Comparison of vascular response of different locations of the gastrointestinal tract to isoproterenol and nifedipine.

In the present study we have compared the effect of intravenous infusion of a calcium channel blocker, nifedipine (1.0 micrograms.kg-1.min-1 for 20 min), with that of isoproterenol (0.1 micrograms.kg-1.min-1 for 20 min) on the hemodynamic parameters and the vascular response of different locations and tissue layers of the gastrointestinal tract. Heart rate increased with isoproterenol but not with nifedipine. Both agents caused a similar increase in cardiac output and a similar fall in mean arterial pressure. After 20 min infusion, nifedipine increased the blood flow of the axillary artery, but isoproterenol had no such effect. Isoproterenol caused vasodilation of the mucosa in the antrum but not in the fundus and the body of the stomach or in the duodenum, jejunum, mid small intestine, ileum, and colon. The mucosal effect of nifedipine was similar, except that it also caused vasodilation in the small bowel and in the ascending colon. Nifedipine caused vasodilation of the muscularis throughout the gastrointestinal tract, but isoproterenol had no such effect. These differences are discussed in relation to the mechanism of action of these two vasodilators. It is suggested that the vascular response of different locations and tissue layers of the gastrointestinal tract to vasodilators is locally regulated by a variety of mechanisms may include beta- and alpha-receptor density and (or) sensitivity, angiotensin II activity, and metabolic need of the tissues.