Evidence that renal arterial-venous oxygen shunting contributes to dynamic regulation of renal oxygenation

Renal blood flow (RBF) can be reduced in rats and rabbits by up to 40 without significant changes in renal tissue PO2. We determined whether this occurs because renal oxygen consumption changes with RBF, or due to some other mechanism. The relationships between RBF and renal cortical and medullary tissue PO2 and renal oxygen metabolism were determined, in the denervated kidneys of anesthetized rabbits under hypoxic, normoxic and hyperoxic conditions. During artificial ventilation with 21 oxygen (normoxia), RBF increased 32+/-8 during renal arterial infusion of acetylcholine and reduced 31+/-5 during angiotensin II infusion. Neither infusion significantly altered arterial pressure, tissue PO2 in the renal cortex or medulla, nor renal oxygen consumption. However, fractional oxygen extraction fell as RBF increased and the ratio of oxygen consumption to sodium reabsorption increased during angiotensin II infusion. Ventilation with 10 oxygen (hypoxia) significantly reduced both cortical and medullary PO2 (60-70 ), while ventilation with 50 and 100 oxygen (hyperoxia) increased cortical and medullary PO2 (by 62-298 and 30-56 respectively). However, responses to altered RBF under hypoxic and hyperoxic conditions were similar to those under normoxic conditions. Thus, renal tissue PO2 was relatively independent of RBF within a physiological range (+/-30 ). This was not due to RBF-dependent changes in renal oxygen consumption. The observation that fractional extraction of oxygen fell with increased RBF, yet renal parenchymal PO2 remained unchanged, supports the hypothesis that preglomerular diffusional shunting of oxygen from arteries to veins increases with increasing RBF, and so contributes to dynamic regulation of intrarenal oxygenation . Key words: arteriovenous shunt, diffusional shunt, hypoxia, ischemia, renal oxygenation.