The role of angiotensin in the intestinal vascular response to hypotension in a canine model.
It was previously shown that the vasoconstrictory response to hypotension was similar in the mucosa of the small bowel and the colon but was significantly higher in the muscularis of the latter than that of the former. To understand the mechanism of this differential response of the muscularis of the small bowel and the colon, the present study investigated the effect of an angiotensin II inhibitor (saralasin) on the hypotension-induced vasoconstriction of the mucosa and the muscularis of these two locations of the gastrointestinal tract. Dogs were used. Hypotension was induced by hemorrhage to reduce blood pressure by 40 mm Hg. Blood flow was measured by 15-microns radiolabeled microspheres. Saralasin was infused intravenously for 20 minutes at a rate of 0.05 mg.kg-1 bolus followed by 1 microgram.kg-1.min-1. Saralasin had no effect on the basal blood flow of the mucosa or the muscularis of the small bowel or on the hypotension-induced vasoconstriction of these two layers of the small bowel. In contrast, saralasin decreased blood flow to the mucosa (-28%; P less than 0.001) and increased blood flow to the muscularis (+140%; P less than 0.001) of the colon under basal conditions and also reduced the hypotension-induced vasoconstriction of the colonic muscularis (P less than 0.01). These and supplementary data indicate that there is a difference between the small bowel and the colon in local activity of vascular angiotensin system and that this system is most active in the colonic muscularis where it plays a significant role in the vasoconstrictory response to hypotension.