Abstract 799: Rapid Modulation of Calcium Channel Open Probability Alters Calcium-induced Calcium Release Gain
Background: Calcium-induced calcium release (CICR) from the sarcoplasmic reticulum (SR) in cardiac myocytes is triggered by the calcium current (ICa), and is proportional to ICa amplitude. We assessed the effects of rapid modulation of the L-type Ca2+ channel open probability on CICR in the presence of controlled SR Ca2+ load.
Methods: Fura-2 loaded rat ventricular myocytes were voltage-clamped at − 40 mV (perforated patch clamp). Voltage-current and voltage-transient curves were obtained by 10 mV step depolarisations. To standardize SR Ca2+ load, a train of 5 conditioning pulses (− 40 to 0 mV, 0.5 Hz) was applied prior to each test pulse. Rapid switching permitted the perfusate solution to be exchanged within a single beat. External solutions containing either 2.5 μM FPL 64176 (calcium channel agonist) or 0.5 μ M nifedipine (calcium channel antagonist) were applied immediately after the last conditioning pulse.
Results: Steady-state application of FPL significantly increased peak ICa amplitude by 39±15 % and calcium transient amplitude by 39±10 %, resulting in no change in calculated CICR gain. Rapid application of FPL for 2.5 seconds increased ICa at all potentials tested. At 0 mV, ICa increased by 13±2 %, with no significant change in SR calcium release, leading to a 15±2 % reduction in CICR gain. At − 20 mV, ICa increased by 344±16%, whereas transient amplitude increased by only 41±11%. Steady-state application of nifedipine significantly decreased peak ICa amplitude by 91±2 %, which was accompanied by a 66±2 % reduction in Ca2+ transient amplitude. Rapid application of nifedipine decreased ICa at all potentials tested. At 0 mV, a 38±5 % reduction in ICa amplitude was not accompanied by a significant change in SR Ca2+ release, leading to a 31±12 % increase in CICR gain. A reduction in SR Ca2+ release was noted only at positive, and not at negative potentials (47±5 % decrease in ICa, 32±4 % decrease in transient amplitude at +30 mV).
Conclusion: Rapid modulation of Ca2+ channel open probability may profoundly modify the gain of CICR. Therefore, the steady-state relationship between ICa and SR Ca2+ release may be mediated via changes in SR load or local Ca2+ concentration.