Metabolim of blood-borne lactate in rat brain in vivo.

L-Lactate uniformly labelled with 14C was administered to rats as a single intravenous injection. In experiments concerning the determination of lactate flux into total forebrain, the tissue was obtained by a freeze-clamping technique; in experiments concerning the determination of lactate flux in discrete brain areas the tissue was coagulated by microwave irradiation of the head. In the acid extract of brain tissue the contents and radioactivities of lactate, glucose and cycle amino acids were measured. The following results were obtained: 1. Labelled lactate in plasma equilibrates rapidly with lactate in a small pool in brain tissue. This accessible pool comprises one quarter of total brain lactate. Its size varies in brain regions being largest in cortex and smallest in pons and medulla. 2. Approximately one half of plasma-borne lactate in brain is used metabolically as is seen from the incorporation of lactate carbon in amino acids. The regional activity of lactate metabolism correlates with the local size of the accessible lactate pool. 3. The rates of lactate flux in total forebrain were approximated, the invasion of lactate from plasma to brain tissue to 0.4 mumol, the metabolisms of plasma-borne lactate to 0.2 mumol per g brain tissue min-1. 4. The labelling pattern of cycle amino acids is intermediate between the pattern from [14C]-glucose and [14c]-bicarbonate as precursors. This gives evidence for the influx of part of lactate into the "small" compartment of the citrate cycle along the CO2 fixation route with a predominant accumulation of lactate carbon in aspartate and glutamine.