K-CA CHANNEL ANTAGONISTS REDUCE NO DONOR-MEDIATED RELAXATION OF VASCULAR AND TRACHEAL SMOOTH-MUSCLE

Electrophysiological studies suggest that activation of large-conductance Ca-activated K channels (K-Ca) with nitric oxide (NO) causes hyperpolarization and relaxation of smooth muscle. We determined whether K-Ca blockers decreased relaxation to the NO donors S-nitroso-N-acetylpenicillamine (SNAP) and 3-morpholinosydnonimine-hydrochloride (SIN-1) in isolated segments from main pulmonary artery (MPA), its left branch (LPA), aorta (Ao), carotid artery (CA), and trachea (Tr). NO donors caused concentration-dependent relaxation of tissues precontracted with histamine whereas the inactive carrier molecule C88-3934 was without effect. The rank order profiles of SNAP and SIN-1 sensitivity were CA = Ao = MPA > LPA = Tr. Compared with histamine, 80 mM KCl precontraction caused variable reductions in tissue sensitivity and maximum relaxation to SNAP. The K-Ca antagonists charybdotoxin, iberiotoxin, and tetraethylammonium decreased sensitivity to SNAP and SIN-1 2- to 11-fold in MPA, LPA, and Tr, with variable shifts in Ao and CA. The effect of iberiotoxin was not altered by removing the endothelium or epithelium. Furthermore, charybdotoxin or iberiotoxin did not alter basal or SNAP-stimulated guanosine 3',5'-cyclic monophosphate content. Glibenclamide, noxiustoxin, and leiurotoxin I, antagonists of ATP-dependent, delayed rectifier, and small-conductance K-Ca channels, respectively, had no effect. In conclusion, antagonists of K-Ca decrease NO donor-mediated relaxation of pulmonary arterial and tracheal smooth muscle.