MECHANISM BY WHICH CYCLIC ADENOSINE 3'-5'-MONOPHOSPHATE-DEPENDENT PROTEIN-KINASE STIMULATES CALCIUM-TRANSPORT IN CARDIAC SARCOPLASMIC-RETICULUM

We examined the mechanism by which cyclic AMP-dependent protein kinase (PK) stimulates the calcium pump of cardiac sarcoplasmic reticulum vesicles. The Ca2+ dependence of calcium uptake rates by 30 μg/ml canine cardiac sarcoplasmic reticulum was measured at 25°C in 120 mM KCl, 40 mM histidine buffer (pH 6.8), 5 mM MgATP, and an ATP-regenerating system (75 μg/ml pyruvate kinase + 5 mM phosphoenolpyruvate) with 50 mM phosphate as calcium-precipitating anion. Preincubation with PK, 100 μg/ml, plus 1 μM cyclic AMP for 10 minutes increased calcium uptake rates from 2- to 3-fold at Ca2+ concentrations between 0.25 and 2.0 μM. This stimulation was associated with a decrease in the Ca2+ concentration needed to produce 50% maximal calcium uptake velocity from 2.38 ± 0.21 to 1.07 ± 0.10 μM(n = 7, P<0.001). The Ca2+ dependence of calcium uptake in nonphosphorylated cardiac sarcoplasmic reticulum vesicles exhibited positive cooperativity, whereas cooperativity was not evident in the corresponding preparations from 'fast' rabbit skeletal muscle. The estimated Hill coefficient for control vesicles was 1.77 ± 0.15; this value decreased significantly to 1.24 ± 0.08 (P<0.01) after preincubation with PK. After exposure to PK, therefore, cardiac sarcoplasmic reticulum exhibited the low Ca2+ cooperativity seen with fast skeletal sarcoplasmic reticulum. Phosphorylation of cardiac sarcoplasmic reticulum by PK thus appears to increase the apparent Ca2+-sensitivity of the calcium pump while decreasing positive cooperativity between the two Ca2+-binding sites of the calcium pump.