Cholinergic vasodilation of intracerebral arterioles in rats

Much morphological and physiological evidence indicates that cholinergic mechanisms play a significant role in the control of cerebral blood flow. Despite in situ data suggesting that an intrinsic cholinergic mechanism produces vasodilation in the intracerebral microcirculation, there is no direct information on the effect of acetylcholine (ACh) on intracerebral arterioles. We investigated cholinergic mechanisms in isolated perfused intracerebral arterioles from pentobarbital sodium-anesthetized Sprague-Dawley rats. In arterioles with resting diameters of 46.8 ± 6.6 μm (mean ± SE) ACh produced no significant dilation at pH 7.30. At pH 7.60, however, a significant dose-dependent dilation to a maximum of 119.0 ± 1.0% of control diameter was observed. Carbachol, a long-acting cholinergic agonist, similarly failed to dilate vessels at pH 7.30 but significantly dilated vessels at pH 7.60. Prostaglandin F(2a) produced a maximum contraction to 68.3 ± 2.7% of control diameter (n = 8). ACh at concentrations of 10^-4 and 2 x 10^-4 M induced a significant dilation of this prostaglandin-induced contraction. In vessels similarly preconstricted with serotonin, 10^-4 M ACh produced significant dilation. Atropine, having no effect on vessel diameter when administered alone, blocked cholinergic vasodilation of intracerebral arterioles at pH 7.60. Attempts at endothelial removal, although successful in eliminating endothelial cells from the preparation, significantly impaired smooth muscle contractility. ACh has no significant effect on the spontaneous cerebrovascular tone in this preparation, but in vessels preconstricted by a variety of means it produced vasodilation mediated by atropine sensitive receptors.