THE SOURCE OF CONTRACTILE 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 thapsi gargin (TG). These cells show Ca2+ transients of nearly normal amplitu de, albeit with the slowed kinetics. We found in the voltage clamped a nd loaded with Indo 1-AM cells that at depolarizing potentials from -3 0 to +70mV, blocking of sarcolemmal Ca2+ channels with 20 mu M nifedip ine or 20 mu M Cd2+ reduced Ca2+ transients and contractions as much i n the cells treated with TG as in the normal cells. The residual Ca2transients were mostly subthreshold for the contractile system. The re sult suggests that in the cells treated with TG, Ca2+ influx by the re versed Na/Ca exchange is not more important for activation of contract ion 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 fro m 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 ce lls pretreated with TG, 20 ms pulses initiated Ca2+ transients which w ere hardly visible in the records of fluorescence. Their amplitude inc reased with increase in the duration of the pulses linearly correlatin g with the charge transfered with the Ca2+ current. We propose that th e direct source of Ca2+ activating contraction in the guinea-pig ventr icular myocytes is sarcolemmal Ca2+ influx mostly through the sarcolem mal Ca2+ channels. The alternative hypothesis is that there is some ye t unidentified cellular source of activator Ca2+ (internal leaflet of sarcolemma?) from which it may be released by sarcolemmal Ca2+ influx.