Depletion of phosphatidylinositol 4,5-bisphosphate by activation of phospholipase C-coupled receptors causes slow inhibition but not desensitization of G protein-gated inward rectifier K+ current in atrial myocytes

G protein-gated inwardly rectifier K+ current in atrial myocytes (I-K(ACh)) upon stimulation with aeetylcholine (ACh) shows a fast desensitizing component (t(1/2) similar to 5 s), After washout of ACh, I-K(ACh) recovers from fast desensitization within < 30 s, A recent hypothesis suggests that fast desensitization is caused by depletion of phosphatidylinositol 4,5-bisphosphate (PtIns( 4,5)P-2), resulting from costimulation of phospholipase C (PLC)-coupled M-3 receptors (M(3)AChR). The effects of stimulating two established PLC-coupled receptors, cy-adrenergic and endothelin (ETA), on I-K(ACh) were studied in rat atrial myocytes. Stimulation of these receptors caused activation of I-K(ACh) and inhibition of the M(2)AChR-activated current. In myocytes loaded with GTP gammaS (guanosine 5 ' -3-O-(thio) triphosphate), causing stable activation of I-K(ACh), inhibition via alpha -agonists and ET-1 was studied in isolation. Stimulation of either type of receptor under this condition, via G(q/11), caused a slow inhibition (t(1/2) similar to 50 s) by about 10%. No comparable effect on GTP gammaS-activated I-K(ACh) was induced by ACh, suggesting that PLC-coupled M(3)AChRs are not functionally expressed in rat myocytes, which was supported by the finding that M(3)AChR transcripts were not detected by reverse transcriptase-polymerase chain reaction in identified atrial myocytes. Supplementing the pipette solution with PtIns(4,5)P-2 significantly reduced inhibition of I-K(ACh) but had no effect on fast desensitization. From these data it is concluded that stimulation of PLC-coupled receptors causes slow inhibition of I-K(ACh) by depletion of PtIns(4,5)P-2, whereas fast desensitization of I-K(ACh) is not related to PtIns(4,5)P-2 depletion. As muscarinic stimulation by ACh does not, exert inhibition of I-K(ACh) comparable to stimulation of alpha (1)- and ETA receptors, expression of functional. PLC-coupled muscarinic receptors in rat atrial myocytes is unlikely.