Effects of cholinergic and adrenergic agonists on phosphorylation of a 165,000-dalton myofibrillar protein in intact cardiac muscle.

The purpose of this investigation was to examine the effects of beta-adrenergic and muscarinic cholinergic agonists on protein phosphorylation in intact frog atrium. beta-Adrenergic agonists increase and muscarinic agonists decrease 32P incorporation into a 165,000-dalton (165K) protein within less than 1 min. The concentrations of isoproterenol that produce increases in 32P incorporation into the 165K protein and in systolic tension are similar. Further, the changes in 32P incorporation and tension produced by isoproterenol occur with similar time courses. Carbamylcholine decreases tension somewhat more quickly and at lower concentrations than it decreases 32P incorporation, however. Isoproterenol-stimulated 32P incorporation is thought to be mediated by cAMP-dependent protein kinase because bath application of dibutyryl cAMP, cholera toxin, or phosphodiesterase inhibitors increase 32P incorporation into the 165K protein in intact atria. When heart homogenates are incubated in the presence of [gamma-32P]ATP, cAMP stimulates the incorporation of 32P into the 165K protein. cGMP is much less effective. We suggest that carbamylcholine decreases 32P incorporation into the 165K protein by a mechanism independent of cAMP levels because carbamylcholine inhibits the stimulation of 32P incorporation into the 165K band produced by 8-bromo cAMP in intact cells. Phosphorylation of the 165K protein occurs in cardiac muscle but not in other tissues. We hypothesize that the 165K protein is C-protein, because the 165K- and C-proteins have similar solubilities and are associated with the myofibril. Further, antibodies produced against the 165K protein bind to C-protein purified from rabbit heart and also bind to the same region of the myofibril where C-protein is found.