Three types of single K channels contribute to the transient outward current in myocardial mouse cells

Whole cell currents and single channel currents were measured by a patch clamp technique in single adult cardiocytes of mice. By stepping from Vh = -70 mV into depolarizing direction nonlinear transient whole cell currents could be elicited at test potentials Vt positive to -40 mV. In cell attached patches three types of unitary currents were isolated to contribute to the whole cell current: (1) a 27 pS channel with a mean open time tau 0 = 3.62 +/- 0.61 ms and a fast mean shut time tau s1 = 2.11 +/- 0.35 ms (n = 15), (2) a 12 pS channel with tau 0 = 24.3 +/- 6.1 ms and tau s1 = 5.51 +/- 1.99 ms (n = 8), and (3) a 5 pS channel with tau 0 = 0.71 +/- 0.14 ms (n = 7) and tau s1 = 2.31 +/- 0.46 ms (n = 6). All times were voltage independent, all open time distributions could be fitted monoexponentially, all shut time distributions needed two exponentials. The reversal potential of all channels was determined at -70 mV. By replacement of all Cl- and 91% Na+ in the pipette the three channels were identified as potassium channels. In inside-out patches the application of 5 mmol/l ATP to the cytoplasmic side of the membrane completely blocked the transient 27 pS and 5 pS channels and incompletely the 12 pS channels. In most cases inside-out patches contained several (2-8) time independent ATP regulated 27 pS channels and sometimes (4%) a time independent 12 pS channel. Those channels could also be blocked by ATP as the corresponding transient channels, however the open times were prolonged markedly (27 pS: open time distribution needed two exponentials with tau 01 = 13.1 +/- 5.2 ms and tau 02 = 87 +/- 49 ms, n = 17; 12 pS; tau 0 = 34.0 +/- 12.9 ms; n = 4). A possible identity of the transient and the time independent channels of the same conductance is discussed.