Adaptation of glucose transport across rat enterocyte basolateral membrane in response to altered dietary carbohydrate intake.

1. The effect of changes in the carbohydrate content of the diet on D-glucose transport across the basolateral membrane of rat enterocytes has been compared with alterations in transport across the brush-border membrane. 2. Measurement of carrier-mediated D-glucose uptake across the jejunal brush border from animals fed a low- or high-carbohydrate diet showed a change in the maximal rate of transport by 7 days which was maintained for 14 days. The low-carbohydrate diet produced a progressive decline in uptake whereas the high-carbohydrate diet increased the transport. There was no alteration in the apparent affinity constant as a result of the dietary manipulations and no discernible trend for changes in the passive permeability to glucose. 3. Transport of D-glucose across the basolateral membrane was also affected by the dietary composition. After 7 days the maximal transport rate was greater in the animals fed the high-carbohydrate diet. However, while this increase was maintained for 14 days, uptake into vesicles prepared after 2 weeks on the low-carbohydrate diet showed a return to control levels. 4. A detailed analysis of the time course of these responses showed the effect on basolateral membrane transport to be inducible within 3 days of switching from the low- to the high-carbohydrate diet and could be reversed within a similar period. 5. Kinetic studies using purified basolateral membrane vesicles confirmed that the change in transport was the result of an increase in the maximal transport rate. Analysis of cytochalasin B binding to these membranes showed a parallel change in the number of glucose-inhibitable binding sites. 6. The component of the diet responsible for these changes was further investigated by replacing the glucose in the high-carbohydrate food with galactose, fructose, mannose or 3-O-methylglucose. Only glucose and fructose produced any significant change in the transport across the basolateral membrane. 7. It is concluded that in response to changes in the carbohydrate content of the diet there are alterations in the capacity for glucose transport across the basolateral membrane of the enterocyte as well as in the brush-border membrane. The change in transport across the basolateral membrane is best explained by an increase in the number of glucose carriers in this membrane.