Allosteric regulation of Na/Ca exchange current by cytosolic Ca in intact cardiac myocytes

The cardiac sarcolemmal Na-Ca exchanger (NCX) is allosterically regulated b y [Ca](i) such that when [Ca](i) is low NCX current (I-NCX) deactivates. In this study we used membrane potential (E-m) and I-NCX to control Ca entry into and Ca efflux from intact cardiac myocytes to investigate whether this allosteric regulation (Ca activation) occurs with [Ca](i) in the physiolog ical range. In the absence of Ca activation, the electrochemical effect of increasing [Ca](i) would be to increase inward I-NCX (Ca afflux) and to dec rease outward I-NCX On the other hand, Ca activation would increase I-NCX i n both directions. Thus, we attributed [Ca](i)-dependent increases in outwa rd I-NCX to allosteric regulation, Ca activation of I-NCX was observed in f erret myocytes but not in wild-type mouse myocytes, suggesting that Ca regu lation of NCX may be species dependent. We also studied transgenic mouse my ocytes overexpressing either normal canine NCX or this same canine NCX lack ing Ca regulation (Delta 680-685). Animals with the normal canine NCX trans gene showed Ca activation, whereas animals with the mutant transgene did no t, confirming the role of this region in the process. In native ferret cell s and in mice with expressed canine NCS, allosteric regulation by Ca occurs under physiological conditions (K-mCaAct = 125 +/- 16 nM SEM approximate t o resting [Ca](i)). This, along with the observation that no delay was obse rved between measured [Ca](i) and activation of INCX under our conditions, suggests that beat to beat changes in NCX function can occur in vivo. These changes in the I-NCX activation state may influence SR Ca load and resting [Ca](i), helping to fine tune Ca influx and efflux from cells under both n ormal and pathophysiological conditions. Our failure to observe Ca activati on in mouse myocytes may be due to either the extent of Ca regulation or to a difference in K-mCaAct from other species. Model predictions for Ca acti vation, on which our estimates of K-mCaAct are based, confirm that Ca activ ation strongly influences outward I-NCX, explaining why it increases rather than declines with increasing [Ca](i).