Analysis of the polyamine-binding sites in inwardly rectifying K^+ channels.

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We analyzed the Mg^<2+>/polyamine sensitivity of the IRK2 (Kir2.2) channel heterologously expressed in HEK293T cells. IRK2 (Kir2.2) is believed to be the subunit of the cardiac inwardly-rectifying K^+ channel, I_<K1>. The IRK2 channels showed strong inward rectification in the cell-attached configuration of the patch-clamp method. However, they gradually lost the inward rectification in the inside-out patch membranes whose intracellular side was continuously perfused with Mg^<2+>-free solution. Mg^<2+> and spermine spplied to the internal side of the patch membrane suppressed the outward channel currents at the potential 40 mV positive to the potassium equilibration potential (E_K) with the IC_<50> of 10muM and 3 nM,respectively. Because millimolar and submillimolar Mg^<2+> and polyamine are known to exist in cytosol of most cell types, the Mg^<2+>/Polyamine block is likely to underlie the inward rectification of IRK2 channels in intact cells. Mg^<2+> caused the instantaneous rectification by inducing the subconducting level of the channel, while spermine time-dependently suppressed the open probability of the channel at potentials positive to E_K. When Mg^<2+> was further applied to the channel in the presence of spermine, the inward rectification of the channel was paradoxically attenuated compared with that in the presence of spermine alone. This phenomenon was well explained by the model in which Mg^<2+> and spermine compete with each other through binding the same binding site (s) on the channel. These data (1) indicate that the physiological inward rectification of the cardiac I_<K1> channel is determined through such competitive binding of intracellular Mg^<2+>/polyamine to the channel and (2) raise a possibility that artificial polyvalent cations introduced into cardiocytes might be utilized to pharmacologically modulate the strong inward rectification of the channels induced by intracellular polyamines.