Relationship between energy requirements for Na+ reabsorption and other renal functions

In the mammalian kidney, the use of the ratio, delta net T-Na+/delta Q-O2, provides an overestimate of the energy requirements for unidirectional active Na+ transport. In the proximal tubule, the overestimate of the energy cost for T-Na+ is due to these phenomena: (1) The "leaky" characteristics of the proximal tubule does not permit an accurate estimate to be made of the active fraction of the unidirectional flux of Na+. Thus, the net Na+ or net HCO3- reabsorption rate alone cannot be used to determine the stoichiometry for unidirectional extrusion of Na+ (with HCO3-) by the Na,K-ATPase, since backflux of HCO3- into the lumen occurs. (2) There is a moiety of active Na+ with Cl- along the pars recta. Whether this reabsorptive rate is altered and O2 uptake also changed when GFR or NaHCO3 reabsorption is varied is not yet known. (3) The occurrence of energy-requiring synthetic functions (substrate-interconversions) in the proximal tubule, coupled, in part, to the rate of Na+ entry into the proximal tubule cells, results in changes in renal O2 uptake proportional to some (undetermined) fraction of the change in Na+ reabsorption. The utilization of a portion of these reabsorbed substrates in endergonic syntheses must account for a portion of the Na+-stimulated suprabasal O2 uptake rate. Hence, the presence of synthetic functions in the proximal tubule also contributes to the overestimation of the energy value of net Na+ reabsorption when the ratio, delta net TNa-+/delta Q-O2, is used.(ABSTRACT TRUNCATED AT 250 WORDS)