Changes in intestinal vascular diameter during norepinephrine vasoconstrictor escape.

The hypothesis that escape from norepinephrine-induced vasoconstriction in the intestine is due to relaxation of initially constricted vessels was tested in 18 anesthetized cats. Intestinal blood flow was measured by an electro magnetic probe on the superior mesenteric artery. Intestinal submucosal and muscle arterioles and small mesenteric arteries were studied by in vivo microscopy with an image-splitting technic. Continuous recordings of mesenteric flow and vessel diameter were made during the infusion into the superior mesenteric artery of norepinephrine (NE) at a rate of 1-2 mug/min for 3 min. Mesenteric flow decreased soon after the NE infusion began but then escaped. Microscopically, arterial constriction and escape were noted in submucosal, muscle, and mesenteric vessels with a time course similar to that for flow. Arteriovenous anastomoses were not seen. These findings support the hypothesis that escape from Ne-induced vasoconstriction is due to relaxation of initially constricted vessels.