Glutamine metabolism in normal and acidotic rats in vivo

The kidney responds to metabolic acidosis by the increased excretion of the ammonium salts of nonvolatile acids in the urine. It is therefore important to maintain an adequate supply of precursors of urinary ammonia in order to maintain acid-base balance. In this work, arterio-venous differences for all amino acids were measured across the kidneys of normal and acidotic rats to determine which amino acids were important in urinary ammonia production. Glutamine was the only amino acid taken up by the acidotic kidney; no amino acids were removed by the normal kidney. -- These measurements were made using whole blood since amino acid levels in whole blood and plasma were found to differ. Glycine, glutamate, lysine, and arginine were concentrated within the blood cells. In addition, levels of serine, threonine and lysine rose and levels of glutamine fell in acidosis in both whole blood and plasma. Whole blood and plasma arterio-venous differences were measured for glutamine across the acidotic rat kidney. Results showed that glutamine is removed exclusively from the plasma. -- In a further study on kidney metabolism, arterio-renal venous differences for lactate were measured. Since both normal and acidotic kidney removed equivalent amounts of lactate, it was concluded that the oxidation of lactate carbon was not replaced by the oxidation of glutamine carbon in acidosis. -- The site of production of glutamine and the changes which occur in glutamine metabolism in response to an increased acid load were then investigated. To this end, measurements of the turnover rate of free glutamine in the body were made using both the single injection and constant infusion methods. However, in order to obtain accurate estimates of the turnover rate, a technique for the measurement of the specific 14 activity of 1- C-glutamine first had to be devised. Results obtained by both methods showed that the turnover rate of glutamine did not change in acidosis. This was taken to indicate that the extra glutamine required by the kidney in acidosis was supplied by a lessening of glutamine utilization by other tissues. Turnover values obtained by the single injection method were considerably higher than those obtained by constant infusion. The pool size of glutamine calculated from the single injection data was also considerably smaller than the total body glutamine pool obtained by analysis of whole body extracts and by a summation of data from other investigators. These differences were thought to be due to the methods used in the analysis of the single injection data. -- Arterio-venous differences for glutamine were measured across the major tissues of the body in order to determine the site of glutamine production. Although glutamine was produced by the muscle of the normal rat, no significant output of glutamine by any tissue was found in acidosis. The kidneys and gastrointestinal tract removed glutamine during acidosis. Arterio-venous differences were also measured for glutamate to find if glutamate uptake paralleled glutamine output. However, glutamate was released by the muscle and brain of the normal rat and by the gastrointestinal tract of the acidotic rat; no uptake of glutamate was found. It thus appears that the glutamate required for glutamine synthesis does not come directly from the blood