Sarcoplasmic reticulum Ca2+ content, L-type Ca2+ current and the Ca2+ transient in rat myocytes during β-adrenergic stimulation

1. The effect of beta-adrenergic stimulation on the relationship between the intracellular Ca2+ transient and the amplitude of the L-type Ca2+ current (I-Ca) has been investigated in ventricular myocytes isolated from rat hearts. Intracellular [Ca2+] was monitored using fura-2 during field stimulation and while membrane potential was controlled using voltage clamp techniques. 2. The increase in the amplitude, and the rate of decline, of the Ca2+ transient produced by isoprenaline (1.0 mu mol l(-1)) was not significantly different in myocytes generating action potentials and in those voltage clamped with pulses of constant duration and amplitude. 3. Under control conditions, the current-voltage (I-V) relationship for I-Ca was bell shaped. The amplitude of the Ca2+ transient also showed a bell-shaped voltage dependence. In the presence of isoprenaline, the amplitude of both I-Ca and the Ca2+ transient was greater at all test potentials and the I-V relationship maintained its bell-shaped voltage dependence. However, the size of the Ca2+ transient was no longer graded with changes in the amplitude of I-Ca: a small I-Ca could now elicit a maximal Ca2+ transient. 4. Rapid application of caffeine (10 mmol l(-1)) was used to elicit Ca2+ release from the sarcoplasmic reticulum (SR). Isoprenaline increased the integral of the subsequent rise in cytoplasmic [Ca2+] to 175 +/- 13 % of control. 5. Abbreviation of conditioning pulse duration in the presence of isoprenaline was used to reduce the amplitude of the Ca2+ transient. to control levels. Under these conditions, the amplitude of the Ca2+ transient was again graded with the amplitude of I-Ca in the same way as under control conditions. 6. Nifedipine (2 mu mol l(-1)) was also used to decrease Ca2+ transient amplitude in the presence of isoprenaline. In the presence of isoprenaline and nifedipine, the amplitude of the Ca2+ transient again showed a bell-shaped voltage dependence. 7. The SR Ca2+-ATPase inhibitor thapsigargin (2.5 mu mol l(-1)) reduced the effect of isoprenaline on the amplitude of the Ca2+ transient. In the presence of thapsigargin, the size of the Ca2+ transient increased as I-Ca increased in response to isoprenaline. 8. These data suggest that the increase in the amplitude of the Ca2+ transient produced by beta-adrenergic stimulation in cardiac muscle is due to an increase in the gain of the SR Ca2+ release process, due principally to an increase in the Ca2+ content of the SR.