Mechanism of relaxing action of the antiasthmatic drug, azelastine, in isolated porcine tracheal smooth muscle

Extracellular acidosis selectively inhibits pharmacomechanical coupling induced by carbachol in strips of rat gastric fundus

NEW FINDINGS

What is the central question of this study? Acute acidosis that results from short-term exercise is involved in delayed gastric emptying in rats and the lower responsiveness of gastric fundus strips to carbachol. Does extracellular acidosis decrease responsiveness to carbachol in tissues of sedentary rats? How? What is the main finding and its importance? Extracellular acidosis inhibits cholinergic signalling in the rat gastric fundus by selectively influencing the G_q/11 protein signalling pathway. Acute acidosis that results from short-term exercise delays gastric emptying in rats and decreases the responsiveness to carbachol in gastric fundus strips. The regulation of cytosolic Ca²⁺ concentrations appears to be a mechanism of action of acidosis. The present study investigated the way in which acidosis interferes with gastric smooth muscle contractions. Rat gastric fundus isolated strips at pH 6.0 presented a lower magnitude of carbachol-induced contractions compared with preparations at pH 7.4. This lower magnitude was absent in carbachol-stimulated duodenum and KCl-stimulated gastric fundus strips. In Ca²⁺ -free conditions, repeated contractions that were induced by carbachol progressively decreased, with no influence of extracellular pH. In fundus strips, CaCl₂ -induced contractions were lower at pH 6.0 than at pH 7.4 but only when stimulated in the combined presence of carbachol and verapamil. In contrast, verapamil-sensitive contractions that were induced by CaCl₂ in the presence of KCl did not change with pH acidification. In Ca²⁺ store-depleted preparations that were treated with thapsigargin, the contractions that were induced by extracellular Ca²⁺ restoration were smaller at pH 6.0 than at pH 7.4, but relaxation that was induced by SKF-96365 (an inhibitor of store-operated Ca²⁺ entry) was unaltered by extracellular acidification. At pH 6.0, the phospholipase C inhibitor U-73122 relaxed carbachol-induced contractions less than at pH 7.4, and this phenomenon was absent in tissue that was treated with the RhoA kinase blocker Y-27632. Thus, extracellular acidosis inhibited pharmacomechanical coupling in gastric fundus by selectively inhibiting the G_q/11 protein signalling pathway, whereas electromechanical coupling remained functionally preserved.

Effects of azelastine on membrane currents in tracheal smooth muscle cells isolated from the guinea-pig

Azelastine [4-(p-chlorobenzyl)-2-(hexahydro-1-methyl -1H-azepin-4-yl)-1-(2H)-phthalazinone hydrochloride], an anti-allergic agent, inhibited the high K(+)-induced contraction in tracheal smooth muscle cells isolated from the guinea-pig. In order to investigate the ionic mechanisms, we examined the effects of azelastine on membrane currents, using the tight-seal whole cell voltage clamp technique. Azelastine (1-100 microM) caused an inhibition of the Ba2+ inward current (IBa) through the voltage-dependent L-type Ca2+ channel in a concentration-dependent manner. The inhibitory effect of azelastine on IBa was fully reversible. The IC50 value for azelastine-induced inhibition of IBa was approximately 8 microM, and 100 microM azelastine completely suppressed IBa. Azelastine exerted mainly a tonic block of IBa but did not show use dependence. Azelastine (10 microM) shifted the quasi-steady-state inactivation curve of IBa to more negative membrane potentials by approximately -20 mV, suggesting that the inhibitory effect of azelastine on IBa was voltage-dependent. In addition, azelastine produced inhibitory actions on other membrane currents (i.e. the voltage-dependent transient outward K+ current and the Ca(2+)-activated oscillatory K+ current) at doses higher than 10 microM. These results suggest that azelastine inhibits the voltage-dependent L-type Ca2+ current in single tracheal smooth muscle cells, which may contribute to the anti-allergic actions of azelastine in airways.