Phosphorylation induced by carp-dependent protein kinase was examined in a plasma membrane-enriched fraction from control and P-adrenergic-stimulated rat aortic myocytes. Phosphorylation of a 16 kDa protein which copurified with the plasma membrane marker (Nab + Kc)-ATPase was most prominent. It was decreased by pretreatment of the myocytes with isoproterenol and the effect of isoproterenol was inhibited by propranolol. Both phosphorylation induced by CAMP-dependent protein kinase and its inhibition by isoproterenoi pretreatment declined in preparations exposed to endogenous phosphatase. These results provide strong evidence that @-adrenergic stimuiation of aortic myocytes induces in situ phosphorylation of a 16 kDa plasma membrane protein.(Rat aorta)

A plasma membrane-enriched fraction of vesicles from rat mesenteric artery possessed ATP-dependent calcium properties similar to those which have been reported previously. The catalytic subunit from a soluble, cyclic AMP-dependent protein kinase was prepared from the same tissue, in partially purified form. Calcium uptake by membrane vesicles increased 30% in the presence of this kinase. This effect was proportional to the kinase concentration, and was blocked by either catalytic subunit inhibitor or by prior boiling of the subunit preparation. Stimulation of calcium uptake by the kinase occurred maximally between 1 and 10 µMfree calcium. These findings suggest that cyclic AMP and its dependent protein kinase may participate in the regulation of smooth muscle tension at high free myoplasmic calcium concentrations.

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