Multiple mechanisms act to maintain kidney oxygenation during renal ischemia in anesthetized rabbits

Evans RG, Eppel GA, Michaels S, Burke SL, Nematbakhsh M, Head GA, Carroll JF, O'Connor PM. Multiple mechanisms act to maintain kidney oxygenation during renal ischemia in anesthetized rabbits. Am J Physiol Renal Physiol 298: F1235-F1243, 2010. First published March 3, 2010; doi:10.1152/ajprenal.00647.2009.-We examined the mechanisms that maintain stable renal tissue PO2 during moderate renal ischemia, when changes in renal oxygen delivery ((D) over dotO(2)) and consumption ((V) over dotO(2)) are mismatched. When renal artery pressure (RAP) was reduced progressively from 80 to 40 mmHg, (V) over dotO2 (- 38 +/- 7%) was reduced more than (D) over dotO(2) (- 26 +/- 4%). Electrical stimulation of the renal nerves (RNS) reduced (D) over dotO(2) (- 49 +/- 4% at 2 Hz) more than (V) over dotO2 (- 30 +/- 7% at 2 Hz). Renal arterial infusion of angiotensin II reduced (D) over dotO(2) (- 38 +/- 3%) but not (V) over dotO2 (- 10 +/- 10%). Despite mismatched changes in (D) over dotO(2) and (V) over dotO2, renal tissue PO2 remained remarkably stable at >= 40 mmHg RAP, during RNS at <= 2 Hz, and during angiotensin II infusion. The ratio of sodium reabsorption to (V) over dotO2 was reduced by all three ischemic stimuli. None of the stimuli significantly altered the gradients in PCO2 or pH across the kidney. Fractional oxygen extraction increased and renal venous PO2 fell during 2-Hz RNS and angiotensin II infusion, but not when RAP was reduced to 40 mmHg. Thus reduced renal (V) over dotO(2) can help prevent tissue hypoxia during mild renal ischemia, but when renal (V) over dotO(2) is reduced less than (D) over dotO(2), other mechanisms prevent a fall in renal PO2. These mechanisms do not include increased efficiency of renal oxygen utilization for sodium reabsorption or reduced washout of carbon dioxide from the kidney, leading to increased oxygen extraction. However, increased oxygen extraction could be driven by altered countercurrent exchange of carbon dioxide and/or oxygen between renal arteries and veins.