Structural antioxidant defense mechanisms in the mammalian and nonmammalian kidney: different solutions to the same problem?

Tissue oxygen levels are tightly regulated in all organs. This poses a challenge for the kidney, as its function requires blood flow, and thus, oxygen delivery to greatly exceed its metabolic requirements. Because superoxide production in the kidney is dependent on oxygen availability, tissue hyperoxia could drive oxidative stress. In the mammalian renal cortex, this problem may have been solved, in part, through a structural antioxidant defense mechanism. That is, arteries and veins are closely associated in a countercurrent arrangement, facilitating diffusional arterial-to-venous (AV) oxygen shunting. Because of this mechanism, a proportion of the oxygen delivered in the renal artery never reaches kidney tissue but instead diffuses to the closely associated renal veins, thus limiting oxygen transport to tissue. In the nonmammalian kidney, arteries and veins are not arranged in an intimate countercurrent fashion as in mammals; thus AV oxygen shunting is likely less important in regulation of kidney oxygenation in these species. Instead, the kidney's blood supply is predominately of venous origin. This likely has a similar impact on tissue oxygenation as AV oxygen shunting, of limiting delivery of oxygen to renal tissue. Thus, we hypothesize the evolution of structural antioxidant mechanisms that are anatomically divergent but functionally homologous in the mammalian and nonmammalian kidney.