DIFFERENTIAL COUPLING OF M1, M2 AND M3-MUSCARINIC-RECEPTOR SUBTYPES TO INOSITOL 1,4,5-TRISPHASPHATE AND ADENOSINE 3',5'-CYCLIC-MONOPHOSPHATE ACCUMULATION IN CHINESE-HAMSTER OVARY CELLS

Agonist-stimulated accumulation of inositol 1,4,5-trisphosphate and ad enosine 3',5'-cyclic monophosphate (cAMP) were measured in Chinese ham ster ovary (CHO) cells expressing mi (CHO-m1), m2 (CHO-m2) or m3 (CHO- m3) muscarinic receptors. At similar levels of expression (approximate ly 1000 fmol of receptor per mg of protein), mi and m3 muscarinic rece ptors mediated similar carbachol-stimulated, biphasic accumulation of inositol-1,4,5-trisphosphate in intact cells and similar release of pr eloaded Ca-45(++) from permeabilized cells. However, CHO-ml cells prod uced a 4-fold greater agonist-stimulated accumulation of cAMP compared with CHO-m3 cells, in a pertussis toxin-insensitive manner. CHO-m2 ce lls (expressing approximately 100 fmol of receptor per mg of protein) coupled to the inhibition of adenylyl cyclase in a pertussis toxin-sen sitive manner. However, after pertussis toxin pretreatment, agonist st imulation mediated a 50% potentiation of forskolin-stimulated cAMP acc umulation. Muscarinic m1, m2 and m3 receptor-mediated stimulation of c AMP accumulation, correlated with the apparent binding affinity of car bachol for these receptors, suggesting a lack of an apparent receptor reserve for this response. Reducing the level of m3 muscarinic recepto rs by approximately 50% resulted in no detectable stimulation of cAMP accumulation. The results suggest that mi and m3 muscarinic receptors, expressed at similar levels in CHO cells, couple to the activation of phospholipase C with similar efficiency. However, mi muscarinic recep tors couple with greater efficiency to the stimulation of adenylyl cyc lase compared with m3 muscarinic receptors. Muscarinic m1, m2 and m3 r eceptor-mediated cAMP accumulation in CHO cells does not appear to be a consequence of phospholipase C activation.