The source of contracticle calcium in guinea-pig cardiac myocytes treated with thapsigargin

We investigated the source of activator Ca2+ in the cells deprived of sarcoplasmic reticulum (SR) Ca2+ by pretreatment with 10(-6) mM thapsigargin (TG). These cells show Ca2+ transients of nearly normal amplitude, albeit with the slowed kinetics. We found in the voltage clamped and loaded with Indo 1-AM cells that at depolarizing potentials from -30 to +70mV, blocking of sarcolemmal Ca2+ channels with 20 microM nifedipine or 20 microM Cd2+ reduced Ca2+ transients and contractions as much in the cells treated with TG as in the normal cells. The residual Ca2+ transients were mostly subthreshold for the contractile system. The result suggests that in the cells treated with TG, Ca2+ influx by the reversed Na/Ca exchange is not more important for activation of contraction than in the normal cells. In the normal cells shortening of one of the depolarizing pulses (+ 5mV) applied at a steady rate of 30/min from 200 ms to 5, 10, 20, 30, 50, or 100 ms little affected amplitude of the respective Ca2+ transients, although their duration was decreased proportionally to the decrease of the duration of the pulse. In the cells pretreated with TG, 20 ms pulses initiated Ca2+ transients which were hardly visible in the records of fluorescence. Their amplitude increased with increase in the duration of the pulses linearly correlating with the charge transfered with the Ca2+ current. We propose that the direct source of Ca2+ activating contraction in the guinea-pig ventricular myocytes is sarcolemmal Ca2+ influx mostly through the sarcolemmal Ca2+ channels. The alternative hypothesis is that there is some yet unidentified cellular source of activator Ca2+ (internal leaflet of sarcolemma?) from which it may be released by sarcolemmal Ca2+ influx.