Magnetic resonance spectroscopy for the determination of renal metabolic rate in vivo

Abstract

The method of magnetic resonance spectroscopy has been validated and applied to the determination of renal metabolic rate in vivo. Using an indwelling detector coil, 31P NMR spectra from one kidney of anesthetized rats were quantified. The concentration of ATP was the same as that determined enzymatically, but both ADP and Pi were substantially lower. Only 25% of renal Pi and virtually none of the ADP were detected by NMR. The remainder is assumed to be bound to proteins. These concentrations of metabolites contributed to a significantly increased phosphorylation potential, which in turn should increase the free energy of hydrolysis of ATP. Saturation transfer, a non-destructive magnetic technique for the measurement of chemical exchange, was readily able to detect synthesis of ATP from ADP and inorganic phosphate. The rate of ATP synthesis determined was comparable to that determined in parallel studies of renal oxygen consumption. An ATP:O ratio of approximately 2 was found, indicating that fatty acid is the preferred fuel of respiration of the rat kidney in vivo.

³¹P Nuclear Magnetic Resonance and Saturation Transfer Studies of the Isolated Perfused Rat Kidney

“…From our saturation transfer experiments and the absolute intensity of the intracellular Pi signal (Mo), we derived a forward rate con stant (kf) for Pi incorporation into ATP of 0.34±0.05 s'1, similar to that reported by Free man et al [5] in the perfused rat kidney (kf = 0.35) but higher than those reported for the rat kidney in vivo [6,14,15]. We have observed even higher k( values (~0.6 s '1) in kidneys of small animals [unpubl.…”

“…The renal content of intracellu lar Pi, calculated here to compare with other values reported in the literature, was 1.50 ¡tmol/g wet weight, which is twice as high as that reported by Freeman et al [5,6] for the isolated perfused rat kidney and for the rat kidney in vivo. Our value is similar to the value of Koretsky et al [14], 1.80 ¡.tmol/g wet weight which was calculated from renal ATP concen trations reported in the literature and the ratio of the intensities of the Pi to the ATP reso nances in th e 31P spectra of the kidneys used in his experiments.…”

“…This value is the highest reported thus far for the kidney. A value of 16.1 was obtained by Freeman et al [5] in the perfused kidney, and values of 9.5 [6], 12.0 [14] and 20 [15] have been reported for the kidney in vivo. Our estimate of T, cf( and kf are similar to others observed in the perfused rat kidney (equation II).…”

“…In the experiments of Freeman et al [5] turnover of ATP was estimated at 16 pmol/min/g, the oxygen consumption was 3.3 pmol/min/g and the P /0 = 2.4 in the isolated perfused rat kid ney. Even lower values of ATP turnover (9.5 umol/g/min), of oxygen consumption (2.57 timol/g/min) and of the P/O ratio (1.7) were measured in the rat kidney in vivo [6]. These values are in contrast with the much higher rates of oxidative metabolism estimated from measurements of the renal oxygen consump tion in the rat kidney in vivo [7][8][9] and in isolat ed perfused rat kidneys [10][11][12][13].…”

Evidence for megalin-mediated proximal tubular uptake of L-FABP, a carrier of potentially nephrotoxic molecules

“…14 Moreover, the precise subcellular localization of L-FABP in the kidney, and the origin and functions of renal L-FABP are not known. L-FABP is putatively involved in the renal metabolism of FFA, which are major substrates for oxidation in the renal cortex [15][16][17] but have harmful effects on the kidney in nephrotic conditions. 18 In this study, we determined the cellular and subcellular localization of L-FABP in the rat kidney by immunohistochemical analyses.…”