Stereospecific permeability of rat blood-brain barrier to lactic acid

Abstract

Blood and whole brain 14C and 32P activities were determined in hepatectomized rats one, two, five and ten minutes after intravenous (I.V.) injection of 14C-labeled L-lactate or D-lactate and 32P-labeled rat red blood cells. Whole brain homogenate 14C was corrected for blood 14C and chemically partitioned into 14C-lactate, 14CO2and other 14C compounds. In controls, lactate was replaced with 14C-D-glucose and 125l-antipyrine. At one minute postinjection, whole brain 14C expressed as percent of total injected 14C 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 14C was 74% lactate, 5% CO2and 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-Iactate uptake to 0.09% (=5% of antipyrine). At two, five and ten minutes, brain contained more 14C with larger fractions metabolized to CO2and 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. © 1974 American Heart Association, Inc.