CALCIUM-ACTIVATED TRANSIENT MEMBRANE CURRENTS ARE CARRIED MAINLY BY CHLORIDE-IONS IN ISOLATED ATRIAL, VENTRICULAR AND PURKINJE-CELLS OF RABBIT HEART

Under physiological conditions, calcium-dependent ([Ca2+](i)-dependent ) Cl- currents (I-Cl(Ca)) have been suggested to participate in the re polarizing processes. In this paper, the possible contribution of I-Cl (Ca) to transient inward currents and, hence to arrhythmias, has been studied in myocytes from the working myocardium and from the conductiv e system. Single atrial, ventricular and Purkinje cells, isolated enzy matically from rabbit heart, have been studied under whole-cell voltag e-clamp and were internally perfused with the fluorescent Ca2+ indicat or, fura-2 (100 mu M). Ca2+ release from the sarcoplasmic reticulum wa s either induced by external application of caffeine or occurred spont aneously in Ca2+-overloaded cells. Membrane currents accompanying Ca2 transients showed linear current-voltage characteristics between -60 and +80 mV as evidenced from fast voltage ramps. When intra- and extra cellular Cl- concentrations were kept symmetrical in the absence of th e Na+-Ca2+ exchange mechanism, transient currents had a reversal poten tial close to 0 mV. Reduction of external Cl- concentration shifted th is reversal potential towards the new Cl- equilibrium potential. Neith er the time course of the transient currents nor the shift in their re versal potentials was significantly affected by the presence of Na+. A pproximately 90 % of this current was blocked by the application of th e Cl- channel blocker, anthracene-9-carboxylic acid (0.5 mM) at +80 mV . It is concluded, that [Ca2+](i)-activated transient membrane current s in atrial, ventricular and Purkinje cells of rabbit heart are mainly due to the activation of a [Ca2+](i)- dependent Cl- current.