Evaluation of cerebral acetate transport and metabolic rates in the rat brain in vivo using 1H-[13C]-NMR

Acetate is a well-known astrocyte-specific substrate that has been used extensively to probe astrocytic function in vitro and in vivo. Analysis of amino acid turnover curves from C-13-acetate has been limited mainly for estimation of first-order rate constants from exponential fitting or calculation of relative rates from steady-state C-13 enrichments. In this study, we used H-1-[C-13]-Nuclear Magnetic Resonance spectroscopy with intravenous infusion of [2-C-13] acetate-Na+ in vivo to measure the cerebral kinetics of acetate transport and utilization in anesthetized rats. Kinetics were assessed using a two-compartment (neuron/astrocyte) analysis of the C-13 turnover curves of glutamate-C4 and glutamine-C4 from [2-C-13] acetate-Na+, brain acetate levels, and the dependence of steady-state glutamine-C4 enrichment on blood acetate levels. The steady-state enrichment of glutamine-C4 increased with blood acetate concentration until 90% of plateau for plasma acetate of 4 to 5 mmol/L. Analysis assuming reversible, symmetric Michaelis-Menten kinetics for transport yielded 27 +/- 2 mmol/L and 1.3 +/- 0.3 mu mol/g/min for K-t and T-max, respectively, and for utilization, 0.17 +/- 0.24 mmol/L and 0.14 +/- 0.02 mu mol/g/min for K-M_util and V-max_util, respectively. The distribution space for acetate was only 0.32 +/- 0.12 mL/g, indicative of a large excluded volume. The astrocytic and neuronal tricarboxylic acid cycle fluxes were 0.37 +/- 0.03 mu mol/g/min and 1.41 +/- 0.11 mu mol/g/min, respectively; astrocytes thus comprised similar to 21%+/- 3% of total oxidative metabolism. Journal of Cerebral Blood Flow & Metabolism (2010) 30, 1200-1213; doi: 10.1038/jcbfm.2010.2; published online 3 February 2010