A slowly inactivating transient outward current in rat ventricular myocytes

In rat ventricular myocytes we found two components of transient outward current, which could be discriminated time- and voltage- dependently. Besides the well known fastly inactivating transient outward current (ito,f, tau = 35 +/- 8 ms, n = 4) we investigated properties of a slowly inactivating transient outward current (ito,s, tau = 1.7 +/- 0.4 s, n = 4). Because of the slow inactivation process of ito,s tail currents were observed at -25 mV. The inactivation curve of ito,f was characterized by a half- inactivation voltage of -58.4 +/- 1.4 mV and a slope factor of 5.6 +/- 0.5 mV (n = 4). The inactivation curves of ito,s and tail currents were nearly identical but significantly different from the ito,f-curve. Half-inactivation voltages of ito,s and tail currents were -87.5 +/- 6 mV and -89.1 +/- 5 mV (n = 4), respectively. Slope factors were 10.3 +/- 2.9 mV and 9.8 +/- 1.7 mV (n = 4). The activation gate of ito,s was half-maximally opened at -11.5 +/- 2.6 mV, and the slope factor was -10.6 +/- 1.7 mV (n = 3). Ito,s tail current reversed its direction at -62 +/- 3.2 mV (n = 5). This indicates, that ito,s- current flow is carried mainly by potassium ions. Ito,s- current was not abolished by Tetrodotoxin (TTX) and Cd.