Inhibition of dihydropyridine-sensitive calcium entry in hypoxic relaxation of airway smooth muscle

Hypoxia dilates airways in vivo and reduces active tension of airway smooth muscle in vitro. To determine whether hypoxia impairs Ca²⁺ entry through voltage-dependent channels (VDC), we tested the ability of dihydropyridines to modulate hypoxia-induced relaxation of KCl-and carbamyl choline (carbachol)-contracted porcine bronchi. Carbachol- or KCl-contracted bronchial rings were exposed to progressive hypoxia in the presence or absence of 1 μM BAY K 8644 (an L-type-channel agonist). In separate experiments, rings were contracted with carbachol or KCl, treated with nifedipine (a VDC antagonist), and finally exposed to hypoxia. BAY K 8644 prevented hypoxia-induced relaxation in KCl-contracted bronchi. Nifedipine (10^-5 M) totally relaxed KCl-contracted bronchi. Carbachol-contracted bronchi were only partially relaxed by nifedipine but were completely relaxed when the O₂ concentration of the gas was reduced from 95 to 0%. These data indicate that hypoxia can reduce airway smooth muscle tone by limiting entry of Ca²⁺ through a dihydropyridine-sensitive pathway, but that other mechanisms also contribute to hypoxia-induced relaxation of carbachol- contracted bronchi.