When the ratio ΔT(Na+/ΔQ(O2) is used to estimate the energy requirements for net Na+ reabsorption (T(Na+), it is assumed that the entire change in renal O2 uptake (ΔQ(O2)) is utilized only for the ΔT(Na+). However, if increases in renal synthetic work also occur when T(Na+) is increased, the energy cost for T(Na+) will be overestimated. We perfused the substrate-limited isolated rat kidney at 38°C, pH 7.4, a mean arterial pressure of 120 mmHg, and mean lactate concentrations between 0 and 8.3 mM. We measured Q(O2), T(Na+), net reabsorption of lactate (T(lac)), net utilization of lactate (Q(lac)), lactate decarboxylation rate (Q(ox)(lac)), as well as the net entry rate of lactate into biosynthetic pathways (Q(xs)(lac)). When no exogenous substrate was present (rates are means, g wet wt-1/min-1) GFR was 351 ± 38 μl, %T(Na+) was 54 ± 2%, and Q(O2) was 2.85 ± 0.31 μmol; there was also a loss of about 20% of renal tissue K+ content. When mean [lactate] ≥ 0.73 mM, the loss of tissue K+ was completely prevented and %T(Na+) increased to and remained at about 85%. At mean [lactate] of 8.3 mM, T(lac) was 5.1 ± 0.6 μmol, Q(O2) was 6.12 ± 1.24 μmol, and GFR was 709 ± 83 μl. Q(lac), ΔQ(ox)lac) and ΔT(Na+) increased in parallel with each other and approached maximal rates as [lactate] was raised. By contrast, T(lac) increased as a linear function of perfusate [lactate] and was not related to changes in Q(lac). The molar increases in T(Na+) were 10- to 20-fold greater than the increases in T(lac). It is more probable, therefore, that lactate enhances T(Na+) by providing energy from its oxidation rather than by a co-transport phenomenon. At all concentrations of lactate, more lactate was utilized (K(m) = 1.2 mM; V(max) = 3.4) than was decarboxylated (K(m) = 1.6 mM; V(max) = 1.7), indicating that as lactate concentration increased, both the synthesis of new products from lactate and Na+ reabsorption increased. We conclude that the ratio ΔT(Na+)/ΔQ(O2) overestimates the energy cost of Na+ reabsorption. In order to obtain an accurate estimate of the energy requirements for T(Na+) in kidney, the simultaneous changes in the rate of net biosynthetic work must also be quantified as T(Na+) is changed.
|Original language||English (US)|
|Number of pages||13|
|Journal||American Journal of Physiology - Renal Fluid and Electrolyte Physiology|
|State||Published - 1980|
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