cAMP, calmodulin-dependent stimulation and stability to proteolysis of Ca 2+ transport in the heart sarcoplasmic reticulum

The calmodulin content in cardiomyocyte cytosol of hypoxic myocardium is increased compared to normal level. This is unaccompanied by differences in the stimulating effect of calmodulin on Ca2+ transport in sarcoplasmic reticulum (SR) of ischemic heart. The decrease of the endogenous cAMP-dependent protein kinase activity in ischemia is associated with the lowered resistance to trypsinolysis of Ca2+ transport in SR (trypsin/microsomal protein ratio is 1:10) with simultaneous Ca-ATPase activation. In the presence of exogenous protein kinase and cAMP the protective effect of phosphorylation on Ca2+ transport in SR vesicles of hypoxic cardiomyocytes treated with trypsin for 10 min reaches the same level as in intact heart.