles and subjected to a protocol of eccentric stretches consisting of ten tetani each with a 40 % stretch. Ten minutes later the fibres showed a reduced force, a shift in the peak of the force-length relation and a steepening of the force-frequency relation. Addition of the fluorescent dye sulforhodamine B to the extracellular space enabled the T-tubular system to be visualized. In unstimulated fibres and fibres subjected to 10 isometric tetani, the T-tubules were clearly delineated. Sulforhodamine B diffused out of the T-tubules with a half-time of 18 +/- 1 s. Following the eccentric protocol, vacuoles connected to the T-tubules were detected in six out of seven fibres. Sulforhodamine B diffused out of the vacuoles of eccentrically damaged fibres extremely slowly with a half-time of 6.3 +/- 2.4 min and diffused out of the T-tubules with a half-time of 39 +/- 4 s. Vacuole production was eliminated by application of 1 mM ouabain to the muscle during the eccentric protocol. On removal of the ouabain, vacuoles appeared over a period of 1 h and were more numerous and more widely distributed than in the absence of ouabain. We propose that T-tubules are liable to rupture during eccentric contraction probably because of the relative movement associated with the inhomogeneity of sarcomere lengths. Such rupture raises intracellular sodium and when the sodium is pumped from the cell by the sodium pump, the volume load of Na(+) and water exceeds the capacity of the T-tubules and causes vacuole production. The damage to the T-tubules may underlie a number of the functional changes that occur in eccentrically damaged muscle fibres.

Effect of eccentric contraction-induced injury on force and intracellular pH in rat skeletal muscles.

The effect of eccentric contraction on force generation and intracellular pH (pH(i)) regulation was investigated in rat soleus muscle. Eccentric muscle damage was induced by stretching muscle bundles by 30% of the optimal length for a series of 10 tetani. After eccentric contractions, there was reduction in force at all stimulation frequencies and a greater reduction in relative force at low-stimulus frequencies. There was also a shift of optimal length to longer lengths. pH(i) was measured with a pH-sensitive probe, 2',7'-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein AM. pH(i) regulation was studied by inducing an acute acid load with the removal of 20-40 mM ammonium chloride, and the rate of pH(i) recovery was monitored. The acid extrusion rate was obtained by multiplying the rate of pH(i) recovery by the buffering power. The resting pH(i) after eccentric contractions was more acidic, and the rate of recovery from acid load post-eccentric contractions was slower than that from postisometric controls. This is further supported by the slower acid extrusion rate. Amiloride slowed the recovery from an acid load in control experiments. Because the Na(+)/H(+) exchanger is the dominant mechanism for the recovery of pH(i), this suggests that the impairment in the ability of the muscle to regulate pH(i) after eccentric contractions is caused by decreased activity of the Na(+)/H(+) exchanger.

Inhibitory effect of tetrabutylammonium ions on endothelium/nitric oxide-mediated vasorelaxation.

Apart from the well-described K+ channel blocking effects in vascular smooth muscle cells, monovalent quaternary ammonium ions may also interact with endothelial cells in the endothelium-intact mammalian arteries. The present study was aimed to examine the effect of tetrabutylammonium ions on endothelium-dependent and -independent relaxation in the rat isolated aortic rings. Pretreatment with tetrabutylammonium concentration dependently reduced the endothelium-dependent relaxation induced by acetylcholine, cyclopiazonic acid and ionomycin. Tetrabutylammonium also inhibited endothelium-independent relaxation induced by hydroxylamine or nitroprusside. Pretreatment of endothelium-denuded rings with tetrabutylammonium did not affect relaxation induced by NS1619 or by diltiazem. In contrast, tetrabutylammonium significantly reduced the pinacidil- or cromakalim-induced relaxation. Tetrabutylammonium also inhibited the acetylcholine- but not nitroprusside-induced increase of tissue content of cyclic GMP in the aortic rings. The present study indicates that tetrabutylammonium ions could inhibit endothelial and exogenous nitric oxide-mediated aortic relaxation while it had no effect on relaxation induced by activation of Ca2+-activated K+ channels (by NS1619) or by inhibition of voltage-gated Ca2+ channels (by diltiazem). The inhibitory effect on pinacidil- and cromakalim-induced relaxation suggests that tetrabutylammonium ions also inhibit ATP-sensitive K+ channels in aortic smooth muscle cells.

Efficacy of muscarinic stimulation and mode of excitation-contraction coupling in bovine trachealis muscle.

