Differential coupling of m1, m3, and m5 muscarinic receptors to activation of neuronal nitric oxide synthase

The selectivity of coupling of m1, m3, and m5 muscarinic receptors to activation of the neuronal type of nitric oxide synthase was investigated. Stimulation with the agonist carbachol of all three receptor subtypes expressed in Chinese hamster ovary cells resulted in a rapid and transient activation of the enzyme, as measured by stimulation of guanylate cyclase in reporter neuroblastoma cells. Carbachol was more potent and efficacious at m5 receptors than at the other two receptor subtypes. Stimulation of all three muscarinic receptors resulted in an increased concentration of intracellular calcium, with a time course that preceded activation of nitric oxide synthase. At each receptor subtype, there was a close relationship between the magnitude of the maximal calcium response and that of enzyme activation.