A novel type of depolarization-activated K⁺ current in isolated adult rat atrial myocytes

To determine the types of voltage-gated K⁺ channels controlling action potential repolarization in atrial cells, we have characterized the properties of depolarization-activated K⁺ channels in isolated adult rat atrial myocytes using the whole cell patch-clamp recording technique. On membrane depolarization, Ca²⁺-independent outward K⁺ currents in these cells begin to activate at approximately -40 mV. At all test potentials, the currents activate rapidly after a delay, and there is little or no decay of the peak outward current amplitude during brief (100 ms) depolarizations. In addition, the currents show little steady-state inactivation at membrane potentials negative to -60 mV. The currents are blocked effectively by 1-5 mM 4-aminopyridine but are relatively insensitive to extracellular tetraethylammonium at concentrations up to 50 mM. Based on the measured time- and voltage-dependent properties and the pharmacological sensitivity of the currents, we suggest that the depolarization-activated K⁺ channels underlying the macroscopic currents in adult rat atrial myocytes are distinct from those described previously in other myocardial preparations, including adult rat ventricular myocytes. Interestingly, the outward K⁺ currents characterized here in isolated adult rat atrial myocytes are remarkably similar to those of several recently described ''delayed rectifier'' K⁺ channel genes isolated from rat brain cDNA libraries and expressed in Xenopus oocytes, suggesting that similar K⁺ currents are likely present in cells of the mammalian central nervous system.