Abstract
Influx of extracellular Ca$^2+$ plays a major role in the activation
of contraction in fish cardiac cells. The relative contributions
of Na$^+$/Ca$^2+$ exchange and L-type Ca$^2+$ channels
to Ca$^2+$ influx are, however, unknown. Using a physiological
action potential as the command pulse in voltage-clamped heart cells,
we examined sarcolemmal Ca$^2+$ influx through Na$^+$/Ca$^2+$
exchange and L-type Ca$^2+$ 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$^+$/Ca$^2+$ 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$^+$/Ca$^2+$ 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$^+$/Ca$^2+$ exchanger and L-type
Ca$^2+$ channels to Ca$^2+$ influx were estimated using two
partly different methods. Integration of the Ni2+-sensitive Na$^+$/Ca$^2+$
exchange current and the verapamil- and Cd2+-sensitive L-type Ca$^2+$
current suggests that, during the action potential, approximately
one-third of the activating Ca$^2+$ comes through Na$^+$/Ca$^2+$
exchange and approximately two-thirds through L-type Ca$^2+$
channels. An alternative method of analysis, using the inward tail
current as a measure of the total sarcolemmal Ca$^2+$ flux from
which the Ni2+-sensitive Na$^+$/Ca$^2+$ exchange current
was subtracted to obtain the Ca$^2+$ influx through the channels,
suggests that L-type Ca$^2+$ channels and Na$^+$/Ca$^2+$
exchange are almost equally important in the activation of contraction.
Furthermore, the time course of cell shortening is not adequately
explained by sarcolemmal Ca$^2+$ influx through the channels
alone, but is well approximated by the sum of Ca$^2+$ influx
through the channels and the exchanger. The present results indicate
that reverse Na$^+$/Ca$^2+$ exchange in crucian carp ventricular
myocytes has sufficient capacity to trigger contraction and suggest
that the exchange current makes a significant contribution to contractile
Ca$^2+$ during the physiological action potential. The relative
significance of channels and exchanger molecules in sarcolemmal Ca$^2+$
entry into crucian carp ventricular myocytes was unaffected by thermal
acclimation when determined at 22 degrees C.
- 0010359679
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