Basal renal O₂ consumption and the efficiency of O₂ utilization for Na⁺ reabsorption

We examined how the presence of a fixed level of basal renal O₂ consumption (V̇O₂ ^basal; O₂ used for processes independent of Na⁺ transport) confounds the utility of the ratio of Na⁺ reabsorption (T_Na+) to total renal V̇O₂ (V̇O₂ total) as an index of the efficiency of O₂ utilization for TNa⁺. We performed a systematic review and additional experiments in anesthetized rabbits to obtain the best possible estimate of the fractional contribution of V̇O₂ basal to V̇O₂ total under physiological conditions (basal percent renal V̇O₂). Estimates of basal percent renal V̇O₂ from 24 studies varied from 0% to 81.5%. Basal percent renal V̇O₂ varied with the fractional excretion of Na⁺ (FE_Na+) in the 14 studies in which FE_Na+ was measured under control conditions. Linear regression analysis predicted a basal percent renal V̇ O2 of 12.7-16.5% when FE_Na+ = 1% (r² = 0.48, P = 0.001). Experimentally induced changes in T_Na+ altered T_Na+/V̇O₂ total in a manner consistent with theoretical predictions. We conclude that, because V ̇O₂ basal represents a significant proportion of V̇O₂ total, T_Na+/V̇O₂ total can change markedly when T_Na+ itself changes. Therefore, caution should be taken when T_Na+/V̇O₂ total is interpreted as a measure of the efficiency of O₂ utilization for T_Na+, particularly under experimental conditions where T_Na+ or V̇O₂ total changes.