Beat Dependent Alteration of Ca²⁺-Activated Cl^- Current during Rapid Stimulation in Rabbit Ventricular Myocytes

Abstract

The transient outward currents (I_to) play an important role in action potential repolarization in cardiac myocytes. Two components of I_to have been identified as 4-AP-sensitive but Ca²⁺-insensitive I_to carried by K⁺, and Ca²⁺-sensitive but 4-AP insensitive I_to carried by Cl^- (I_{Cl (Ca)}). It is known that the amplitudes of I_to change depending on the stimulation frequency. In this study we investigated the beat dependent alteration of ICl (Ca) during rapid stimulation using the whole cell patch clamp technique in rabbit ventricular myocytes. The cells were internally perfused with a solution containing 0.1 mM free Ca²⁺ to develop I_{Cl (Ca)} and all internal K⁺ was replaced with Cs⁺ to block 4-AP-sensitive I_to and other K⁺ currents. By applying depolarizing pulses at a high frequency of 2.5 Hz, the amplitudes of I_{Cl (Ca)} gradually increased as the number of pulses increased following a transient decrease in the 2nd pulse and reached a plateau level at the 20th pulse. The shape of the current-voltage curve of I_{Cl (Ca)} was not overly different for different numbers of preceding pulses. The recovery from inactivation of I_{Cl (Ca)}could be fitted to a single expon-ential curve and full recovery was achieved after > 1 sec with a time constant of 368 ms. The ramp clamp experiments showed that the conductance of the background I_{Cl (Ca)} increased with the preceding pulse numbers, indicating that the resting level of [Ca²⁺]_i increased with the pulses applied. From these results, we conclude that beat dependent alteration of ICl (Ca) is determined by not only its apparent kinetic property, but also the resting level of [Ca²⁺]_i during rapid stimulation.