STIMULATION OF CA2+ UPTAKE BY CYCLIC-AMP AND PROTEIN-KINASE IN SARCOPLASMIC RETICULUM-RICH AND SARCOLEMMA-RICH MICROSOMAL FRACTIONS FROM RABBIT HEART

  • 1 January 1976
    • journal article
    • research article
    • Vol. 35  (5) , 529-541
Abstract
The effect of cyclic[c]AMP on Ca2+ uptake by rabbit heart microsomal vesicular fractions representing mainly fragments of either sarcoplasmic reticulum or sarcolemma was investigated in the presence and absence of soluble cardiac protein kinase and with microsomes prephosphorylated by cAMP-dependent protein kinase. The acceleration of oxalate-promoted Ca2+ uptake by fragmented sarcoplasmic reticulum following cAMP-dependent membrane protein phosphorylation was confirmed. The acceleration was greatest at pH 7.2 and almost negligible at pH 6.0 and pH 7.8. A marked increase in Ca2+ uptake by cAMP-dependent membrane protein phosphorylation was observed in the presence of boric acid, a reversible inhibitor of Ca2+ uptake. Besides the microsomal fraction thought to represent mainly fragments of the sarcoplasmic reticulum, the effect of protein kinase and cAMP on Ca2+ uptake was investigated in a cardiac sarcolemma-enriched membrane fraction. Ca2+ uptake by sarcolemmal vesicles, unlike Ca2+ uptake by sarcoplasmic reticulum vesicles, was inhibited by low doses of digitoxin. The acceleration of oxalate-promoted Ca2+ uptake by cAMP and soluble cardiac protein kinase was quite similar to that seen in preparations of fragmented sarcoplasmic reticulum, suggesting that it may reflect an acceleration of active Ca2+ transport across the myocardial cell surface membrane.

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