The Contribution of Blood Lactate to Brain Energy Metabolism in Humans Measured by Dynamic 13C Nuclear Magnetic Resonance Spectroscopy

To determine whether plasma lactate can be a significant fuel for human brain energy metabolism, infusions of [3-C-13] lactate and H-1-C-13 polarization transfer spectroscopy were used to detect the entry and utilization of lactate. During the 2 h infusion study, C-13 incorporation in the amino acid pools of glutamate and glutamine were measured with a 5 min time resolution. With a plasma concentration ([Lac](P)) being in the 0.8-2.8 mmol/L range, the tissue lactate concentration ([Lac](B)) was assessed as well as the fractional contribution of lactate to brain energy metabolism (CMRlac). From the measured relationship between unidirectional lactate influx (V-in) and plasma and brain lactate concentrations, lactate transport constants were calculated using a reversible Michaelis-Menten model. The results show that (1) in the physiological range, plasma lactate unidirectionaltransport(V-in) and concentration in tissue increase close to linearly with the lactate concentration in plasma; (2) the maximum potential contribution of plasma lactate to brain metabolism is 10% under basal plasma lactate conditions of similar to 1.0 mmol/L and as much as 60% at supraphysiological plasma lactate concentrations when the transporters are saturated; (3) the half-saturation constant K-T is 5.1 +/- 2.7 mmol/L and V-MAX is 0.40 +/- 0.13 mu mol.g(-1).min(-1) (68% confidence interval); and (4) the majority of plasma lactate is metabolized in neurons similar to glucose.