SARCOLEMMAL CA INFLUX THROUGH L-TYPE CA CHANNELS IN VENTRICULAR MYOCYTES OF A TELEOST FISH

The whole cell patch clamp method was used to measure Ca current throu gh L-type Ca channels in enzymatically isolated ventricular myocytes o f crucian carp (Carassius carassius L.) heart. Fish were acclimated to 22 degrees C for more than 4 wk, and properties of Ca current were me asured at room temperature (21 +/- 1 degrees C). Depolarizing voltage steps from -50 mV evoked rapidly activating Ca currents, which exhibit ed a bell-shaped voltage dependence with peak amplitude at 0 mV. The c urrents were suppressed by nifedipine (5 mu), verapamil (2.5 mu M), an d Cd2+ (175 mu M). The current amplitude was increased by 67.5 +/- 17. 2% (n = 5) in the presence of 1 mu M isoproterenol. Steady-state inact ivation and activation curves showed half-maximal inactivation at -31. 3 +/- 0.95 mV, with a slope factor of 5.88 +/- 0.51, and half-maximal activation at -10.6 +/- 1.65 mV, with a slope factor of 7.84 +/- 0.54 (n = 9). The overlap of inactivation and activation curves suggests th e presence of a small window current, which is maximally 4% of the pea k current at -27 mV. The density of L-type Ca current was 6.95 +/- 0.7 9 pA/pF at 0 mV (n = 35). A total increment in cellular Ca contributed by L-type Ca current during a 500-ms voltage clamp pulse was calculat ed from the integral of Ca current and cell volume. The charge transfe r through L-type Ca current was 0.325 +/- 0.023 pC/pF, and the mean ce ll volume was 1,377 +/- 44 mu m(3). The increment in total cellular Ca by Ca influx through L-type Ca channels was calculated to be 39.3 +/- 2.8 mu M. These findings imply that Ca influx through L-type Ca chann els can contribute significantly to the activation of contraction in t he ventricular myocytes of fish heart.