Glutamate interconversions and glucogenicity in the sheep.

Simultaneous measurements were made of net and unidirectional glutamate metabolism by the portal-drained viscera, liver, kidneys, and hindquarters of fed, acidotic, and fasted sheep. The contribution of glutamate to glutamine and glucose production also was estimated. Of the glutamate present in whole blood, 45% was in plasma and 55% in the cellular fraction. Acidosis and fasting reduced blood glutamate concentrations, but did not change the plasma:cellular ratio. [14C]glutamate exchanged only little between plasma and blood cells. Clearly, this demonstrates a lack of the alpha-amino acid transport system in blood cells. Net rates of plasma glutamate flux by the portal-drained viscera and kidneys were less than 0.5 mmol/h, but, in fed sheep, the liver released 2--3 mmol/h into the plasma and the hindquarters removed an average of 0.9 mmol/h. Both were reduced by acidosis and fasting. Unidirectional rates were highly significant and greater than net rates. Acidosis and fasting primarily seemed to affect production by the liver, but only utilization by the hindquarters. Plasma glutamate turnover averaged 6--9 mmol/h, but interconversions with glutamine were low; only 12--25% was converted to glutamine and most of this occurred in extrarenal tissues. A similar rate of 20--26% of the glutamate was converted to glucose, which accounted for about 4% of the total glucose produced. The kidneys seemed to play an important gluconeogenic role; whereas the liver possibly could account for 100% of the glucose produced from glutamate in fed sheep, the kidneys accounted for 40--45% during acidosis and fasting.