Ford, G. C.

The contribution of the kidneys to postabsorptive endogenous glucose production is a matter of controversy. To assess whether this could relate to the use of various isotopical methods with different analytical performance capabilities, we measured glucose kinetics in 12 healthy subjects. Blood samples were taken from the femoral artery and the renal vein after 4 h of [6,6-2H2]glucose infusion (for gas chromatography [GC]/mass spectrometry [MS] analysis), and renal plasma flow was determined with paraaminohippurate. In addition, six subjects received uniformly labeled [13C]glucose (for GC/combustion/isotope ratio MS [IRMS]) and [3-3H]glucose (for counting of radioactive disintegrations). Arterial glucose concentrations (means ± SD) were 4.2 ± 0.1 mmol/l, and endogenous glucose production rates using [2H2]glucose were 2.2 ± 0.1 mg · kg–1 · min–1 or 818 ± 50 μmol/min. Dilution of [2H2]glucose across the kidney was 0.79 ± 1.32%, and renal glucose production (RGP) rates were 27 ± 72 μmol/min. In the six subjects receiving additional tracers, dilutions across the kidney were 2.83 ± 0.72 and 0.54 ± 1.20 (for [U-13C]glucose and [3-3H]glucose, respectively, the dilution with [U-13C] being higher than that with [2H2] (P = 0.007). Corresponding RGP values were 144 ± 39 and 43 ± 76 μmol/min for [U-13C] and [3-3H], respectively. In conclusion, we found that the highly sensitive [U-13C] GC/Combustion/IRMS technique showed consistent dilution of label across the kidney, whereas the less sensitive techniques gave some negative values and smaller RGP rates. Thus, depending on which technique is being used, a fivefold difference in calculated RGP values may be encountered. The methodological variability of our data suggests that extrapolation from regional renal measurements to the whole-body level should be perfumed with caution.

To define the mechanism of insulin's anticatabolic action, the effects of three different dosages of insulin (0.25, 0.5, and 1.0 mU x kg(-1) x min(-1)) versus saline on protein dynamics across splanchnic and skeletal muscle (leg) beds were determined using stable isotopes of phenylalanine, tyrosine, and leucine in 24 healthy subjects. After an overnight fast, protein breakdown in muscle exceeded protein synthesis, causing a net release of amino acids from muscle bed, while in the splanchnic bed protein synthesis exceeded protein breakdown, resulting in a net uptake of these amino acids. Insulin decreased (P < 0.003) muscle protein breakdown in a dose-dependent manner with no effect on muscle protein synthesis, thus decreasing the net amino acid release from the muscle bed. In contrast, insulin decreased protein synthesis (P < 0.03) in the splanchnic region with no effect on protein breakdown, thereby decreasing the net uptake of the amino acids. In addition, insulin also decreased (P < 0.001) leucine nitrogen flux substantially more than leucine carbon flux, indicating increased leucine transamination (an important biochemical process for nitrogen transfer between amino acids and across the organs), in a dose-dependent manner, with the magnitude of effect being greater on skeletal muscle than on the splanchnic bed. In conclusion, muscle is in a catabolic state in human subjects after an overnight fast and provides amino acids for synthesis of essential proteins in the splanchnic bed. Insulin achieves amino acid balance across splanchnic and skeletal muscle beds through its differential effects on protein dynamics in these tissue beds.