M₃ and M₁ receptors in cerebral arterioles in vivo: Evidence for downregulated or ineffective M₁ when endothelium is intact

Pial arterioles of mice were monitored by intravital television microscopy. Responses to acetylcholine (ACh) and to McN-A-343, a selective M₁ activator, were monitored before and after endothelial injury by a laser- dye technique in the presence or absence of different reference antagonists to the M₁, M₂, and M₃ muscarinic receptors. ACh and McN-A-343 produced endothelium-dependent dilation that was blocked by 4-diphenyl-N-methyl- piperidine methiodide, an inhibitor of the M₃ receptor. Dilations were not augmented by blocking the M₁ receptor with pirenzepine. Endothelial injury unmasked a constricting effect of both ACh and McN-A-343. The constricting effect was blocked by pirenzepine. The results support the literature suggesting an action of McN-A-343 at more than one site. They indicate that ACh causes endothelium-dependent relaxation via the M₃ receptor and directly constricts vascular smooth muscle via the M₁ receptor. There does not appear to be continuous competition between endothelium-dependent relaxation and endothelium-independent constriction. Rather the M₁ receptors mediating constriction in vascular smooth muscle appear downregulated and/or uncoupled from the contractile machinery as long as the overlying endothelium synthesizes/releases the mediator (endothelium-derived relaxing factor) of ACh's dilating action.