The rate of ketone body (beta-hydroxybutyrate and acetoacetate) metabolism was measured in individual cerebral structures of fed, starved, and diabetic rats. This was done by infusing beta-[3-14C]hydroxybutyrate intravenously and measuring the incorporation of 14C into brain by quantitative autoradiography. The capacity of the brain to use ketone bodies, expressed as plasma clearance, increased in starvation and diabetes by approximately 50-60%. Plasma clearance was near maximal after 2 days starvation and was not significantly increased after 4 days starvation, 6 days of diabetes or 28 days of diabetes. In all situations the ketone bodies provided only a modest amount of fuel for brain energy metabolism; 3.2% after 2 days starvation and 6.5 and 9.9% after 6 and 28 days of diabetes. The fraction of their energy requirement which the various structures could derive from the ketone bodies differed widely. In general the telencephalon made greatest use of ketone bodies, whereas the hindbrain used least. There was no correlation between the energy requirement of structures (estimated from glucose use in fed rats) and the fraction of energy they could derive from ketone bodies.

The influx of phenylalanine, tryptophan, leucine, and lysine across the blood-brain barrier of individual brain structures was studied in rats 7--8 weeks after a portacaval shunt or sham operation. The method involved a brief infusion of labeled amino acid in tracer quantity and quantitative autoradiography. The clearance rates of phenylalanine, tryptophan, and leucine were increased in proportion to each other in every region examined, but not by the same factor. Tryptophan clearance increased the most (about 200%) and leucine the least (about 30%), compared with phenylalanine (about 80%). This was unexpected, as all three amino acids are believed to be transported by the same mechanism. The changes were most marked in several limbic structures and the reticular formation, whereas the hypothalamus was least affected. Plasma clearance of lysine was decreased in all areas by about 70%. Since the circulating lysine concentration was decreased by 13%, the actual rate of lysine influx was even more reduced. The results demonstrate specific alterations in two different amino acid transport systems. The resulting excess brain neutral amino acids, some of which are neurotransmitter precursors, as well as reduced basic amino acid availability, may be of etiological significance in heptic encephalopathy.

Liver failure, or shunting of intestinal blood around the liver, results in hyperammonemia and cerebral dysfunction. Recently it was shown that ammonia caused some of the metabolic signs of hepatic encephalopathy only after it was metabolized by glutamine synthetase in the brain. In the present study, small doses of methionine sulfoximine, an inhibitor of cerebral glutamine synthetase, were given to rats either at the time of portacaval shunting or 3-4 weeks later. The effects on several characteristic cerebral metabolic abnormalities produced by portacaval shunting were measured 1-3 days after injection of the inhibitor. All untreated portacaval-shunted rats had elevated plasma and brain ammonia concentrations, increased brain glutamine and tryptophan content, decreased brain glucose consumption, and increased permeability of the blood-brain barrier to tryptophan. All treated rats had high ammonia concentrations, but the brain glutamine content was normal, indicating inhibition of glutamine synthesis. One day after shunting and methionine sulfoximine administration, glucose consumption, tryptophan transport, and tryptophan brain content remained near control values. In the 3-4-week-shunted rats, which were studied 1-3 days after methionine sulfoximine administration, the effect was less pronounced. Brain glucose consumption and tryptophan content were partially normalized, but tryptophan transport was unaffected. The results agree with our earlier conclusion that glutamine synthesis is an essential step in the development of cerebral metabolic abnormalities in hyperammonemic states.

The efficacy of [14C]glucose molecules labeled in various positions as tracers of regional cerebral glucose utilization (rCMRGlc) was examined in rats. Arteriovenous differences of different [14C]-glucose species and 14CO2 were measured across brain to determine the relative rates of 14CO2 loss. As anticipated, 14CO2 evolution decreased in the order: [U-14C]glucose greater than [2-14C]glucose greater than [1-14C]glucose greater than [6-14C]glucose. Release of 14CO2 from [6-14C]glucose was undetectable at 5 min and barely detectable at 10 min, and release from [1-14C]glucose, which includes the pentose phosphate pathway, was only slightly greater. rCMRGlc was measured with [1-14C]-,[2-14C]-, or [6-14C]glucose in 5-min experiments. The results of [1-14C]- and [6-14C]glucose were indistinguishable; no difference due to the activity of the pentose phosphate pathway was found. Both [1-14C]- and [6-14C]-glucose gave values similar to, but on the whole slightly higher than, [2-14C]glucose. It was concluded that when knowledge of total rCMRGlc is required, [6-14C]glucose is the labeled substrate of choice. When the experimental objective is measurement of energy metabolism, use of [1-14C]glucose avoids inclusion of the nonenergy-yielding pentose phosphate pathway.