L-type Ca 2+ channel recovery from inactivation in rabbit atrial myocytes.

Adaptation of the myocardium to varying workloads critically depends on the recovery from inactivation (RFI) of L-type Ca 2+ channels (LCCs) which provide the trigger for cardiac contraction. The goal of the present study was a comprehensive investigation of LCC RFI in atrial myocytes. The study was performed on voltage-clamped rabbit atrial myocytes using a double pulse protocol with variable diastolic intervals in cells held at physiological holding potentials, with intact intracellular Ca 2+ release, and preserved Na + current and Na + /Ca 2+ exchanger (NCX) activity. We demonstrate that the kinetics of RFI of LCCs are co-regulated by several factors including resting membrane potential, [Ca 2+ ] i , Na + influx, and activity of CaMKII. In addition, activation of CaMKII resulted in increased I Ca amplitude at higher pacing rates. Pharmacological inhibition of NCX failed to have any significant effect on RFI, indicating that impaired removal of Ca 2+ by NCX has little effect on LCC recovery. Finally, RFI of intracellular Ca 2+ release was substantially slower than LCC RFI, suggesting that inactivation kinetics of LCC do not significantly contribute to the beat-to-beat refractoriness of SR Ca 2+ release. The study demonstrates that CaMKII and intracellular Ca 2+ dynamics play a central role in modulation of LCC activity in atrial myocytes during increased workloads that could have important consequences under pathological conditions such as atrial fibrillations, where Ca 2+ cycling and CaMKII activity are altered.