Effects of arterial hypoxia and isoproterenol on in vitro postnatal intestinal circulation.

We have previously speculated that intestinal vasodilation and hyperemia that occur in response to moderate arterial hypoxia in newborn swine in vivo are mediated by factors intrinsic to the intestinal circulation. To test this speculation, we vascularly perfused in vitro loops of jejunum from postnatal swine with control (PO2 98 +/- 4 mmHg) and hypoxic (PO2 38 +/- 2 mmHg) blood obtained from donor swine. In response to hypoxic perfusion, jejunal vascular resistance decreased 12 +/- 2, 13 +/- 3, 33 +/- 5, and 42 +/- 3% in in vitro loops from 1-, 7-, 14-, and 30-day-old swine, respectively, whereas jejunal oxygen uptake decreased 53 +/- 6, 29 +/- 6, 31 +/- 4, and 13 +/- 6% in these age groups. To clarify whether this age-dependent vasodilation was unique to the stimulus of arterial hypoxia or a response characteristic of the postnatal swine intestine to other vasodilator stimuli, we also determined the effect of intra-arterial isoproterenol infusion at rates of 0.01, 0.1, and 1.0 micrograms/min on jejunal hemodynamics and oxygenation in vitro. In jejunal loops taken from 7- and 30-day-old swine, isoproterenol caused a similar degree of vasodilation at each drug-infusion rate. We conclude that vasodilation of the postnatal swine intestine in response to moderate arterial hypoxia is mediated, at least in part, by intrinsic vascular regulation. We speculate that the age dependency of hypoxic vasodilation may reflect a relative inability of the intestine from very young swine to respond to the stimulus of arterial hypoxia.