Abstract

Suspensions of individual smooth muscle cells were isolated by enzymatic digestion of stomach muscle of Bufo marinus. The role of cyclic 3',5'-AMP as a mediator of beta adrenergic-induced decreases in the contractility of smooth muscle was assessed by comparison of the time course and dose dependence for the effects of the beta adrenergic agent isoproterenol on both parameters. Drug-induced changes in the contractile state of isolated smooth muscle cells were determined utilizing a Coulter counter. The ability of isoproterenol to inhibit contractions produced by subsequent exposure to the excitatory agent carbamylcholine was used as an index of decreases in contractility. Exposure to isoproterenol for more than 5 sec was necessary to demonstrate significant inhibition of contraction. Maximal effects of isoproterenol were noted after exposure for 15 sec. Basal cyclic AMP levels, calculated from measurements of cyclic AMP content of cell suspensions and intracellular fluid volume, were 4.5 µM. Exposure to isoproterenol for as little as 2.5 sec produced a significant increase in cyclic AMP levels; maximal changes were noted after exposure for 5 sec or more. The changes in cyclic AMP and contractility exhibited similar dose-response curves, with Ed₅₀ values of approximately 0.4 µM. Prior incubation of the cells with theophylline, cyclic AMP, or N⁶,2'-O-dibutyryl cyclic AMP also inhibited contractions induced by carbamylcholine. These findings strongly support the hypothesis that decreases in the contractility of smooth muscle produced by beta adrenergic agents may be mediated by an increase in cyclic AMP. The Coulter counter results also reveal that individual cells are capable of responding to both cholinergic and adrenergic transmitters. Thus it is likely that integration of the cholinergic excitatory and adrenergic inhibitory input to the whole tissue takes place in part, if not entirely, in individual cells.