GIRK (G protein-activated inward-rectifying K+ channel) channels, important regulators of membrane excitability in the heart and in the central nervous, are activated by interaction with βγ subunits from heterotrimeric G proteins upon receptor stimulation. For activation interaction of the channel with phosphatidylinositol 4,5-bisphosphate (PtIns(4,5)P2) is conditional. Previous studies have provided evidence that in myocytes PtIns(4,5)P2 levels relevant to GIRK channel regulation are under regulatory control of receptors activating phospholipase C. In the present study a phosphatidyl-4-phosphate 5-kinase was expressed in atrial myocytes by transient transfection. This did not affect basal properties of GIRK current activated by acetylcholine via M2 receptors but completely abolished inhibition of guanosine triphosphate-γ-S activated current by endothelin-1 or α-adrenergic agonists. We conclude that though PtIns(4,5)P2 is conditional for channel gating, its normal level in the membrane is not limiting basal function of GIRK channels. Moreover, our data provide further evidence for a regulation of GIRK channels by α1A receptors and endothelin-A receptors, endogenously expressed in atrial myocytes, via depletion of PtIns(4,5)P2.
