ELECTROGENIC NA-CA EXCHANGE CLEARS CA2+ LOADS FROM RETINAL AMACRINE CELLS IN CULTURE

Calcium influx into cultured retinal amacrine cells is followed by a s mall, slow, inward current that we show here results from the operatio n of electrogenic Na-Ca exchange, The activity of the exchanger is sho wn to correlate with the magnitude of the Ca2+ load and to depend on b oth the Ca2+ and Na+ gradients, Li+ is unable to substitute for Nac an d in the absence of Na+, slow tail currents are almost entirely suppre ssed. A rapid change in [K+](o) does not affect the activity of the ex changer, suggesting that only Nai and Ca2+ are transported, The ratio of charge entering as Ca2+ current to the charge entering as exchange current is highly variable between cells. We suggest that variability results from a variable fraction of Ca2+ load, we estimate typically 4 0%, being removed by a process other than Na-Ca exchange, This process is likely to involve internal buffering or sequestration since inhibi tion of the plasmalemmal Ca2+-ATPase does not increase the fraction of Ca2+ expelled by the exchanger, Ca2+ loading performed in the absence of Na-o(+) generates smaller exchange charge the longer the delay in returning Na-o(+) to the neuron, About 30% of exchange charge is lost for a delay of 1 sec.