1. The metabolism and transport of glutamine and glucose were investigated in a preparation of rat small intestine perfused through the vascular bed in vitro and in situ. 2. With glucose (7.5mm) or glutamine (4.5mm) in the lumen, approx. 40% of the substrate taken up appears unchanged on the vascular side. When glutamine (1.5mm) is also added to the vascular perfusate, metabolism of glutamine is increased and there is uptake of glutamine from both the vascular bed and lumen. Orientation of substrate (vascular bed or lumen) influences the value of alanine production/glutamine utilization and lactate production/glucose utilization. 3. Deprivation of food and metabolic acidosis have no effect upon the utilization of glutamine by unit length of jejunum. In fed rats, glutamine utilization is 44% of glucose utilization, but in rats deprived of food it is 112% of glucose utilization. 4. Glucose utilization and lactate production are not significantly altered by the presence of glutamine in the vascular bed or lumen. 5. With glucose only in the vascular perfusate, glucose utilization is the same in jejunum and ileum. Glutamine metabolism in the ileum is 28% lower than in the jejunum. 6. Glutamine utilization is dependent on the concentration of glutamine in the vascular perfusate, but is not significantly affected by the absence of glucose. 7. Results are discussed in relation to the role of intestinal glutamine metabolism and with respect to some problems of the transepithelial movement of substrates that are both transported and metabolized.

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