We have compared the efficacy of cromakalim and nifedipine to inhibit acetylcholine (ACh) and pilocarpine-induced tonic contractions in control preparations and in tissues where a fraction of the muscarinic receptor population had been removed by alkylation with phenoxybenzamine (PBZ). Both agonists induced contractions by stimulating pharmacologically similar receptors, probably of the M3 muscarinic subtype. The receptor reserve was larger, and the coupling between stimulation and contraction (E-C coupling) more efficient when ACh was the stimulating agonist. For stimulations that produced equal levels of muscle response, cromakalim was more efficacious in inhibiting contractions induced by pilocarpine. The efficacy of cromakalim in relaxing contractions induced by ACh increased when the number of functional receptors decreased. Cromakalim and nifedipine decreased the efficiency of E-C coupling for ACh and pilocarpine. Cromakalim efficacy decreased in a sigmoid manner when stimulating concentrations of ACh (and receptor occupancy) increased, and there was an inverse relationship between receptor occupancy by ACh and cromakalim efficacy. In the presence of TEA, a K+ channel blocker, nifedipine almost completely inhibited contractions induced by the M3 muscarinic agonist bethanechol. These data indicate that in bovine tracheal smooth muscle, electro-mechanical coupling is an inherent part of muscarinic E-C coupling, but its functional expression is dependent upon the efficacy of stimulation. The data also suggest that the M3 receptor is coupled to a cellular pathway linked with the activation of K+ channels that exerts a potent functional antagonism against activation of voltage-dependent Ca2+ entry.

Control of the mode of excitation-contraction coupling by Ca(2+) stores in bovine trachealis muscle.

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.

Endothelium-dependent effect of bradykinin and ATP on rabbit ventricular myocytes.

We have investigated the effects of bradykinin (BK) and ATP on Ca2+ transient induced by electrical-field stimulation in freshly isolated rabbit ventricular myocytes, in the presence or absence of rabbit aortic endothelial cells. BK and ATP induced an increase in intracellular Ca2+ concentration ([Ca2+]i) in the endothelial cells, but had no significant effect on Ca2+ transient in electrical-field stimulated ventricular myocytes. In the presence of cultured endothelial cells, the amplitude of Ca2+ transient induced by electrical stimulation in ventricular myocytes was decreased. BK and ATP further reduced the amplitude of Ca2+ transient induced by electrical stimulation in ventricular myocytes. These data show that BK and ATP have endothelium-dependent depressing effects on ventricular myocytes and indicate that substances released from endothelial cells in response to BK and ATP stimulation can modulate ventricular myocytes excitation-contraction coupling. However, identification of the cardioactive mediators produced by the endothelial cells requires further study.

Two distinct pathways for refilling Ca2+ stores in permeabilized bovine trachealis muscle.

Calcium entry from extracellular space to acetylcholine (ACh)-sensitive internal stores was investigated in beta-escin permeabilized bovine tracheal smooth muscle. Cyclopiazonic acid (CPA), a selective inhibitor of the sarcoplasmic reticulum (SR) calcium pump, and nifedipine, both inhibited the refilling, and inhibition was larger when these compounds were used simultaneously. BayK 8644 enhanced the refilling and completely reversed the inhibition induced by cyclopiazonic acid. In pCa 7 solution containing CPA, there was a spontaneous time-dependent decrease of ACh-induced transient contraction. In the presence of nifedipine or verapamil in the incubation solution reduced this time-dependent decrease in contractile responses to ACh stimulation, suggesting that these calcium-entry blockers decreased calcium leakage from internal stores to the extracellular space. These results suggest that in addition to the active calcium uptake in the SR, another pathway controlled by an L-type like calcium channel (dihydropyridine-sensitive) may exist between the extracellular compartment and the lumen of the SR in airway smooth muscle, and contributes significantly to the loading of ACh-sensitive calcium stores.

Dual effect of cobra cardiotoxin on vascular smooth muscle and endothelium.

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.

Probing excitation-contraction coupling in trachealis smooth muscle with the mycotoxin cyclopiazonic acid.

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.

Hyporesponsiveness to Ca2+ of aortic smooth muscle in endotoxin-treated rats: no-dependent and -independent in vitro mechanisms.

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.

Internal calcium stores and norepinephrine overflow from isolated, field stimulated rat vas deferens.

Ryanodine has been shown to selectively inhibit the initial phase of contraction of rat vas deferens smooth muscle stimulated by endogenous release of norepinephrine (NE) (1), and part of this effect could be pre-junctional. To assess this, its effect on NE overflow was measured in the same preparation. NE overflow from electrical field-stimulated isolated rat vas deferens was quantified by electrochemical detection using HPLC. In order to limit pre-junctional autoregulatory mechanisms, alpha2-adrenergic receptors were blocked and P2x purinergic receptors were desensitized. In these experimental conditions, NE overflow was directly proportional to extracellular Ca2+ concentration. Ryanodine only induced a modest decrease in NE overflow. Cyclopiazonic acid (CPA), an inhibitor of sarcoplasmic reticulum Ca2+-ATPase, slightly increased NE overflow but decreased smooth muscle contraction induced by electrical field stimulation. It is concluded that part of the effect of ryanodine on field stimulation-induced contraction may be due to an inhibition of NE release, although the major inhibitory effect of this alkaloid is post-junction. For CPA, its inhibitory effect on field stimulation-induced contraction is entirely post-junctional. Its effect on NE overflow suggests that, in this preparation, internal Ca2+ stores could function to accelerate termination of neurotransmitter release by sequestering cytosolic Ca2+.

