METHOD

The method employed has been described in detail (6, 7) and is only briefly summarized here. Wistar rats of either sex, 250-350 g, were anesthetized with intraperitoneal pentobarbital. The right common carotid artery was surgically exposed, cannulated with a 0.38-mm od needle, and 0.15-0.2 ml of buffered Ringer solution (McGaw Laboratories, Glendale, Calif.) was injected during approximately 0.2 sec. The injected Ringer solution contained 10 mM of HEPES buffer (Calbiochem, La Jolla, Calif.), approximately 0.25 PC 14C-labeled organic acid, and approximately 1 PC water-3H. In some of the studies known concentrations of unlabeled solutes were also included to study competition for saturable transport systems. The pH was adjusted to the pK of the buffer at pH 7.55 prior to injection. Fifteen seconds after carotid injection, the rat was decapitated and the entire brain rostra1 to midbrain was subjected to routine liquid scintillation counting for 3H and 14C. Some of the injected solution was similarly counted. The amount of 14C in brain was calculated as a percentage of 3H present, the 3H water serving as a highly diffusible internal standard of reference. This percentage was taken as an index of brain uptake which is some nonlinear but monotonic function of BBB permeability. The uptake index approximates the percentage of the labeled species that is lost to brain during a single capillary passage. Some representative uptakes are: approximately 100% for ethanol and nicotine, 2-55% for amino acids, and 32% for D-ghCO%(7, 8). Nonpenetrating substances such as mannitol or inulin have an uptake of 1.8-2%, and they provide the background level for the method. This background has not been subtracted in the results presented here. Low injected concentrations. The 14C-labeled acids listed in Table 1 were studied at the low injected concentrations shown. The injected concentrations were those present in radiolabeled material obtained from the radiochemical supplier (New England Nuclear Corp., Billerica, Mass., or Amersham/Searle, Arlington Heights, Chicago, Ill.). Elevated injected concentrations. In addition to the low injected concentrations shown in Table 1, unlabeled substances were added to the injected solution to demonstrate self-inhibition and cross-inhibition for several of the acids and lack of cross-inhibition between pyruvate and glucose, phenylalanine, or arginine.

The self-inhibiting effects of the addition to the injected solution of unlabeled acid on the uptake of the same 14C-labeled acid were studied using acetate, propionate, pyruvate, L-lactate, and butyrate as test substances. The maximum concentration of unlabeled acid added was 10 mM except for L-lactate where it was carried out to 15 mM. The cross-inhibiting effects of the addition to the injected