TRANSIT-TIME HETEROGENEITY IN CANINE SMALL-INTESTINE - SIGNIFICANCE FOR OXYGEN-TRANSPORT

We previously found that local O-2 extraction efficacy in isolated pum p-perfused intestine was enhanced when systemic reflex vasoconstrictio n was stimulated by hypovolemia (Samsel, R.W., and P.T. Schumacker. 19 94. J. Appl. Physiol. 77: 2291-2298). The microvascular mechanism unde rlying this beneficial effect could involve a redistribution of now be tween mucosa and serosa, or an adjustment in the heterogeneity of perf usion within those regions. We measured regional blood flows and distr ibutions of now and capillary erythrocyte transit times in two segment s of small intestine in anesthetized dogs (n = 10). Each vascularly is olated segment of intestine was pump-perfused under high now (O-2 supp ly-independent VO2) and low flow (O-2 supply-dependent) conditions. Du ring the first gut segment, the animal was kept normovolemic using i.v . fluids to minimize reflex vasoconstriction. During the second, the a nimal was hemorrhaged to augment vasoconstriction (n = 7), or kept nor movolemic to control for the effects of time (n = 3). Blood flow distr ibutions were measured using 15 mu m radiolabeled microspheres. Tissue blood volume was measured using Tc-99m-labeled red blood cells. Capil lary volume was determined as the product of tissue blood volume and t he histologically derived fraction of vascular volume in the capillari es. Transit times were calculated as the ratio of capillary volume to flow. Each gut segment was fixed and sectioned into 350 similar to 100 mg tissue pieces for analysis. Data revealed significant spatial hete rogeneity of blood flow and capillary transit times in both mucosa and muscularis, with relative dispersions (SD/Mean) ranging from 23 to 97 %, Hypovolemia caused an increase in flow heterogeneity in muscularis at both high and low Bow states, and in mucosa under high flow conditi ons. However, hypovolemia also elicited changes in capillary volume, s uch that transit time heterogeneity remained unchanged. Augmentation o f vasoconstrictor tone caused a redistribution of now toward mucosa (P < 0.003) under high and low now conditions. This redistribution corre lated with the improvements in O-2 extraction ratio (P = 0.022). Thus, the improvement in gut O-2 extraction efficacy seen with increased va soconstriction may be explained mostly by an intramural redistribution of flow between mucosa and muscularis. Capillary transit time heterog eneity remained unchanged, suggesting that this variable is tightly re gulated.