[Existence and properties of a chloride-sensitive conductivity in atrial myocardium]

Investigated are the intracellularly recorded action potentials (A. P.) of the rabbit atrial trabeculae (length 2-4 mm, diameter 150-800 micrometers). After a pacing pause the first A. P. shows a rapid initial repolarization (I. R.) up to -40 mV within 4-6 ms. By increasing the duration of the pacing pause the I. R. is accentuated. It is diminished by repetitive stimulation after the pause and disappears within 2-4 s. At variation of the extracellular Ca++ concentration and under verapamil, tetraethyl ammonium chloride load (TEA load) the I. R. is unchanged. On the other hand, the I. R. is very sensitive to the variation of the external Cl- concentration. When decreasing the external Cl- concentration, the I. R. is diminished. At 10 mmol x 1(-1) the I. R. disappears. Assuming a Cl- current ICl the experimental findings are modelled. During the A. P. a Cl- conductivity gCl is activated instantaneously and inactivated slowly (rate constant 5.09 s-1). Because of the repriming of gCl the I. R. disappears after the post rest pacing. During the pacing pause a recovery of gCl takes place (rate constant 0.06 s-1). The gCl recovery induces the post rest I. R. Estimation of the maximum gCl yields 0.24 mmho . cm-2.