Effect of Na+ current on excitation-contraction coupling in ventricular myocytes of guinea pig heart.

We investigated the effect of Na+ current on the Ca2+ current and Ca2+ transients in cardiac myocytes. Myocytes were isolated from the ventricles of guinea-pig hearts by enzymatic dispersion. The membrane currents were recorded by the whole-cell voltage clamping. The Ca2+ current was activated by depolarisation from -80 to +5 mV preceded by the prepulses to -40 mV. Cellular action potentials (APs) were recorded by current clamping. Intracellular [Ca2+] was assessed by recording of fluorescence of Indo-1 loaded into cells. In current clamped cells (APs recorded) 20 microM tetrodotoxin (TTX) reduced the time to 75% of amplitude of Ca2+ transients from 50 +/- 6.6 ms to 32 +/- 5 ms (n = 7). In voltage clamped cells prepulses from the holding potential of -80 mV to -40 mV 50-100 ms long activated the Na+ current and initiated step increase in [Ca2+] reaching 30-50% of the total amplitude of the transient. Prepulses 10-20 ms long initiated increase in [Ca2+] merging with that elicited by Ca2+ current into smooth rising phase. Blocking of Na+ current with TTX or by switching the holding potential from -80 to -40 mV increased the amplitude of the Ca2+ current by 38 +/- 3.2% (n = 8) and 43 +/- 9% (n = 7), respectively, and eliminated the initial step increase in [Ca2+]. When 10-20 ms prepulses were used, blocking of Na+ current with TTX or switching of the holding potential decreased the time to 75% of amplitude of Ca2+ transients from 27 +/- 3.7 ms to 12 +/- 1.2 ms (n = 5) and from 25 +/- 3.1 ms to 14 +/- 1.1 ms (n = 9), respectively. 100 microM Cd2+ inhibited the initial rise in [Ca2+], however, the inhibition did not correlate with degree of inhibition of Ca2+ current. The Na+ current activated prior to Ca2+ current reduces its amplitude and decreases the rate of release of Ca2+ from sarcoplasmic reticulum. In voltage clamped cells this could result from Ca2+ influx prior to onset of Ca2+ current initiated by Na+ current escaping from the voltage control and/or reversal of Na/Ca exchange due to increase in subsarcolemmal [Na+]. In cells in which Ca2+ transients were initiated by APs only the second mechanism is conceivable.