Vol. 56, Issue 4, 813-823, October 1999

Cross Talk Between m3-Muscarinic and ₂-Adrenergic Receptors at the Level of Receptor Phosphorylation and Desensitization

David C. Budd, R. A. John Challiss, Kenneth W. Young, and Andrew B. Tobin

Department of Cell Physiology and Pharmacology, University of Leicester, Leicester, United Kingdom

In this study we investigated cross talk between m3-muscarinic and ₂-adrenergic receptors coexpressed in Chinese hamster ovary (CHO-m3/₂) cells, focusing on two possible mechanisms of regulation. The first mechanism is based on recent in vitro studies demonstrating that G protein-coupled receptor kinase (GRK) activity, the protein kinase responsible for ₂-adrenergic receptor homologous phosphorylation and desensitization, may be regulated by calcium/calmodulin and membrane phosphatidylinositol 4,5-bisphosphate. Stimulation of the phospholipase C signaling pathway via m3-muscarinic receptors in CHO-m3/₂ cells increased intracellular free calcium by ~10 fold and membrane phosphatidylinositol 4,5-bisphosphate levels decreased by ~74%. However, despite these changes the ability of endogenous kinases, possibly the GRKs, to phosphorylate the ₂-adrenergic receptor was not altered. The second mechanism investigated involves a direct heterologous phosphorylation of the ₂-adrenergic receptor after muscarinic receptor stimulation. Activation of m3-muscarinic receptors did mediate heterologous phosphorylation of ₂-adrenergic receptors in a GRK-independent fashion, via protein kinase C. Heterologous ₂-adrenergic receptor phosphorylation correlated with receptor desensitization as measured by a loss in guanine-nucleotide sensitive-high affinity agonist binding and reduction in maximal cAMP response. This receptor cross talk may have a profound physiological importance in a wide variety of cell types, for example smooth muscle, where these two receptors are known to be coexpressed.