Abstract

Agonist-stimulated accumulation of inositol 1,4,5-trisphosphate and adenosine 3',5'-cyclic monophosphate (cAMP) were measured in Chinese hamster ovary (CHO) cells expressing m1 (CHO-m1), m2 (CHO-m2) or m3 (CHO-m3) muscarinic receptors. At similar levels of expression (approximately 1000 fmol of receptor per mg of protein), m1 and m3 muscarinic receptors mediated similar carbachol-stimulated, biphasic accumulation of inositol-1,4,5-trisphosphate in intact cells and similar release of preloaded 45Ca++ from permeabilized cells. However, CHO-m1 cells produced a 4-fold greater agonist-stimulated accumulation of cAMP compared with CHO-m3 cells, in a pertussis toxin-insensitive manner. CHO-m2 cells (expressing approximately 100 fmol of receptor per mg of protein) coupled to the inhibition of adenylyl cyclase in a pertussis toxin-sensitive manner. However, after pertussis toxin pretreatment, agonist stimulation mediated a 50% potentiation of forskolin-stimulated cAMP accumulation. Muscarinic m1, m2 and m3 receptor-mediated stimulation of cAMP accumulation, correlated with the apparent binding affinity of carbachol for these receptors, suggesting a lack of an apparent receptor reserve for this response. Reducing the level of m3 muscarinic receptors by approximately 50% resulted in no detectable stimulation of cAMP accumulation. The results suggest that m1 and m3 muscarinic receptors, expressed at similar levels in CHO cells, couple to the activation of phospholipase C with similar efficiency. However, m1 muscarinic receptors couple with greater efficiency to the stimulation of adenylyl cyclase compared with m3 muscarinic receptors. Muscarinic m1, m2 and m3 receptor-mediated cAMP accumulation in CHO cells does not appear to be a consequence of phospholipase C activation.