Basal renal O2consumption and the efficiency of O2utilization for Na+reabsorption

We examined how the presence of a fixed level of basal renal O2consumption (V̇o2basal; O2used for processes independent of Na+transport) confounds the utility of the ratio of Na+reabsorption (TNa+) to total renal V̇o2(V̇o2total) as an index of the efficiency of O2utilization for TNa+. We performed a systematic review and additional experiments in anesthetized rabbits to obtain the best possible estimate of the fractional contribution of V̇o2basalto V̇o2totalunder physiological conditions (basal percent renal V̇o2). Estimates of basal percent renal V̇o2from 24 studies varied from 0% to 81.5%. Basal percent renal V̇o2varied with the fractional excretion of Na+(FENa+) in the 14 studies in which FENa+was measured under control conditions. Linear regression analysis predicted a basal percent renal V̇o2of 12.7–16.5% when FENa+= 1% ( r2= 0.48, P = 0.001). Experimentally induced changes in TNa+altered TNa+/V̇o2totalin a manner consistent with theoretical predictions. We conclude that, because V̇o2basalrepresents a significant proportion of V̇o2total, TNa+/V̇o2totalcan change markedly when TNa+itself changes. Therefore, caution should be taken when TNa+/V̇o2totalis interpreted as a measure of the efficiency of O2utilization for TNa+, particularly under experimental conditions where TNa+or V̇o2totalchanges.