A further study on constrictor responses of dog coronary arterial strips to acetylcholine under potassium-contracture

Cholinergic constriction in the general circulation and its role in coronary artery spasm

The release of acetylcholine from autonomic nerves in those tissues that receive a cholinergic innervation is widely believed to dilate blood vessels. Exogenously administered acetylcholine in vivo does dilate vascular beds and produce hypotension; however, this latter effect is indirect and probably the result of liberation of endothelium-derived relaxing factor (EDRF) from endothelial cells. Some blood vessels contain a substantial population of medial constrictor receptors for acetylcholine, and the implications of this presence for vascular control systems has been largely ignored, although it needs to be considered. A survey of the evolution of vasomotor control systems indicates that acetylcholine serves principally as an excitatory transmitter to blood vessels. Neurally mediated cholinergic constriction and not dilation is found in fish, amphibians, reptiles, and birds, with responses initiated by medial muscarinic receptors. Acetylcholine constricts many vascular preparations from these lower animals, but some vessels relax, reflecting the emergence of an EDRF responsive to acetylcholine. An examination of cholinergic responses in mammalian vessels reveals that cholinergic (neurogenic) dilation is limited to a very few vascular beds and to only a few species. Both experimental evidence and evolutionary considerations support the likelihood that cholinergic (neural) constriction operates in some vascular regions in mammals and, in particular, in the coronary circulation of some species, including humans. In fact, constriction, and not dilation, may be the dominant vascular response to activation of the cholinergic axis in most mammals, including humans. The complications and contradictions introduced by the simultaneous presence of both EDRF and a cholinergic constrictor innervation involving medial muscarinic receptors are discussed. A variety of evidence is also presented that implicates cholinergic constriction in at least some instances of coronary artery spasm and sudden death.

Responses of isolated and perfused dog coronary arteries to acetylcholine, norepinephrine, KCl, and diltiazem before and after removal of the endothelial cells by saponin

The vascular responses to acetylcholine (ACh), norepinephrine (NE), KCl, and diltiazem were examined before and after removal of endothelial cells by an intraluminal bolus injection of saponin (1 mg) in isolated and perfused dog coronary arteries. Without any precontraction, ACh induced a long-lasting vasodilation in small doses (less than 1 microgram), and an initial brief vasoconstriction was occasionally accompanied in large doses. These vascular responses to ACh were not significantly affected by the pretreatment with propranolol (5 X 10(-6) mol/l). The endothelial removal by intraluminal saponin was confirmed electron microscopically. After 20-60 min of saponin treatment, the responses to drugs were observed and compared with the control. The ACh-induced vasodilation was significantly attenuated by saponin (P less than 0.01), but the ACh-induced vasoconstriction was not affected by it. The vasodilation was blocked by atropine. The NE- and KCl-induced vasoconstrictions and diltiazem-induced vasodilation were not affected by saponin treatment. It is suggested that: (1) ACh produced a vasodilation in the nonpreconstricted condition of dog coronary arteries; (2) the vasodilation caused by ACh is mostly endothelium-dependent, which is considered to be mediated by muscarinic receptors; and (3) the vascular responses to NE, KCl, and diltiazem and the vasoconstriction produced by ACh are not influenced by removal of the endothelium in a relatively large epicardial coronary artery of the dog.