Acetylcholine-induced Phosphatidylinositol 4,5-Bisphosphate Depletion Does Not Cause Short-term Desensitization of G Protein-gated Inwardly Rectifying K+ Current in Mouse Atrial Myocytes

Depletion of phosphatidylinositol 4,5-bisphosphate (PIP2) induced by phenylephrine or endothelin causes the inhibition of acetylcholine-activated K+ current (IKACh) in atrial myocytes. In the present study, we have investigated the hypothesis that muscarinic receptor induced PIP2 depletion also causes inhibition of IKACh, resulting in desensitization. We confirmed the expression of Gq-coupled muscarinic receptors in mouse atrial myocytes using reverse transcriptase-polymerase chain reaction. The involvement of M1 and M3receptors in desensitization is examined using specific antagonists, 4-DAMP and pirenzepine, but they significantly reduced peak IKACh, implying nonspecific M2 blockade. When ACh-induced phosphoinositide depletion was specifically inhibited using PLCβ1 knock-out mice, the extent of desensitization during 4 min was 47.5 ± 3.2%, which was not different from that in wild type (46.8 ± 2.1%). Phenylephrine-induced phosphoinositide hydrolysis and phenylephrine-induced inhibition of IKACh were not affected by PLCβ1 knock-out. To facilitate PIP2depletion, replenishment of PIP2 was blocked by wortmannin. Wortmannin did not affect the desensitization and the recovery from desensitization. These results suggest that PIP2 depletion by acetylcholine does not contribute to short-term desensitization of IKACh. The differential regulation of IKACh by different phospholipase C-linked receptors may imply that receptor co-localization is required for PIP2 to act as a signaling molecule.