Inhibition and Rapid Recovery of Ca 2+ Current During Ca 2+ Release From Sarcoplasmic Reticulum in Guinea Pig Ventricular Myocytes

Abstract

We have investigated the modulation of the L-type Ca ²⁺ channel by Ca ²⁺ released from the sarcoplasmic reticulum (SR) in single guinea pig ventricular myocytes under whole-cell voltage clamp. [Ca ²⁺ ] _i was monitored by fura 2. By use of impermeant monovalent cations in intracellular and extracellular solutions, the current through Na ⁺ channels, K ⁺ channels, nonspecific cation channels, and the Na ⁺ -Ca ²⁺ exchanger was effectively blocked. By altering the amount of Ca ²⁺ loading of the SR, the time course of the Ca ²⁺ current (I _Ca ) could be studied during various amplitudes of Ca ²⁺ release. In the presence of a large Ca ²⁺ release, fast inhibition of I _Ca occurred, whereas on relaxation of [Ca ²⁺ ] _i , fast recovery was observed. The time course of this transient inhibition of I _Ca reflected the time course of [Ca ²⁺ ] _i . However, the inhibition seen in the first 50 ms, ie, the time of net Ca ²⁺ release from the SR, exceeded the inhibition observed later during the pulse, suggesting the existence of a higher [Ca ²⁺ ] near the channel during this time. Transient inhibition of I _Ca during Ca ²⁺ release was observed to a similar degree at all potentials. It could still be observed in the presence of intracellular ATP-γ-S and of cAMP. Therefore, we conclude that the modulation of I _Ca by Ca ²⁺ release from the SR is not related to dephosphorylation. It could be related to a reduction in the driving force and to a direct inhibition of the channel by [Ca ²⁺ ] _i . The observation that the degree of inhibition does not depend on membrane potential suggests that the Ca ²⁺ binding site for this modulation is located outside the pore. The transient nature of the modulation of I _Ca by Ca ²⁺ release will contribute to the recovery of I _Ca during prolonged action potentials.