Objective: Contractions of isolated, single myocytes of guinea pig heart stimulated at 37 degreesC consist of a phasic component and a voltage dependent tonic component. In this study we investigated the source of Ca2+ activating the tonic component. Methods: Experiments were performed at 37 degreesC in ventricular myocytes of guinea pig heart. Voltage-clamped cells were stimulated by the pulses from the holding potential of -40 to +5 mV. [Ca2+](i) was monitored as fluorescence of Indo 1-AM and contractions were recorded with the TV edge-tracking system. Results: Superfusion of 5 mmol/l Ni2+ during 30 s pause did not inhibit subsequent biphasic Ca2+ transients and contractions despite inhibition of Ca2+ current and Na+/Ca2+ exchange. KB-R7943 (5 mu mol/l) or intracellular dialysis with 0 Na+ solution, both of which inhibit reversed Na+/Ca2+ exchange, decreased amplitude of Ca2+ transients and contractions by similar to 40%. The ratio of amplitudes of tonic to phasic component was increased by Ni2+ and was not changed by KB-R7943 or 0 Na-i(+). Ryanodine (200 mu mol/l) inhibited both components of contractions in cells superfused with Ni2+. The phasic component but not the tonic component was inhibited by 20 mu mol/l nifedipine in cells superfused with Ni2+. Conclusions: Tonic component of contraction of single myocytes of guinea pig heart is not activated by Ca2+ current or by the reverse mode Na+/Ca2+ exchange as currently proposed in literature. Rather, it is activated by Ca2+ released from the sarcoplasmic reticulum. However, kinetics and mechanism of release seem to be quite different from those of Ca2+ fraction activating the phasic component of contraction. (C) 2001 Elsevier Science B.V. All rights reserved.
