Mild trypsin treatment of canine cardiac microsomes consisting largely of sarcoplasmic reticulum vesicles produced a severalfold activation of oxalate-facilitated calcium uptake. The increase in calcium uptake was associated with an increase in ATP hydrolysis. Proteases other than trypsin were also effective although to a lesser degree. Trypsin produced a shift of the Ca2+ concentration dependency curve for calcium uptake toward lower Ca2+ concentrations, which was almost identical with that produced by phosphorylation of microsomes by cyclic AMP dependent protein kinase when the trypsin and the protein kinase were present at maximally activating concentrations. The Hill numbers (+/- SD) of the Ca2+ dependency after treatment of microsomes with trypsin (1.5 +/- 0.1) or protein kinase (1.7 +/- 0.1) were similar and were not significantly different from those for untreated control microsomes (1.6 +/- 0.1 and 1.8 +/- 0.1, respectively). Autoradiograms of sodium dodecyl sulfate-polyacrylamide electrophoretic gels indicate that 32P incorporation into phospholamban (Mr 27.3K) or its presumed monomeric subunit (Mr 5.5K) was markedly reduced when trypsin-treated microsomes were incubated in the presence of cyclic AMP dependent protein kinase and [gamma-32P]ATP compared to control microsomes incubated similarly but pretreated with trypsin inhibitor inactivated trypsin. The activation of calcium uptake by increasing concentrations of trypsin was paralleled by the reduction of phosphorylation of phospholamban. Trypsin treatment of microsomes previously thiophosphorylated in the presence of cyclic AMP dependent protein kinase and [gamma-35S]thio-ATP did not result in a loss of 35S label from phospholamban, which suggests that phosphorylation of phospholamban protects against trypsin attack.(ABSTRACT TRUNCATED AT 250 WORDS)

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Protein kinase A- (PKA-) catalyzed phosphorylation of phospholamban (PLN), the protein regulator of the cardiac Ca pump, mediates abbreviation of systole in response to beta-adrenergic agonists. Investigators previously, however, have been unsuccessful in demonstrating an effect of PLN phosphorylation or anti-PLN monoclonal antibody (mAb), which is considered to mimic phosphorylation's well-known effect on Km(Ca), on microsomal Ca uptake at the (high) Ca2+ concentrations found intracellularly at peak systole. We therefore compared the effects of the catalytic subunit of PKA and anti-PLN mAb on the kinetics of Ca uptake in sucrose gradient-purified cardiac microsomes. Both treatments produced a 33-44% increase in Vmax(Ca) at 25 and 37 degrees C, and an 11-31% decrease in Km(Ca) with comparable changes in Ca2+-ATPase activity. An acceleration of E2P decomposition upon PLN phosphorylation may contribute to the increased Vmax(Ca) of Ca uptake at 25 degrees C but not at 37 degrees C, based on measurement of the kinetics of E2P decomposition and steady-state E2P formation from Pi at different temperatures. Our data document almost identical increases in Vmax(Ca) of microsomal Ca uptake with PLN phosphorylation or addition of anti-PLN mAb and hence provide insight into the kinetic mechanism of PLN's regulation of the cardiac sarcoplasmic reticulum Ca pump protein.