Effect of ionic environment on intestinal sugar transport

Abstract

Glucose is absorbed from the intestine by two different mechanisms, diffusion and active transport. The former is effected by a concentration difference between the lumen and the blood; the latter proceeds against a concentration gradient. Active transport hastens the absorption up to a certain limit of concentration and causes the glucose to be completely absorbed even if the concentration in the lumen falls below that in the blood. Whereas the experimental approach to diffusion is relatively simple and clear-cut, active transport must be studied in 'living' preparations such as surviving isolated loops of intestine (Fisher & Parsons, 1949; Darlington & Quastel, 1953; Wilson & Wiseman, 1954). In such a preparation the sugar can be present in identical initial concentrations on both serosal and mucosal sides: an increase in concentration of sugar on either side, not accounted for through changes in volume, indicates that active transport is involved. This kind of experimental approach in mammalian gut has several drawbacks. One of them is the quantitative recovery of the sugar. This is complicated by the rapid metabolism of part of the glucose by the surviving gut tissue (Fisher & Parsons, 1950). Instead of glucose, however, 3-0-methylglucose (3-methylglucose) can be used. This sugar behaves like glucose in regard to intestinal absorption (Csaky, 1942), but since it is not metabolized (CsAky & Wilson, 1956; CsAky & Glenn, 1957), it can be quantitatively recovered. Another tiresome factor in connexion with the study of sugar transport in surviving mammalian gut is that the preparation has to be kept at body temperature and well supplied with oxygen. This difficulty can be eliminated by the use of the intestine of a poikilothermic animal, which transports both glucose and 3-methylglucose at room temperature and requires less vigorous oxygenation (Csaky & Fernald, 1960).