Overexpression of monomeric and multimeric GIRK4 subunits in rat atrial myocytes removes fast desensitization and reduces inward rectification of muscarinic K+ current (I-K(ACh)) - Evidence for functional homomeric GIRK4 channels

K+ channels composed of G-protein-coupled inwardly rectifying K+ channel (G IRK) (Kir3.0) subunits are ex. pressed in cardiac, neuronal, and various en docrine tissues. They are involved in inhibiting excitability and contribut e to regulating important physiological functions such as cardiac frequency and secretion of hormones. The functional cardiac (K-(ACh)) channel activa ted by G(i)/G(o)-coupled receptors such as muscarinic M-2 or purinergic A., receptors is supposed to be composed of the subunits GIRK1 and GIRK4 in a heterotetrameric (2:2) fashion. In the present study, we have manipulated t he subunit composition of the K(ACh) channels in cultured atrial myocytes f rom hearts of adult rats by transient transfection of vectors encoding for GIRK1 or GIRK4 subunits or GIRK4 concatemeric constructs and investigated t he effects on properties of macroscopic IK(Ach). Transfection with a GIRK1 vector did not cause any measurable effect on properties Of IK(ACh), wherea s transfection with a GIRK4 vector resulted in a complete loss in desensiti zation, a reduction of inward rectification, and a slowing of activation. T ransfection of myocytes with a construct encoding for a concatemeric GIRK4( 2) subunit had similar effects on desensitization and inward rectification. Following transfection of a tetrameric construct (GIRK4,), these changes i n properties Of IK(ACh) were still observed but were less pronounced. Heter ologous expression in Chinese hamster ovary cells and human embryonic kidne y 293 cells of monomeric, dimeric, and tetrameric GIRK4 resulted in robust currents activated by co-expressed A(1) and M-2 receptors, respectively. Th ese data provide strong evidence that homomeric GIRK4 complexes form functi onal G(beta gamma) gated ion channels and that kinetic properties of GIRK c hannels, such as activation rate, desensitization, and inward rectification , depend on subunit composition.