Stereospecific permeability of rat blood-brain barrier to lactic acid.

Stereospecific Permeability of Rat BloodBrain Barrier to Lactic Acid • Blood and whole brain C and P activities were determined in hepatectomized rats one, two, five and ten minutes after intravenous (I.V.) injection of C-labeled L-lactate or D-lactate and P-labeled rat red blood cells. Whole brain homogenate C was corrected for blood "C and chemically partitioned into "C-lactate, CO2 and other "C compounds. In controls, lactate was replaced with C-D-glucose and I-antipyrine. At one minute postinjection, whole brain C expressed as percent of total injected C activity and as percent of the antipyrine value were: antipyrine 1.78% (100%); D-glucose 1.45% (81%); L-lactate 0.36% (20%); and D-lactate 0.13% (7%). One minute after L-lactate injection, brain C was 74% lactate, 5% CO2 and 21% other compounds. Preloading rats with cold racemic Na-lactate reduced L-lactate uptake to 0.14% of the injectate (8% of antipyrine), and reduced D-lactate uptake to 0.09% (= 5% of antipyrine). At two, five and ten minutes, brain contained more C with larger fractions metabolized to CO2 and other compounds from both L-lactate and D-lactate. The blood-brain barrier appears to contain a saturable lactate carrier exhibiting threefold L-stereospecificity to D-stereospecificity, but resulting in far less net transport than the comparable glucose carrier. Lactate transport may be limited by the scarcity of neutral lactic acid at normal blood pH. Additional