Calcium compartmentalization in cultured and adult myocardium: activation of a caffeine-sensitive component

Calcium (Ca) exchange was studied under various perfusion conditions in monolayer myocardial culture and in the interventricular septum of the rabbit. In cultured cells perfused in HEPES buffered medium or in 10 mM phosphate (Pi), pH less than 7.2, 10 mM caffeine produced no change in 45Ca uptake rate. By contrast, increase of pH to 7.35 in the presence of 10 mM Pi caused 45Ca uptake rate to increase by more than two-fold when caffeine was added. Control 45Ca uptake (prior to caffeine) was markedly increased in 10 mM Pi, pH = 7.35 as compared to the two other perfusion conditions in the cultured cells. The same sequence of response of 47Ca uptake rate to caffeine was found in the rabbit septum, i.e. no increased uptake under HEPES or Pi, pH less than 7.2 perfusion, but significant increase under 10 mM Pi, pH 7.35 with development of progressive contracture only in the last case. Two other conditions produced sensitivity (both in 47Ca uptake and contracture) to caffeine in the septum. Preperfusion with ouabain in HEPES buffer increased caffeine sensitivity proportional to ouabain concentration (5 X 10(-7) to 10(-5) M) as did preperfusion with vanadate at low concentration (1 to 3 X 10(-6) M). The results suggest that activation of Ca uptake by the sarcoplasmic reticulum (SR) is dependent upon a threshold of cellular Ca and that a stable contractile state is possible in the absence of SR activation in both cultured cells and adult ventricular tissue.