Mechanism of activation of the tonic component of contraction in myocytes of guinea pig heart.

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Contractions of myocytes of guinea pig heart consist of a phasic component relaxing independently on the voltage and a tonic component relaxing upon repolarization. We found previously that Ca(2+) activating the tonic component is released from the sarcoplasmic reticulum. In this study, we analysed the mechanism of activation and maintenance of this release. Experiments were performed at 37 degrees C in ventricular myocytes of guinea pig heart. Voltage-clamped myocytes were stimulated by the pulses of the duration of 300 ms to 15-45 s from the holding potential of -40 to +5 mV. [Ca(2+)](i) was monitored as fluorescence of Indo-1 and contractions were recorded with the TV edge-tracking system. Myocytes responded to the short and long pulses with phasic contraction or Ca(2+) transient followed by the sustained contraction or increase in [Ca(2+)](i). Repolarization brought about relaxation. 10 mmol L(-1) Ni(2+) blocking Na(+)/Ca(2+) exchange superfused during the tonic component increased its amplitude. Superfusion of Ca(2+)-free solution during sustained contraction brought about relaxation both in normal cells and in cells superfused with Ni(2+) despite preserved sarcoplasmic reticulum Ca(2+) content assessed with caffeine spritz. Relaxing effect of Ca(2+)-free solution was not affected by carboxyeosin, a blocker of sarcolemmal Ca(2+)-ATPase. Tonic component of contraction and of Ca(2+) transient was inhibited by 200 micromol L(-1) ryanodine, a blocker of Ca(2+) release channels of the sarcoplasmic reticulum and by 20 micromol L(-1) nifedipine, a blocker of L-type I(Ca). Tonic component of contraction results from Ca(2+) release via the sarcoplasmic reticulum Ca(2+) channels activated by sustained, nifedipine-sensitive and Ni(2+)-insensitive Ca(2+) influx. Alternatively, the SR Ca(2+) release is activated by voltage, the dihydropyridine receptors acting like voltage sensors.