Muscarinic cholinergic signaling in cardiac myocytes: dynamic targeting of M2AChR to sarcolemmal caveolae and eNOS activation.

The isoform of nitric oxide synthase (eNOS or NOS3) originally described in endothelial cells is also expressed in a number of other cell types, including cardiac myocytes. eNOS is activated in both atrial and ventricular myocytes, including specialized pacemaker cells, by M2AChR agonists, among other stimuli. In cardiac myocytes, as in endothelial cells, eNOS is targeted to sarcolemmal caveolae, due to both co-translational myristoylation and later palmitoylation, and by the presence of a caveolin binding domain in eNOS which interacts with the caveolin scaffolding domain. In the absence of ligand, the M2AChR is not associated with caveolar microdomains, but translates into caveolae upon agonist (but not antagonist) binding. Finally, the role of M2AChR-induced eNOS activation in regulating I(Ca-L) via activation of guanylyl cyclase has been confirmed in ventricular myocytes of mice that lack functional eNOS (i.e., eNOS(null)).