The decay of inward currents was characterized using the giant patch-clamp technique in the cloned inward rectifier K+ channels Kir2.1 expressed in Xenopus laevis oocytes. 2. The degree of decay was increased by strong hyperpolarization and reduced by increases in external [K+]. This voltage (membrane potential, Vm)- and K+-dependent decay is referred to as inactivation. The dissociation constant for the protective effects of external K+ ions against inactivation was about 5 mM and was not Vm dependent. 3. Internal K+ ions also showed mildly protective effects against inactivation when external K+ sites were not saturated. Results from variations in [K+] suggest that the hyperpolarization-induced inactivation of the Kir2.1 channels is not dependent on the driving force for K+ ions. 4. In the mutant which demonstrates higher external K+ affinity, the degree of inactivation was reduced. These results suggest that binding of K+ ions in the external channel pore mouth stabilizes channel opening. 5. Internal Mg2+ and polyamines induced time-dependent decay of inward currents in a dose-dependent but Vm-independent manner between -150 and -60 mV. The order of potency for Mg2+- and polyamine-induced decay was different from that for inward rectification. Furthermore, mutations with reduced inward rectification did not show parallel reduction of Mg2+- and polyamine-induced decay. These results suggest that the effects of internal Mg2+ and polyamines on Kir2.1 channels involve different binding sites. 6. This study provides evidence for Vm-dependent processes controlling the inactivation of the Kir2.1 channels.
