Effects of three different L-type Ca2+ entry blockers on airway constriction induced by muscarinic receptor stimulation.

The crucial role of L-type Ca(2+) channels in airway smooth muscle contraction suggests that these channels could be an important therapeutic target. There are three separate drug binding sites on this channel: those for dihydropyridines, benzothiazepines and phenyl alkylamines. In this study, we examined the effects of the dihydropyridines nifedipine and nicardipine, the benzothiazepine diltiazem, and the phenylalkylamine verapamil on airway constriction. Tension of guinea-pig tracheal strips was measured isometrically in vitro with a force displacement transducer. Strips were precontracted with carbachol 10(-7) M with or without 4-aminopyridine 10(-3) M, a voltage-sensitive K(+ )channel blocker. Then, nifedipine 10(-8)-10(-4) M, diltiazem 10(-8)-3 x 10(-4) M or verapamil 10(-8)-3 x 10(-4) M was added cumulatively to the organ bath (n=6 each). The bronchial cross-sectional area of pentobarbital-anaesthetized dogs was assessed using a bronchoscopy method. Bronchoconstriction was elicited with methacholine 0.5 micro g kg(-1) plus 5 micro g kg(-1) min(-1), and then nicardipine 0-1000 micro g kg(-1), diltiazem 0-3000 micro g kg(-1) or verapamil 0-3000 micro g kg(-1) were given i.v. (n=7 each). In the in vitro experiments, nifedipine and diltiazem fully reversed carbachol-mediated tracheal contraction with logIC(50) values of 4.76 (SEM 0.22) (mean 17.5 micro M) and 4.60 (0.33) (mean 24.8 micro M), respectively. Although verapamil 10(-6)-10(-4) M reversed the contraction by 87.2%, strip tension re-increased by 18.1% following maximal relaxation with verapamil 3 x 10(-4 )M. This re-increase was almost fully abolished by pretreatment with 4-aminopyridine. In the in vivo experiments, nicardipine and diltiazem dose-dependently reversed methacholine-induced bronchoconstriction, with logID(50) values of 3.22 (0.05) (mean 0.60 mg kg(-1)) and 1.85 (0.32) (mean 14.0 mg kg(-1)), respectively. Verapamil worsened methacholine-induced bronchoconstriction. Although supraclinical doses of dihydropyridines and benzothiazepines can produce airway relaxant effects, these agents are unlikely to be used in the treatment of bronchoconstriction. In addition, verapamil may aggravate airway constriction.