Sarcoplasmic reticulum Ca2+ content, L-type Ca2+ current and the Ca2+ transient in rat myocytes during beta-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 (ICa) 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 mumol 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 ICa 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 ICa 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 ICa: a small ICa 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 ICa in the same way as under control conditions. 6. Nifedipine (2 mumol 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 Ca(2+)-ATPase inhibitor thapsigargin (2.5 mumol 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 ICa 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.
Hussain
M
M
Department of Physiology, University of Leeds, UK.
Orchard
C H
CH
D000318:Adrenergic beta-Agonists / Q000494:pharmacology*; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000502:physiology*; D020746:Calcium Channels, L-Type; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D002478:Cells, Cultured; D006321:Heart / Q000187:drug effects / Q000502:physiology*; D007545:Isoproterenol / Q000494:pharmacology*; D007700:Kinetics; D008297:Male; D008564:Membrane Potentials; D009206:Myocardium / Q000378:metabolism*; D009543:Nifedipine / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D051381:Rats; D017208:Rats, Wistar; D011943:Receptors, Adrenergic, beta / Q000502:physiology; D012519:Sarcoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*; D019284:Thapsigargin / Q000494:pharmacology
D000318:Adrenergic beta-Agonists; D015220:Calcium Channels; D020746:Calcium Channels, L-Type; D011943:Receptors, Adrenergic, beta; D002110:Caffeine; D019284:Thapsigargin; D000252:Calcium-Transporting ATPases; D009543:Nifedipine; D007545:Isoproterenol; D002118:Calcium