A SLOWLY INACTIVATING TRANSIENT OUTWARD CURRENT IN RAT VENTRICULAR MYOCYTES

In rat ventricular myocytes we found two components of transient outwa rd current, which could be discriminated time- and voltage- dependentl y. Besides the well known fastly inactivating transient outward curren t (i(to,f), tau=35 +/- 8ms, n=4) we investigated properties of a slowl y inactivating transient outward current (i(to,s), tau=1.7 +/- 0.4s, n =4). Because of the slow inactivation process of i(to,s) tail currents were observed at -25mV The inactivation curve of i(to,f) was characte rized by a half- inactivation voltage of -58.4 +/- 1.4mV and a slope f actor of 5.6 +/- 0.5mV (n=4). The inactivation curves of i(to,s) and t ail currents were nearly identical but significantly different from th e i(to,f)-curve. Half- inactivation voltages of i(to,s) and tail curre nts were -87.5 +/- 6mV and -89.1 +/- 5mV (n=4), respectively. Slope fa ctors were 10.3 +/- 2.9mV and 9.8 +/- 1.7mV (n=4). The activation gate of I-to,I-s was half-maximally opened at -11.5 +/- 2.6mV, and the slo pe factor was -10.6 +/- 1.7mV (n=3). I-to,I-s tail current reversed it s direction at -62 +/- 3.2mV (n=5). This indicates, that i(to,s)- curr ent flow is carried mainly by potassium ions. I-to,I-s- current was no t abolished by Tetrodotoxin (TTX) and Cd.