Functional responses of cloned muscarinic receptors expressed in CHO-K1 cells

The physiological responses to activation of the m5 muscarinic acetylcholine receptor were compared with those of m3 and m4 in transformed Chinese hamster ovary cells, using patch-clamp electrophysiological and biochemical techniques. Stimulation of the m5 receptor induced increases in both a calcium-dependent potassium conductance and phosphoinositide (PI) metabolism of similar magnitude to those activated by m3. Raising of intracellular calcium or injection of inositol-1,4,5-trisphosphate mimicked the activation of the calcium-dependent potassium conductance by both of these receptors. Although similar regarding these responses, the m3 and m5 receptors induced different cAMP responses. Stimulation of m5 receptors induced a 2-fold increase in cAMP levels, whereas m3 induced a 20-fold increase. These cAMP responses required greater than 100-fold more agonist than the PI responses, and both PI and cAMP responses were insensitive to pertussis toxin. Stimulation of m4 receptors caused little increase in PI metabolism and no electrophysiological effects. Stimulation of m4 receptors with low concentrations of agonist decreased cAMP levels, but at high agonist concentrations cAMP levels were elevated. After treatment with pertussis toxin, the decrease in cAMP levels induced by m4 was blocked and a marked increase in cAMP levels, comparable to those observed for m3 receptors, was uncovered at higher doses. The data indicate that each of the receptors has distinct functional properties.