A 3,4-diaminopyridine-insensitive, Ca(2+)-independent transient outward K+ current in cardiac ventricular myocytes

A previously unrecognized current that initially is not present and requires at least 25 min of intracellular access to develop can be found in approximately 75% of cardiac myocytes isolated from cat ventricle within 90 min after intracellular access is obtained with conventional suction patch pipette electrodes. We refer to this patch-duration-dependent (PDD) current as IK(PDD). IK(PDD) can be elicited with depolarizing test steps (Vt) ranging between -40 and +60 mV applied after a hyperpolarizing conditioning step to -140 mV for 200 ms from a holding potential of -40 mV. It shows an ohmic voltage dependence and appears to be an essentially pure K+ current. At Vt = 30 mV, the current is a time-dependent, transient current with a time to peak of 1.06 +/- 0.10 ms (n = 5) and a decay phase that can be fit to the sum of two decaying exponentials (tau f = 3.30 +/- 0.51 ms and tau s = 2.48 +/- 5.6 ms; n = 5). The voltage dependence of the steady-state inactivation can be fit to a single exponential Boltzmann distribution with a slope factor of 8.97 mV, and the voltage at which 50% of the channels are inactivated is -78 mV. The current can be blocked by 0.2 mM Ba2+ extracellularly applied or Cs+ intracellularly applied but is insensitive to 0.5 mM 3,4-diaminopyridine. These characteristics are unlike those for other known K+ currents. The lack of similarity between IK(PDD) and any currently documented cardiac K+ current suggests that IK(PDD) is either a previously undescribed K+ current or a modification of IK1 that makes it adopt an ohmic nature transiently, even in the presence of millimolar internal Mg2+.