INTESTINAL MICROVASCULAR RESPONSIVENESS TO NOREPINEPHRINE IN CHRONIC PORTAL-HYPERTENSION

Effects of chronic prehepatic portal hypertension on intestinal microvascular sensitivity to norepinephrine (NE) were studied. Normal and portal hypertensive rats were anesthetized, and the intestine was prepared for in vivo microscopic observation. The preparation was transferred to a video microscope and a first-, second-, or third-order submucosal arteriole (i.e., 1A, 2A, or 3A, respectively) selected for study. Microvascular diameter and arteriolar erythrocyte velocity were measured on-line, and arteriolar blood flow was subsequently calculated as the product of velocity and vessel cross-sectional area. Once steady-state conditions were reached, the preparation was exposed to incremental doses of NE and microvessel responses were recorded. Cumulative log dose-response curves relating the change in arteriolar blood flow and vessel diameter to NE concentration were constructed for each group of arterioles and the ED50 for maximal response obtained from each dose-response relationship. NE ED50 for 1A blood flow was significantly higher in portal hypertensive rats (2.57 +/- 0.25-mu-M) compared with control rats (1.48 +/- 0.19-mu-M). Analysis of the diameter responses of 1A, 2A, and 3A indicated that the loss of vascular NE sensitivity in chronic portal hypertension was localized to the terminal submucosal arterioles (2a and 3A). No differences in the diameter response of 1A were observed between normal and portal hypertensive rats. Separate experiments were conducted to test if glucagon, a known mediator of the hyperdynamic intestinal circulation in portal hypertension, could acutely alter NE responsiveness in normal animals. Glucagon significantly increased intestinal blood flow (15%) by selectively dilating 2A and 3A; vascular responsiveness to a single concentration of NE was significantly attenuated by glucagon at all levels of the microcirculation studied. The ability of glucagon to acutely modulate vasoconstrictor effectiveness was not unique and could be mimicked by equipotent doses of nitroprusside. Results of these studies suggest that vasodilators modulate vascular NE responsiveness in chronic portal hypertension.