Epinephrine enhances Ca2+ current-regulated Ca2+ release and Ca2+ reuptake in rat ventricular myocytes

The voltage dependence of the intracellular Ca2+ transients was measured in single rat ventricular myocytes with the fluorescent Ca2+ indicator dye fura-2. The whole-cell voltage clamp technique was used to measure the membrane current, and 0.9 mM fura-2 was loaded into the cell by including it in the dialyzing solution of the patch electrode. A mechanical light chopper operating at 1200 Hz was used to obtain simultaneous measurements of the intracellular Ca2+ activity with fluorescence excitation on either side of the isosbestic point (330 nm and 410 nm). The symmetry of the two optical Ca2+ signals was used as a criterion to guard against artifacts resulting, for instance, from motion. The voltage dependence of peak Ca2+ current and the Ca2+ transient measured 25 ms after depolarizing clamps from a holding potential of -40 mV were bell-shaped and virtually identical. The Ca2+ entry estimated from the integral of the Ca2+ current (0 mV, 25 ms) corresponds to a 5-10 microM increase in the total intracellular Ca2+ concentration, whereas the optical signal indicated a 100 microM increase in total intracellular Ca2+. Repolarization of clamp pulses from highly positive potentials were accompanied by a second Ca2+ transient, the magnitude of which, when summed with that measured during depolarization, was nearly constant. Ryanodine (10 microM) had little or no effect on the peak Ca2+ current but reduced the magnitude of the early Ca2+ transients by 70-90%. Epinephrine (1 microM) increased the Ca2+ current and the Ca2+ transients, accelerated the rate of decline of the Ca2+ transients at potentials between -30 and +70 mV, and reduced the intracellular [Ca2+] below baseline at potentials positive to +80 or negative to -40 mV, where clamp pulses did not elicit any Ca2+ release. Elevation of intracellular cAMP mimicked the relaxant effect of epinephrine at depolarizing potentials, whereas elevation of extracellular [Ca2+] did not. These results suggest that most of the activator Ca2+ in rat ventricular cells is released from the sarcoplasmic reticulum as a graded response to sarcolemmal Ca2+ influx. Consistent with a graded Ca2+-induced Ca2+ release we find that epinephrine increases the internal Ca2+ release by increasing the Ca2+ current. Epinephrine may also increase the Ca2+ content of the sarcoplasmic reticulum that may, in turn, increase the Ca2+-induced Ca2+ release. The relaxant effect of epinephrine appears to be caused by enhanced rate of Ca2+ resequestration and is mediated by adenylate cyclase system.