Metabolism of Glucose, Amino Acids, and Some Related Metabolites in the Brain of Toads (Bufo Boreas) Adapted to Fresh Water or Hyperosmotic Environments

The levels and specific radioactivities (SA) of glucose, lactate, pyruvate, α‐oxoglutarate and seven amino acids in the brain of toads adapted to fresh water or to an hyperosmotic environment were analysed at various times (5 min–4 h) after an injection of [U‐14C]glucose into the bloodstream. The concentrations and SA of glucose, lactate and five amino acids in blood plasma also were measured. In addition, the SA of glutamine, glutamate, aspartate and GABA in brain were determined 30 min after an injection of [1,5‐14C]citrate into the cisterna magna. The flow of labelled carbon atoms from glucose to amino acids and related metabolites in the toad brain was qualitatively similar to that in the mammalian brain, but quantitatively less than one‐tenth of the rate in the brain of rats. Hyperosmotic adaptation induced a large increase in the levels of glucose and amino acids in the brain without affecting the rate of glucose utilization. The SA of several amino acids relative to the SA of glucose were initially lower in hyperosmotically‐adapted toads than in toads adapted to fresh water, presumably because of a greater dilution of isotope by the larger amino acid pools in the hyperosmotically‐adapted toads. The rates of synthesis of alanine and glutamine from pyruvate and glutamate, respectively, appeared to increase with hyperosmotic adaptation, but the rate of GABA synthesis from glutamate was unaltered. The SA of α‐oxoglutarate and glutamate were similar at all time periods in both groups of toads, an indication that these compounds were interconverted much more rapidly than the rate at which α‐oxoglutarate was formed from isocitrate. The SA of lactate in comparison to that of glucose varied but was always considerably lower, even at 4 h after the [14C]glucose injection. After[U‐14C]glucose, glutamine had a SA lower than that of glutamate, whereas after the injection of [14C]citrate, glutamine was formed with a SA much higher than that of glutamate. Hence, glutamate in the toad brain exhibited metabolic compartmentation similar to that in rat brain.