NA-RETICULUM DURING ACTION-POTENTIALS IN GUINEA-PIG VENTRICULAR MYOCYTES( CURRENT AND CA2+ RELEASE FROM THE SARCOPLASMIC)

1. Ca2+ release from the sarcoplasmic reticulum (SR) was examined in e nzymatically isolated single guinea-pig ventricular myocytes by monito ring [Ca2+](i) with fura-2 during whole-cell recording of action poten tials at room temperature (23-25 degrees C). Modulation of Ca2+ releas e by the Na+ current (I-Na) was studied by manipulating Na+ influx thr ough the Na+ channel. 2. For a comparable Ca2+ loading of the SR, brie f hyperpolarizing currents applied at the peak of the action potential increased Ca2+ release, while depolarizing pulses had the opposite ef fect. Similar currents applied before the action potential did not aff ect Ca2+ release. 3. Application of tetrodotoxin (TTX; 60 mu M) modera tely reduced Ca2+ release from the SR, but this effect was delayed in comparison with the immediate block of I-Na. An early effect of TTX wa s to increase Ca2+ release. 4. Replacement of Na+ with Li+ did not red uce Ca2+ release, but led to a progressive increase in Ca2+ release, r esulting in spontaneous activity. 5. Ca2+ channel blockers (CdCl2, 100 mu M; nisoldipine, 20 mu M; or nifedipine, 20 mu M) drastically reduc ed Ca2+ release from the SR. 6. Voltage clamp experiments confirmed th at TTX blocked I-Na and its associated [Ca2+](i) transient during volt age steps from -90 to -50 mV. I-Na and its associated [Ca2+](i) transi ent were equally suppressed following replacement of Na+ with N-methyl -D-glucamine (NMDG(+)), but the [Ca2+](i) transient was not suppressed following replacement of Na+ with Li+. 7. The I-Na-associated transie nt was sensitive to Ca2+ channel blockers. During steps from -50 to 0 mV, it appeared that the dihydropyridine antagonists often did not pro vide full block of the calcium current (I-Ca). 8. During current clamp stimulation at 1 Hz in the presence of TTX (60 mu M), the Ca2+ conten t of the SR was decreased, due to the changes in action potential conf iguration and to changes in [Na+](i). 9. Our experiments indicate that the Ca2+ entry coupled to Na+ influx via the Na+ channel does not con tribute substantially to the trigger for Ca2+ release from the SR duri ng action potentials (23-25 degrees C). However, I-Na modulates Ca2+ r elease by affecting the Ca2+ load of the SR.