Relation of Na+ reabsorption to utilization of O2 and lactate in the perfused rat kidney

Publisher Website
Google Scholar
Abstract
When the ratio .DELTA..ovrhdot.TNa+/.DELTA..ovrhdot.QO2 is used to estimate the energy requirements for net Na+ reabsorption (.ovrhdot.TNa+), it is assumed that the entire change in renal O2 uptake (.DELTA..ovrhdot.QO2) is utilized only for the .DELTA..ovrhdot.TNa+. If increases in renal synthetic work also occur when .ovrhdot.TNa+ is increased, the energy cost for .ovrhdot.TNa+ will be overestimated. The substrate-limited isolated rat kidney was perfused at 38.degree. C, pH 7.4, a mean arterial pressure of 120 mmHg, and mean lactate concentrations between 0 and 8.3 mM. .ovrhdot.QO2, .ovrhdot.TNa+, net reabsorption of lactate (.ovrhdot.Tlac), net utilization of lactate (.ovrhdot.Qlac), lactate decarboxylation rate .**GRAPHIC**. as well as the net entry rate of lactate into biosynthetic pathways .**GRAPHIC**. were measured. When no exogenous substrate was present (rates are means, g wet wt-1 .cntdot. min-1), GFR [glomerular filtration rate] was 351 .+-. 38 .mu.l, %TNa+ was 54 .+-. 2%, and .ovrhdot.QO2 was 2.85 .+-. 0.31 .mu.mol; there was also a loss of about 20% of renal tissue K+ content. When mean [lactate] .gtoreq. 0.73 mM, the loss of tissue K+ was completely prevented and %TNa+ increased to and remained at about 85%. At mean [lactate] of 8.3 mM, .ovrhdot.Tlac was 5.1 .+-. 0.6 .mu.mol, .ovrhdot.QO2 was 6.12 .+-. 1.24 .mu.mol and GFR was 709 .+-. 83 .mu.l. .ovrhdot.Qlac, .**GRAPHIC**. and .DELTA..ovrhdot.TNa+ increased in parallel with each other and approached maximal rates as [lactate] was raised. .ovrhdot.Tlac increased as a linear function of perfusate [lactate] and was not related to changes in .ovrhdot.Qlac. The molar increase in .ovrhdot.TNa+ were 10- to 20-fold greater than the increases in .ovrhdot.Tlac. Lactate may enhance .ovrhdot.TNa+ by providing energy from its oxidation rather than by a co-transport phenomenon. At all concentrations of lactate, more lactate was utilized (Km = 1.2 mM; Vmax = 3.4) than was decarboxylated (Km = 1.6 mM; Vmax = 1.7), indicating that as lactate concentration increased, both the synthesis of new products from lactate and Na+ absorption increased. The ratio .DELTA..ovrhdot.TNa+/.DELTA..ovrhdot.QO2, overestimates the energy cost of Na+ reabsorption. To obtain an accurate estimate of the energy requirements for .ovrhdot.TNa+ in kidney, the simultaneous changes in the rate of net biosynthetic work must also be quantified as .ovrhdot.TNa+ is changed.

This publication has 12 references indexed in Scilit: