Beat dependent alteration of Ca2+-activated Cl- current during rapid stimulation in rabbit ventricular myocytes

The transient outward currents (I-to) play an important role in action pote ntial 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 C a2+-sensitive but 4-AP insensitive I-to carried by Cl (=) (I-Cl (Ca)). It i s known that the amplitudes of I-to change depending on the stimulation fre quency. In this study we investigated the beat dependent alteration of I-Cl (Ca) during rapid stimulation using the whole cell patch clamp technique i n rabbit ventricular myocytes. The cells were internally perfused with a so lution containing 0.1 muM 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 amplitu des of I-Cl (Ca) gradually increased as the number of pulses increased foll owing 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 f rom inactivation of I-Cl (Ca) could be fitted to a single exponential curve and full recovery was achieved after > 1 sec with a time constant of 365 m s. The ramp clamp experiments showed that the conductance of the background I-Cl(Ca) increased with the preceding pulse numbers, indicating that the r esting level of [Ca2+](i) increased with the pulses applied. From these res ults, we conclude that beat dependent alteration of I-Cl (Ca) is determined by not only its apparent kinetic property, but also the resting level of [ Ca2+](i) during rapid stimulation.