CA2+ LOAD OF GUINEA-PIG VENTRICULAR MYOCYTES DETERMINES EFFICACY OF BRIEF CA2+ CURRENTS AS TRIGGER FOR CA2+ RELEASE

1. In guinea-pig ventricular cells, the concentration of ionized cytosolic calcium ([Ca2+](c)) was estimated from the fluorescence of 100 mu M K-5-indo-1. At 36 degrees C and 2 mM [Ca2+](o), the Ca2+ load of the cells was varied by 1 Hz trains of conditioning clamp pulses to -30 mV (low Ca2+ load), 0 mV (intermediate Ca2+ load) and paired pulses (high Ca2+ load). After seven pulses potentiation was steady and short test pulses to 0 mV were tested for their efficacy in triggering [Ca2+](c) transients. The influx of trigger Ca2+ was graded by varying the test-pulse duration between 1 and 180 ms. 2. After a 3 min rest period, [Ca2+](c) was 100 +/- 20 nM (mean +/- S.E.M.) and 2 ms test pulses were unable to induce [Ca2+](c) transients. Test pulses of 2 ms duration, however, induced [Ca2+](c) transients after potentiation with single or paired pulses. 3. At high cellular Ca2+ load, the amplitude of the [Ca2+](c) transients (Delta[Ca2+](c)) gradually increased with pulse durations up to 8 ms. Pulse durations between 8 and 160 ms, however, did not further increase Delta[Ca2+](c) as if the largest part of the [Ca2+](c) transient was due to regenerative contribution of Ca2+-induced Ca2+ release. 4. Pulses of 180 ms duration induced 'saturating' responses whose amplitudes Delta[Ca2+](c,t=infinity) decreased from 938 +/- 120 nM at high Ca2+ load, to 610 +/- 90 and 350 +/- 120 nar at intermediate and low Ca2+ loads, respectively. 5. Delta[Ca2+](c) was more sensitive to the duration of Ca2+ influx at low or intermediate Ca2+ loads than at high Ca2+ load. When Delta[Ca2+](c) was plotted against the test-pulse duration, 50% of Delta[Ca2+](c,t=infinity) was found to be at 9 +/- 2 ms (low), 4 8 +/- 1 ms (intermediate) or 1.8 +/- 0.5 ms pulses (high Ca2+ load). Correspondingly, the efficacy of 2 ms test pulses in triggering [Ca2+](c) transients increased with the Ca2+ load. 6. At high Ca2+ load, [Ca2+](c) peaked nearly independently of pulse duration at 19 +/- 3 ms. At intermediate or low Ca2+ load, time to peak increased with pulse duration. 7. The results confirm the theory that sarcoplasmic reticulum (SR) Ca2+ release contributes an amount to the [Ca2+](c) transient that increases with the cellular Ca2+ load. The results are compatible with the hypothesis that SR Ca2+ release can be activated by both Ca2+ influx and by SR Ca2+ release and that the latter mechanism constitutes a positive feedback, the amplification of which increases with the amount of releasable Ca2+.