Protein phosphorylation and cardiac function: cholinergic-adrenergic interaction

The muscarinic inhibition of cyclic AMP (cAMP) mediated contractile effects may not only be the result of a reduction in cAMP levels, but may also involve cAMP linked protein phosphorylation in the heart. To show that a cholinergic agonist may antagonise the effects of agents that induce the cAMP signalling sequence at the level of protein phosphorylation, the effect of carbachol was investigated on the in vivo phosphorylation of phospholamban, troponin I, and a 15 kDa protein during positive inotropic intervention with isoprenaline, forskolin, and a phosphodiesterase inhibitor in the isolated rat heart.

The contractile activity of the heart was monitored during drug administration, and the hearts were freeze clamped for assay of protein phosphorylation and metabolites [cAMP, cyclic guanosine monophosphase (cGMP), cAMP dependent protein kinase (cA-PK), and phosphorylase a]. For estimation of the amount of in vivo phosphorylation, a "back phosphorylation" method was used.

Carbachol attenuated in vivo phosphorylation, induced by cAMP increasing drugs, of all the phosphoproteins studied, whereas the accumulation of cAMP was not affected or was decreased to 61% after exposure to isoprenaline alone. In addition, the cAMP mediated phosphorylase a activation was completely reversed by carbachol. similarly, sodium nitroprusside reduced phosphorylation as well as the contractile force of the investigated phosphoproteins.

These data confirm the reduction of cAMP mediated cardiac protein phosphorylation by carbachol. The cholinergic antagonism may primarily involve cellular target systems distal to sites of cAMP generation. The observed similarity in the cardiac response to carbachol and sodium nitroprusside indicates a role of cGMP in the antiadrenergic action of carbachol in the myocardium.