Matching Ca Efflux and Influx to Maintain Steady-state Levels in Cultured Cardiac Cells. Flux Control in the Subsarcolemmal Cleft

The study examines the feed-back mechanism by which Ca efflux via Na/Ca exchange adjusts to match Ca influx via Ca channels—the condition which must obtain if cellular steady-state Ca level is to be maintained. It is proposed that variation in Ca channel influx produces changes in sarcoplasmic reticulum content which, in turn, through interaction within the subsarcolemmal cleft space, sets the level of Na/Ca exchange-mediated efflux. We measured Ca channel influx with whole-cell voltage-clamp, Ca efflux specifically via Na/Ca exchange by high resolution, non-perfusion-limited45Ca wash-out technique and sarcoplasmic reticulum (SR) Ca content by combination of the latter with measurement of sarcolemmal Ca-binding by use of instantaneous “gas-dissected” sarcolemmal membrane isolation. Ca channel influx was varied by Bay K 8644 or nifedipine. Ca influx via Ca channels was increased by 37.8% with Bay K and decreased by 68% with nifedipine. The SR contribution to Na/Ca exchange-mediated efflux was increased by 36.7% with Bay K and reduced by 66.8% by nifedipine—virtually identical changes. Application of the cleft space model to determine beat-to-beat efflux via Na/Ca exchange predicted values which closely matched beat-to-beat channel influx.