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 degrees C) and 2.38+/−0.42 pA pF-1 for cold-acclimated fish (4 degrees 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 degrees C.
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JOURNAL ARTICLE| 01 July 1999
Na+/Ca2+ exchange current in ventricular myocytes of fish heart: contribution to sarcolemmal Ca2+ influx
In collection: Cardiology articles
Online Issn: 1477-9145
Print Issn: 0022-0949
© 1999 by Company of Biologists
J Exp Biol (1999) 202 (13): 1763–1775.
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M. Vornanen; Na+/Ca2+ exchange current in ventricular myocytes of fish heart: contribution to sarcolemmal Ca2+ influx. J Exp Biol 1 July 1999; 202 (13): 1763–1775. doi: https://doi.org/10.1242/jeb.202.13.1763
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