The effects of isoproterenol on intracellular calcium concentration

beta-Adrenergic agonist, isoproterenol (ISO), is a potent relaxant of tracheal smooth muscle and inhibits carbachol-induced contraction. The effect of ISO on intracellular free Ca2+ concentration ([Ca2+]i) was examined in bovine tracheal smooth muscle strips, employing aequorin as Ca2+ indicator. Surprisingly, 10 microM ISO induces a 5-fold increase in [Ca2+]i which then gradually declines but still remains higher than basal after 1 h of stimulation. The ISO-induced increase in [Ca2+]i is dose-dependent, and the ED50 is approximately 50 nM. The ISO-induced increase in [Ca2+]i is inhibited by a beta-receptor blocker, propranolol, not by an alpha-blocker, phentolamine. The ISO-induced rise in [Ca2+]i is dependent on extracellular Ca2+. Forskolin, an adenylate cyclase activator, and vasoactive intestinal peptide, which is known to stimulate adenylate cyclase via a specific receptor in this tissue, have similar effects on [Ca2+]i, suggesting that a rise in cyclic AMP concentration mediates this effect of ISO on [Ca2+]i. Pretreatment of muscle with 10 microM ISO inhibits both the initial Ca2+ transient and the contractile response induced by 0.3 microM carbachol. Conversely, in carbachol-pretreated muscle strips, addition of ISO causes a fall rather than a rise in [Ca2+]i, and an inhibition of contraction. These results indicate that ISO has effects on cellular Ca2+ metabolism at more than a single site in bovine tracheal smooth muscle, that these effects are different in control and carbachol-pretreated muscle, and that the relaxing effect of ISO is not due solely to its effect on Ca2+ metabolism.