Acetylcholine (ACh) can activate at least five classes of muscarinic receptor (M1-5). Some cells, like atrial cells of the heart, express multiple subtypes of muscarinic receptor and, thus, the cellular response to ACh is the result of integration of the pathways activated by the different receptors. G protein-gated inwardly rectifying potassium channels (GIRKs) are one of the targets for multiple muscarinic receptor pathways. Kobrinsky et al. found that rat atrial cells responded to ACh by activation of a GIRK that exhibited short-term desensitization, leading to a reduction in the amplitude of the K+ current during the ACh stimulation. (M2 and M4 receptors activate the channel through the βγ subunits of the G proteins to which they are coupled). Using pharmacological inhibitors, Kobrinsky et al. showed that the desensitization of the GIRK channels was the result of activation of phospholipase C by M1 (in transfected cells) or M3 receptors (atrial cells) and was not dependent on protein kinase C activity. The authors transfected cells with GIRK channels, muscarinic receptors, and a fusion protein between green fluorescent protein and a pleckstrin homology domain to monitor changes in phosphatidylinositol diphosphate (PIP2) levels in response to ACh. GIRK desensitization correlated with the hydrolysis of PIP2. Using channels with known differences in affinity for PIP2, the authors show that channels that have a lower affinity for PIP2 can be desensitized in response to stimuli that decrease PIP2 levels. The authors propose that changes in PIP2 levels regulate GIRK channel activity by altering the channel's interaction with the activating βγ subunits.
Kobrinsky, E., Mirshahi, T., Zhang, H., Jin, T., and Logothetis, D.E. (2000) receptor-mediated hydrolysis of plasma membrane messenger PIP2 leads to K+-current desensitization. Nature Cell Biol. 2: 507-514. [Online Journal]