Na⁺/Ca²⁺ exchange current in ventricular myocytes of fish heart: contribution to sarcolemmal Ca²⁺ influx

Influx of extracellular Ca²⁺ plays a major role in the activation of contraction in fish cardiac cells. The relative contributions of Na⁺/Ca²⁺ exchange and L-type Ca²⁺ channels to Ca²⁺ influx are, however, unknown. Using a physiological action potential as the command pulse in voltage-clamped heart cells, we examined sarcolemmal Ca²⁺ influx through Na⁺/Ca²⁺ exchange and L-type Ca²⁺ channels in crucian carp (Carassius carassius L.) ventricular myocytes. When other cation conductances were blocked, a Ni²⁺-sensitive current with the characteristic voltage- and time-dependent properties of the Na⁺/Ca²⁺ exchange current could be distinguished. At the maximum overshoot voltage of the ventricular action potential (+40 mV; [Na⁺]_i=10 mmol l⁻¹), the density of the Na⁺/Ca²⁺ exchange current was 2.99±0.27 pA pF⁻¹ for warm-acclimated fish (23 °C) and 2.38±0.42 pA pF⁻¹ for cold-acclimated fish (4 °C) (means ± S.E.M., N=5–6; not significantly different, P=0.26). The relative contributions of the Na⁺/Ca²⁺ exchanger and L-type Ca²⁺ channels to Ca²⁺ influx were estimated using two partly different methods. Integration of the Ni²⁺-sensitive Na⁺/Ca²⁺ exchange current and the verapamil- and Cd²⁺-sensitive L-type Ca²⁺ current suggests that, during the action potential, approximately one-third of the activating Ca²⁺ comes through Na⁺/Ca²⁺ exchange and approximately two-thirds through L-type Ca²⁺ channels. An alternative method of analysis, using the inward tail current as a measure of the total sarcolemmal Ca²⁺ flux from which the Ni²⁺-sensitive Na⁺/Ca²⁺ exchange current was subtracted to obtain the Ca²⁺ influx through the channels, suggests that L-type Ca²⁺ channels and Na⁺/Ca²⁺ exchange are almost equally important in the activation of contraction. Furthermore, the time course of cell shortening is not adequately explained by sarcolemmal Ca²⁺ influx through the channels alone, but is well approximated by the sum of Ca²⁺ influx through the channels and the exchanger. The present results indicate that reverse Na⁺/Ca²⁺ exchange in crucian carp ventricular myocytes has sufficient capacity to trigger contraction and suggest that the exchange current makes a significant contribution to contractile Ca²⁺ during the physiological action potential. The relative significance of channels and exchanger molecules in sarcolemmal Ca²⁺ entry into crucian carp ventricular myocytes was unaffected by thermal acclimation when determined at 22 °C.