We have previously reported a depolarization-activated 4-aminopyridine-resistant transient outward K(+) current with inward rectification (I (to.ir)) in canine and guinea pig cardiac myocytes. However, molecular identity of this current is not clear. The present study was designed to investigate whether Kir2.1 channel carries this current in stably transfected human embryonic kidney (HEK) 293 cells using whole-cell patch-clamp technique. It was found that HEK 293 cells stably expressing human Kir2.1 gene had a transient outward current elicited by voltage steps positive to the membrane potential (around -70 mV). The current exhibited a current-voltage relationship with intermediate inward rectification and showed time-dependent inactivation and rapid recovery from inactivation. The half potential (V (0.5)) of availability of the current was -49.4 +/- 2.1 mV at 5 mM K(+) in bath solution. Action potential waveform clamp revealed two components of outward currents; one was immediately elicited and then rapidly inactivated during depolarization, and another was slowly activated during repolarization of action potential. These properties were similar to those of I (to.ir) observed previously in native cardiac myocytes. Interestingly, inactivation of the I (to.ir) was strongly slowed by increasing intracellular free Mg(2+) (Mg(2+) ( i ), from 0.03 to 1.0, 4.0, and 8.0 mM). The component elicited by action potential depolarization increased with the elevation of Mg(2+) ( i ). Inclusion of spermine (100 muM) in the pipette solution remarkably inhibited both the I (to.ir) and steady-state current. These results demonstrate that the Mg(2+) ( i )-dependent current carried by Kir2.1 likely is the molecular identity of I (to.ir) observed previously in cardiac myocytes.