A 3,4-DIAMINOPYRIDINE-INSENSITIVE, CA2-INDEPENDENT TRANSIENT OUTWARD K+ CURRENT IN CARDIAC VENTRICULAR MYOCYTES()

A previously unrecognized current that initially is not present and re quires at least 25 min of intracellular access to develop can be found in similar to 75% of cardiac myocytes isolated from cat ventricle wit hin 90 min after intracellular access is obtained with conventional su ction patch pipette electrodes. We refer to this patch-duration-depend ent (PDD) current as I-K(PDD). I-K(PDD) can be elicited with depolariz ing test steps (V-t) ranging between -40 and +60 mV applied after a hy perpolarizing conditioning step to -140 mV for 200 ms from a holding p otential of -40 mV. It shows an ohmic voltage dependence and appears t o be an essentially pure K+ current. At V-t = 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 decayi ng exponentials (tau(f) = 3.30 +/- 0.51 ms and tau(s) = 24.8 +/- 5.6 m s; n = 5). The voltage dependence of the steady-state inactivation can be fit to a single exponential Boltzmann distribution with a slope fa ctor of 8.97 mV, and the voltage at which 50% of the channels are inac tivated is -78 mV. The current can be blocked by 0.2 mill Ba2+ extrace llularly 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 I-K(PDD) and any currently documented cardiac K+ current suggests that I-K(PDD) is either a previously undescribed K+ current or a modification of I-K1 t hat makes it adopt an ohmic nature transiently, even in the presence o f millimolar internal Mg2+.