The contractile system of rat cardiac muscle that has been made hyperpermeable by soaking the tissue in EGTA (McClellan and Winegrad. 1978. J. Gen. Physiol. 72:737-764) can be probed directly with Ca buffer from the bathing solution without significant interference from either sarcoplasmic reticulum or mitochondria on the Ca concentration. Changes in Ca-activated force are due therefore to changes in the properties of the contractile system itself and not to regulation of Ca concentration. The addition of cAMP, cGMP, and GTP, guanylyl imidodiphosphate (GMP-PNP), or epinephrine to the bath does not alter maximum Ca-activated force, but when these drugs are added with 1% nonionic detergent to the bath, contractility increases by as much as 180%. An inhibitor of phosphodiesterase must be present for the inotropic effect of cAMP but not cGMP, GTP, GMP-PNP, or epinephrine. The inotropic response to cAMP is independent of the Ca sensitivity of the contractile system, but guanine nucleotides enhance contractility only when Ca sensitivity is not high. The inotropic effect of epinephrine is inhibited to a large extent by cGMP but not by GMP-PNP. These data can be explained by a model in which contractility is enhanced by a cAMP-regulated phosphorylation that can be controlled through the beta-receptor adenylate cyclase complex in the sarcolemma. The regulation involves two reactions, one a phosphorylation and a second that occurs in the presence of detergent. Phosphorylation of neither the myosin light chain nor the inhibitory subunit of troponin appears to be involved in this mechanism for regulating contractility.

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