Dual Vasoconstrictor and Vasodilator Innervation

The innervation of the extrinsic uterine arterial supply of the guinea pig has been studied using an isolated perfused preparation. The preparation was normally at minimal tone and responded to periarterial stimulation by a vasoconstriction which was mimicked by norepinephrine and abolished by bretylium. Constrictor responses both to stimulation and to norepinephrine were similar in both pregnant and non-pregnant states. In pregnant preparations, raising the tone of the vessels with norepinephrine revealed a powerful dilator response to periarterial stimulation. This response was mimicked by acetylcholine, potentiated by anticholinesterases, reduced by hyoscine, and abolished by local anesthetic treatment. In contrast, virgin preparations showed only a weak dilator response to stimulation, while the response to acetylcholine even at high concentrations was usually negligible. Histochemical examination of the vessels revealed a dense plexus of fine nerve fibers exhibiting high acetylcholinesterase activity along the main uterine artery. The fibers lay in close apposition to the vascular muscle layer. Nerve fibers exhibited fluorescence for catecholamines were also abundant, but were distributed along the secondary as well as the main arteries. It is concluded that the uterine arterial supply of the guinea pig is innervated by both adrenergic constrictor and cholinergic dilator fibers. The dilator fibers appear to be functional only during pregnancy. The possibility of a noncholinergic dilator innervation is also discussed. ADDITIONAL