Renal medullary tissue oxygenation is dependent on both cortical and medullary blood flow

Abstract

. Renal medullary tissue oxygenation is dependent on both cortical and medullary blood flow. Am J Physiol Renal Physiol 290: F688 -F694, 2006. First published October 11, 2005 doi:10.1152/ajprenal.00275.2005The aim of the current study was to determine whether renal medullary oxygenation is independent of the level of cortical blood flow by testing responses to stimuli that selectively reduce blood flow in either the cortex or medulla. In anesthetized rabbits, renal arterial infusion of [Phe 2 ,Ile 3 ,Orn 8 ]-vasopressin selectively reduced medullary perfusion and PO 2 (Ϫ54 Ϯ 24 and Ϫ50 Ϯ 10%, respectively) but did not significantly affect cortical perfusion or tissue oxygenation. In contrast, stimulation of the renal nerves resulted in renal cortical ischemia with reductions in total renal blood flow (Ϫ76 Ϯ 3% at 4 Hz), cortical perfusion (Ϫ57 Ϯ 17%), and cortical PO 2 (Ϫ44 Ϯ 12%). Medullary tissue PO2 was reduced by Ϫ70 Ϯ 5% at 4 Hz, despite medullary perfusion being unaffected and distal tubular sodium reabsorption being reduced (by Ϫ83.3 Ϯ 1.2% from baseline). In anesthetized rats, in which renal perfusion pressure was maintained with an aortic constrictor, intravenous infusion of ANG II (0.5-5 g ⅐ kg Ϫ1 ⅐ min Ϫ1 ) dose dependently reduced cortical perfusion (up to Ϫ65 Ϯ 3%; P Ͻ 0.001) and cortical PO 2 (up to Ϫ57 Ϯ 4%; P Ͻ 0.05). However, medullary perfusion was only significantly reduced at the highest dose (5 g ⅐ kg Ϫ1 ⅐ min Ϫ1 ; by 29 Ϯ 6%). Medullary perfusion was not reduced by 1 g ⅐ kg Ϫ1 ⅐ min Ϫ1ANG II, but medullary PO2 was significantly reduced (Ϫ12 Ϯ 4%). Thus, although cortical and medullary blood flow may be independently regulated, medullary oxygenation may be compromised during moderate to severe cortical ischemia even when medullary blood flow is maintained.acute renal failure; acute tubular necrosis; hypoxia; ischemia; laserDoppler flowmetry; fluorescence oximetry; oxygen; rabbits; rats; renal circulation THE UNIQUE VASCULAR architecture of the kidney allows differential regulation of cortical and medullary blood flow (7,10,14). Indeed, many vasoconstrictor factors that profoundly reduce total renal blood flow (TRBF) and cortical blood flow appear to have considerably less impact on medullary blood flow (6). Thus the renal medulla may be at least partially protected from hypoxic insults during periods of reduced overall (total) renal blood flow, if medullary blood flow, and so presumably oxygen delivery to this region, is maintained. However, this hypothesis is predicated on renal cortical and medullary oxygenation being relatively independent. Changes in glomerular filtration, tubular sodium reabsorption, and/or the possibility of oxygen loss from preglomerular arterial blood (27) may render medullary oxygenation dependent on the level of cortical blood flow.The aim of the current study was to examine the relationship between tissue oxygenation and perfusion in the cortex and medulla. We tested the hypothesis that medullary oxygenation is independent of cortical blood flow. To achieve this...

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