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 (I_KACh) in atrial myocytes. In the present study, we have investigated the hypothesis that muscarinic receptor induced PIP₂ depletion also causes inhibition of I_KACh, resulting in desensitization. We confirmed the expression of G_q-coupled muscarinic receptors in mouse atrial myocytes using reverse transcriptase-polymerase chain reaction. The involvement of M₁ and M₃ receptors in desensitization is examined using specific antagonists, 4-DAMP and pirenzepine, but they significantly reduced peak I_KACh, 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 I_KACh were not affected by PLCβ1 knock-out. To facilitate PIP₂ depletion, 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 I_KACh. The differential regulation of I_KACh by different phospholipase C-linked receptors may imply that receptor co-localization is required for PIP₂ to act as a signaling molecule.