The effects of feeding glucose, galactose, and alpha-methyl glucoside on the kinetics of absorption of these sugars in the jejunum of the fasting rat were assessed by recently developed chemical and electrical methods for characterizing the electrogenic active transfer mechanisms in vivo. The parameters of 'apparent Km' (an index of carrier affinity) and Vmax (an index of the maximum jejunal transfer capacity) were obtained from the kinetic data to describe quantitatively the active transport mechanisms in rats fed their normal solid diet, in rats deprived of solid food for three days, and in rats deprived of solid food but allowed ad libitum access to isotonic solutions of glucose, galactose, or alpha-methyl glucoside for up to three days. Feeding the sugars to fasting rats produced complex differential effects on the 'apparent Km's' and Vmax's of the various transport mechanisms. The changes, although complex, can best be explained by alterations in the transport mechanisms per se rather than by non-specific changes in intestinal morphology or metabolism. The differential effects induced by the feeding sugars are further evidence for the concept that jejunal enterocytes possess multiple carriers or mechansims for the absorption of actively transported sugars in vivo. The multiple forms of the sugar carriers (isocarriers?) MAY BE INVOLVED IN THE FACULTATIVE RESPONSES OF THE ENTEROCYTES TO CHANGES IN THE DIETARY LEVELS OF SUGAR AND MAY BE IMPORTANT IN UNDERSTANDING THE INFLUENCES OF DISEASE AND NUTRITIONAL STATUS ON ABSORPTION PROCESSES IN MAN.
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