Cross talk between plasma membrane and sarcoplasmic reticulum in canine airway smooth muscle.

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.

Distinct pathways to refill ACh-sensitive internal Ca2+ stores in canine airway smooth muscle.

The ability of extracellular Ca2+ to refill internal Ca2+ stores of canine tracheal smooth muscle after a prior depletion by acetylcholine (ACh) was assessed using a novel sarcoplasmic reticulum (SR) Ca2+ pump inhibitor, cyclopiazonic acid (CPA). The transient contraction induced by ACh in a medium free of Ca2+ was used as an index for the content of agonist-sensitive intracellular Ca2+ stores. CPA inhibited in a concentration-dependent manner the refilling of the stores occurring during high KCl stimulation, and this inhibitory effect was independent of the external Ca2+ concentration. On the other hand, CPA was less effective in inhibiting the refilling occurring during prolonged ACh stimulation, especially when external Ca2+ concentration was raised. At 5.0 mM external Ca2+ or when 0.1 microM BAY 8644 was present in the medium, CPA was ineffective in inhibiting the refilling occurring during prolonged ACh stimulation. The maximum ACh-induced contraction in Ca(2+)-containing medium was independent of the extent of internal store Ca2+ load in the absence of L-type Ca2+ channel blocker but was highly dependent on the extent of internal Ca2+ load in the presence of the Ca2+ channel blocker. Hyperpolarization of the plasma membrane with the K+ channel opener cromakalim reduced the amplitude of ACh tonic contraction. Subsequent addition of nifedipine further reduced ACh tonic contraction. It is concluded that two different pathways for external Ca2+ are used to refill ACh-sensitive internal stores. One involves active Ca2+ uptake via a CPA-sensitive Ca2+ pump, and the other involves a CPA-insensitive pathway whose nature remains to be determined.

NADPH-diaphorase histochemistry identifies isolated endothelial cells at sites of traumatic injury in the adult rat brain.

In addition to labelling endothelium, some ependymal cells (including tanycytes), and a subpopulation of neurons, nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase histochemistry of stab lesion sites in the neocortex revealed a large population of cells concentrated within several hundred micrometers of the lesion site. To determine the identity of these cells, NADPH-diaphorase reactivity was compared to binding with either the I-B4 isolectin from Bandeiraea simplicifolia (which has previously been shown to identify endothelial cells and activated mononuclear phagocytes), or a monoclonal antibody (OX-42) that recognizes activated mononuclear phagocytes. Many I-B4 lectin-labelled cells were also NADPH-diaphorase reactive, and other I-B4 lectin-labelled cells were also OX-42 immunoreactive, but co-existence of OX-42 immunoreactivity and NADPH-diaphorase reactivity was not observed. Only a small minority of NADPH-diaphorase-reactive cells did not exhibit I-B4 lectin binding. In contrast to the simple somatic morphology of the majority of NADPH-diaphorase-reactive cells, the I-B4 lectin-negative cells had a ramified appearance, and while readily observed at two days postlesion, they were only rarely seen at three days postlesion. Primary cultures of bovine aortic endothelial cells also exhibited NADPH-diaphorase reactivity which occupied most of the cytoplasm in a filamentous web pattern. Endothelial cells possess a constitutive form of nitric oxide synthase which, as demonstrated in NADPH-diaphorase-reactive neurons, may be the basis of their NADPH-diaphorase reactivity. These findings indicate that NADPH-diaphorase-reactive cells observed at lesion sites are probably angiogenic endothelial cells not associated with extant blood vessels. Thus, NADPH-diaphorase histochemistry offers an effective method of visualizing neovascularization in the brain and other tissues.

Modification of excitation-contraction coupling in cat ventricular myocardium following endocardial damage.

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)

Chloride and depolarization by acetylcholine in canine airway smooth muscle.

The role of chloride channels has been examined in canine tracheal smooth muscle by recording mechanical responses to field stimulation and to acetylcholine (ACh) and by sucrose gap recording of excitatory junction potentials and ACh-induced electrical changes. The results of substitution studies using isethionate for chloride provided evidence that a chloride conductance contributes to the resting potential. The extrapolated reversal potential for ACh-induced depolarization was positive to the resting potential. Isethionate substitution inhibited ACh-induced depolarization, consistent with a contribution from increased Cl- conductance to the depolarization induced by ACh. However, closure of K+ channels and opening of a non-specific cation channel could also contribute to depolarization. Further study of the effects of isethionate substitution during prolonged tissue exposure to chloride-free medium showed that retention or the accumulation of Ca2+ in intracellular stores was impaired. We conclude that effects of chloride deprivation on responses to ACh may reflect an early increase in Cl- conductance, but longer term changes reflect the requirement for this anion to maintain internal Ca2+ stores.