The muscarinic acetylcholine receptor family is composed of five subtypes denoted M
1 to M
5.
1 In most vascular beds, activation of muscarinic receptors induces powerful vasodilation via the release of vasorelaxing agents from the endothelium.
2,3 Previous studies reported that the M
3 receptor subtype mediates cholinergic vasodilation in the choroid of pigeons and in ocular blood vessels of mice.
4–6 Based on these findings, the M
3 receptor may represent a potential pharmacologic target to modulate blood flow in diseases associated with disturbances of ocular perfusion, such as diabetic retinopathy, age-dependent macular degeneration, nonarteritic anterior ischemic optic neuropathy (NAION), and glaucoma.
7–10 However, in various nonocular vascular beds, cholinergic agonists were shown to exert only limited vasodilation effects or even to induce vasoconstriction when applied in pathologic conditions associated with endothelial dysfunction.
11–17 These effects have been attributed to activation of muscarinic acetylcholine receptors localized on vascular smooth muscle and to an influence of vasoconstrictor agents released from vascular endothelium.
18,19 Because several diseases associated with impaired ocular perfusion also have been associated with endothelial dysfunction,
20,21 one may argue that nonsubtype-selective muscarinic receptor agonists may exert a limited vasodilation effect or even to cause vasoconstriction when applied in these diseases. Subtype-selective muscarinic receptor ligands might be useful to circumvent this problem. Thus, it is important to define the functional role of individual muscarinic receptor subtypes in conditions of endothelial damage or dysfunction. Although it has been shown that the M
3 acetylcholine receptor subtype mediates vasodilation in endothelium-intact ocular blood vessels,
4–6 to our knowledge there are at present no studies reporting on its functional role in ocular blood vessels with damaged or dysfunctional endothelium. Hence, the goal of the present study was to test the hypothesis that the M
3 muscarinic acetylcholine receptor subtype mediates responses in ophthalmic arteries after endothelial removal. We used real-time PCR to determine mRNA expression of all five muscarinic receptor subtypes in murine ophthalmic arteries with and without endothelium. Because the selectivity of agonists and antagonists for individual muscarinic receptor subtypes was shown to be limited,
1,22 we used muscarinic receptor knockout mice to perform functional studies on isolated ophthalmic arteries.