The relationship between calcium flux and beating in hyperpermeable heart muscle cells

Beating hyperpermeable myocardial fragments, suspended in fresh medium, show a Ca uptake rate which greatly exceeds the release rate. Under these conditions, Ca influx and contraction frequency co-vary with temperature. Ca efflux, however, is not temperature responsive. In addition, influx, but not efflux, is ATP-dependent. This implies that influx, but not efflux, is a controlled process, the activity of which has the same determinants as does contraction. However, under conditions approximating the steady state, the Ca influx rate was found to decrease, while the efflux rate increased. This suggests the possibility, at least, that the source of contraction-activating Ca can vary with the prevailing conditions.