Cyclic adenosine monophosphate dependent and independent phosphorylation of sarcolemma membrane proteins in perfused rat heart

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Abstract
The mechanism by which epinephrine modulates cardiac function via protein phosphorylation was studied. A membrane fraction was isolated from freeze-clamped perfused rat heart that contains 2 phosphoproteins. The proteins have MW of 36,000 (A protein) and 27,000 (B protein). The phosphorylation of the A protein occurs during the equilibration of the heart with [32P]phosphate. The phosphorylation of the B protein occurs in response to epinephrine. The A and B proteins are identical with 2 phosphoproteins in enriched preparations of sarcolemma. The protein of the sarcolemma preparation equivalent to the A protein is phosphorylated in vitro by c[cyclic]AMP-independent and cAMP-dependent protein kinases. The phosphorylation of the protein of the sarcolemma preparation equivalent to the B protein is catalyzed by the cAMP-dependent protein kinase. The patterns of phosphorylation of the proteins in vivo and in vitro are compatible. The phosphorylation of the B protein was documented in vitro to modulate Ca-transport, but the response to epinephrine in the perfused heart is not apparently coordinated with the catecholamine-induced inotropic effect.

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