Activation of neuronal nitric oxide synthase by M2 muscarinic receptors associated with a small increase in intracellular calcium.

We investigated the coupling of the M2 muscarinic acetylcholine receptors expressed in Chinese hamster ovary cells to activation of neuronal nitric oxide (NO) synthase. Stimulation of guanylate cyclase activity in detector neuroblastoma cells was used as an indirect measure of the generation of NO in Chinese hamster ovary cells. The muscarinic agonist carbachol induced marked time- and concentration-dependent enhancement of the activity of NO synthase. Activation of neuronal NO synthase by M2 muscarinic receptors was associated with a small increase in the concentration of intracellular Ca2+. These data suggest the presence of alternate mechanisms of activation of neuronal NO synthase which might be operative in the absence of large changes in the concentration of cellular Ca2+. These findings help to understand the mechanisms of activation of NO synthase.