Ca uptake by heart cells: II. Most entering Ca appears to leave without mixing with the sarcoplasmic reticulum Ca pool.

The rate of verapamil-sensitive uptake of 45Ca by rat heart cells stimulated to beat in suspension with 0.2 mM Ca and isoproterenol was increased > 2-fold by cell loading with the chelator Quin-2. No effect of Quin-2 loading was observed on the rate of uptake of trace levels of 54Mn, present in addition to Ca, which was used as an index of Ca channel activity. Quin-2 loading also had little effect on the rate of 45Ca uptake by cells diluted into a high K/low Na medium, where Ca uptake was primarily by Na/Ca exchange. The fast chelator 1,2-bis(o-aminophenoxy)ethane-N,N,-N',N'-tetraacetic acid (BAPTA) was 3-fold more effective than the slow chelator EGTA at preventing Ca efflux. BAPTA loading also caused an increase in sarcoplasmic reticulum (SR) Ca content. These results suggest that chelator loading had little effect on the rate of Ca influx by Ca channels or by Na/Ca exchange, and that the increased rate of 45Ca uptake seen with Quin-2 loading was caused by an inhibition of Ca efflux, either directly by chelation or by increased Ca uptake by the SR or by other intracellular organelles. This further suggests that most of the Ca entering the cell without chelator leaves again within the same beat, and that this may result from Ca efflux from a kinetically limited Ca pool in or around the diad cleft.