O2 affinity of cross-linked hemoglobins modifies O2 metabolism in proximal tubules

Abstract

Previous experiments using cross-linked tetrameric hemoglobins (XLHb) to perfuse isolated rat kidneys showed that high-O2-affinity XLHb improved proximal tubule function more effectively than low-O2-affinity XLHb. To determine how function was improved, proximal tubule fragments were incubated with albumin, Hb34 [half-saturation point (P50) 34 Torr], or Hb13 (P50 13 Torr) with Po2 values ranging from 22 to 147 Torr. ATP content reflected O2 delivery to mitochondria. Both XLHb increased ATP, Hb34 with Po2 ≥ 47 Torr and Hb13 with Po2 ≤ 47 Torr. XLHb increased Na-K-ATPase activity (86Rb uptake) in similar Po2-dependent patterns. O2 consumption (Q̇o2) was measured in a closed, well-stirred chamber. Ouabain- and oligomycin-inhibited Q̇o2, reflecting Na-K-ATPase activity and oxidative phosphorylation, respectively, mirrored the Po2-dependent patterns of ATP and 86Rb uptake. As Po2 fell below the midpoint of XLHb desaturation, Q̇o2, uncoupled from oxidative phosphorylation, transiently increased. The increase was most pronounced with Hb34. Nitro-l-arginine methyl ester had no effect on Q̇o2. Inhibitors of NAD(P)H oxidases and diamine oxidase partially prevented the Q̇o2 surge with Hb34. In conclusion, facilitated diffusion accounts for Po2-dependent XLHb effects on ATP content and Na-K-ATPase and for Hb13's effectiveness in hypoxic perfused kidneys. NO scavenging was not a factor. O2-binding characteristics influence XLHb effects on mitochondria and O2-sensitive enzymes such as oxidases.