Small intestinal glucose transport. Proximal-distal kinetic gradients.

Proximal and distal small intestinal segments of the rat were perfused in situ at two different rates with isotonic solutions containing glucose in concentrations ranging from 25 to 600 mg/100 ml. Absorption was measured as glucose disappearance rate from the lumen. Glucose absorption had not previously been studied at intraluminal concentrations above and below blood glucose. Absorption was more rapid from the proximal segment. In both segments absorption was independent of perfusion rate and of whether glucose was analyzed by counting (14)C or by the Somogyi method. The latter finding suggests that of the unidirectional fluxes, flux out of the bowel is much greater than flux into the bowel. In contrast to the findings in previous studies neither segment showed rate-limiting kinetics, and the Michaelis-Menten analysis was not applicable. The form of the curve depicting absorption rate in relation to concentration differed between the two segments. At the higher concentrations absorption rate continued to increase much more rapidly in the proximal than in the distal segment. The observations could not be explained by known mechanisms of glucose transport and illustrate the difficulties of achieving biochemically and physiologically meaningful in vivo studies of intestinal absorption.