Mechanisms of cyclic nucleotide-induced relaxation in canine tracheal smooth muscle.

The effects of exogeneous cyclopiazonic acid (CPA, 10 microM), a selective inhibitor of the sarcoplasmic reticulum (SR) Ca2+ adenosinetriphosphatase, on cyclic nucleotide-induced relaxations of canine airway smooth muscle were examined. Strips of tracheal muscle were precontracted with carbachol (50% median effective concentration, 0.1 microM) or with 60 mM KCl. The beta-agonist isoproterenol (ISO, 10 microM) relaxed the tissue by approximately 50%. The relaxation was reduced in the presence of CPA when L-type Ca2+ channels were available but not when these were blocked by 0.1 microM nifedipine. Forskolin (1.0 microM), an adenylate cyclase activator, was less effective at inhibiting the contraction than ISO, and addition of CPA did not block its inhibitory effect as effectively as when ISO was used. Radioimmunoassay indicated that both these agents raised adenosine 3',5'-cyclic monophosphate (cAMP) levels to the same degree. Very little relaxation of the precontracted smooth muscle was elicited by 3 mM 8-bromo-adenosine 3',5'-cyclic monophosphate (8-BrcAMP), and addition of CPA had no effect. Sodium nitroprusside (100 microM) and 8-bromo-guanosine 3',5'-cyclic monophosphate (10 mM) inhibited contraction to a greater degree than any agent that raised cAMP. These inhibitions were greatly reduced in the presence of CPA when L-type Ca2+ channels were available. We conclude that pumping of Ca2+ into SR plays a major role guanosine 3',5'-cyclic monophosphate-produced but not cAMP-induced relaxation; L-type Ca2+ channels must be available for the relaxant role of Ca2+ pumping into the SR to be expressed; and ISO-induced relaxation may not involve primarily elevation of the cAMP.(ABSTRACT TRUNCATED AT 250 WORDS)