PRIMARY STRUCTURE AND FUNCTIONAL EXPRESSION OF AN INWARD RECTIFIER POTASSIUM CHANNEL AND A G-PROTEIN-COUPLED MUSCARINIC POTASSIUM CHANNEL

A cDNA encoding an inward rectifier K+ channel (IRK1) was isolated by expression cloning using Xenopus oocytes. The IRK1 channel activated rapidly when the membrane was hyperpolarized below the potassium equilibrium potential (E(K)) Its mRNA was detected in the mouse brain and in the heart and skeletal muscle. The IRK1 channel contains only two putative transmembrane segments and corresponds to the inner core structure (S5, H5, and S6) of voltage-gated outward Kf channels with six transmembrane regions. Based on sequence homology with IRK1, cDNA for a G-protein-coupled inwardly rectifying Kt channel (GIRK1) was isolated. The GIRK1 channel was activated by a patch-delimited interaction with G-protein. Its single-channel conductance and kinetic properties resemble those of the cardiac muscarinic K+ channel. Its mRNA was detected not only in the heart (atrium > ventricle) but also in the brain.