Regulation of carbohydrate metabolism in lymphoid tissue. Nature of the endogenous substrates and their contribution to the respiratory fuel of the sliced rat spleen in vitro

1. Tissue glycogen contributes, maximally, only 10% of the respiratory fuel of the rat spleen slice in the absence of an added carbon source, and makes no significant contribution when glucose (3mM) is added. 2. The reserves of fatty acid in the form of triglyceride (35.5mumol of fatty acid/g dry wt. of tissue) fall by approx. 25% after incubation of spleen slices with or without added glucose for 2h, and, on this basis, account for 32% of the oxidative fuel. 3. In contrast, the total oxidative contribution of fatty acid reserves to the respiratory fuel, determined on the basis of inhibiton of respiration by 2-bromostearate, is 42-52%. This range includes tissue from both starved and well-fed animals and is not significantly altered by the presence of added glycose (3mM). 4. Large quantities of NH3 (31-35mumol//h per g dry wt. of tissue) are produced by spleen slices incubated in the absence of added substrates, and this value is suppressed by approx. 50% on incubation with glucose (3mM). Adenine nucleotide breakdown can account for only 17% of the total ammonia produced. 5. Individual free amino acid concentrations in spleen were determined, both in vivo and in slices before and after 60 min of incubation. Although the total free amino acid pool size increases by 45% during incubation, owing to protein breakdown, the tissue concentrations of aspartate, glutamate, glutamine and alanine do not increase. It is suggested that these amino acids areoxidized in a net sense to CO2 and water with the liberation of free NH3 via transamination reactions, glutaminase, the purine nucleotide cycle and the tricarboxylic acid cycle. 6. It is concluded that the normal endogenous metabolism of sliced rat spleen (43-52% due to lipids, 30% due to amino acids and 10% due to glycogen) is modified by added glycose only to the extent that glycogen oxidation and 50% of the contribtion made by ino acids are suppressed; endogenous lipid metabolism is unaffected.