EFFECT OF NA-CONTRACTION COUPLING IN VENTRICULAR MYOCYTES OF GUINEA-PIG HEART( CURRENT ON EXCITATION)

We investigated the effect of Na+ current on the Ca2+ current and Ca2 transients in cardiac myocytes. Myocytes were isolated from the ventr icles of guinea-pig hearts by enzymatic dispersion. The membrane curre nts were recorded by the whole-cell voltage clamping. The Ca2+ current was activated by depolarisation from -80 to +5 mV preceded by the pre pulses to -40 mV. Cellular action potentials (APs) were recorded by cu rrent clamping, Intracellular [Ca2+] was assessed by recording of fluo rescence of Indo-1 loaded into cells. In current clamped cells (APs re corded) 20 mu M tetrodotoxin (TTX) reduced the time to 75% of amplitud e 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 m V 50-100 ms long activated the Na+ current and initiated step increase in [Ca2+] reaching 30-50% of the total amplitude of the transient, Pr epulses 10-20 ms long initialed increase in [Ca2+] merging with that e licited by Ca2+ current into smooth rising phase. Blocking of Na+ curr ent 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 4 3+/-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 m u< Cd2+ inhibited the initial rise in [Ca2+], however, the inhibition did not correlate with degree of inhibition of Ca2+ current. The Na+ c urrent activated prior to Ca2+ current reduces its amplitude and decre ases the rate of release of Ca2+ from sarcoplasmic reticulum. In volta ge clamped cells this could result from Ca2+ influx prior to onset of Ca2+ current initiated by Naf current escaping from the voltage contro l and/or reversal of Na/Ca exchange due to increase in subsarcolemmal [Na+]. In cells in which Ca2+ transients were initiated by APs only th e second mechanism is conceivable.