Failure of glutamate dehydrogenase system to predict oxygenation state of human skeletal muscle

In a recent study, the total tissue contents of glutamate (Glu), ammonium (NH+4), and 2-oxoglutarate (2-OG) were used to estimate changes in the mitochondrial redox state ([NAD+]/[NADH]) of contracting skeletal muscle with intact circulation [Am. J. Physiol. 253 (Cell Physiol. 22): C263-C268, 1987]. These metabolites participate in the glutamate dehydrogenase (GDH) reaction, which, based on a number of assumptions, theoretically enables calculation of the mitochondrial redox state as follows (brackets indicate concentrations): [NAD+]/[NADH] = ([NH+4] [2-OG])/[( Glu]Kapp), where Kapp is the apparent equilibrium constant for GDH. The purpose of this study was to determine whether changes in the total tissue contents of Glu, NH+4, and 2-OG could be used to predict a reduction of the mitochondrial redox state in anoxic skeletal muscle. Anoxia was induced in the quadriceps femoris muscle by 10 min of circulatory occlusion (low metabolic rate) and isometric contraction to fatigue (high metabolic rate). The mean (+/- SE) value for the metabolite ratio ([NH+4][2-OG]/[Glu]) at rest was 6 +/- 3 mmol/kg dry wt (x 10(-4]. No significant change occurred after circulatory occlusion (4 +/- 2 x 10(-4); P greater than 0.05), whereas an almost 60-fold increase was observed after isometric contraction (P less than 0.05). Because the muscle was anoxic under both conditions, a significant decrease in the metabolite ratio should have occurred. These data demonstrate that changes in total tissue contents of Glu, NH+4, and 2-OG cannot be used to estimate changes in the redox and oxygenation state of mitochondria in intact human skeletal muscle.