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Mechanisms for the time-dependent decay of inward currents through cloned Kir2.1 channels expressed in Xenopus oocytes

Ru-Chi Shieh

1. 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, V_m)- 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 V_m 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 V_m-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 V_m-dependent processes controlling the inactivation of the Kir2.1 channels.

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