Active Sugar Transport Research Articles

The rate of absorption of glucose from isolated surviving guinea pig intestine increases with increase of the concentration of glucose in the lumen until a maximum rate is obtained. The relation between absorption rate of glucose and initial glucose concentration conforms to an equation of the Michaelis–Menten type. The apparent Km(half saturation concentration) is 7 × 10−3M. Increase of the concentration of potassium ions in the Ringer–bicarbonate solution bathing the intestine leads to an increase of the rate of glucose absorption, this being most marked with 15.6 meq./liter K+and 14 mM glucose. No such stimulating action of potassium ions is observed on glucose absorption under anaerobic conditions. The effect of increased potassium ion concentration is to accelerate the rate of transport found with low concentrations of glucose to the maximum value found with high concentrations of the sugar. Sodium ions must be present for glucose absorption to take place and omission of magnesium ions from a Ringer–bicarbonate solution, containing 15.6 meq./liter K+, brings about a decreased rate of active glucose transport. Magnesium ions are necessary for the stimulated rate of glucose absorption obtained in the presence of potassium ions. The presence of ammonium ions decreases the rate of glucose absorption. Potassium ions may be effectively replaced by rubidium ions for stimulation of glucose transport. Cesium ions do not activate. The proportion of glucose to fructose appearing in the serosal solution, when fructose is absorbed from the mucosal solution, depends on the concentration of fructose present. The proportion may be as high as 9:1 with low (7 mM) fructose concentrations; it decreases with increasing fructose concentrations. The active transport of fructose, as demonstrated by the conversion of fructose in the isolated surviving guinea pig intestine, is enhanced by the presence of potassium ions (15.6 meq./liter). The rate of transport of fructose itself is unaffected by potassium. Using radioactive glucose and fructose, it is shown that the total amount of sugar transferred through the intestine as estimated by the radioactivity appearing in the serosal solution is approximately that calculated from chemical analyses. Potassium ions have no activating action on the transport of sugars such as sorbose, mannose, and D-glucosamine, but have a marked effect on galactose transport. The results support the conclusion that potassium ions do not influence active transport of glucose, fructose, and galactose by a change of intestinal permeability to these sugars, but do so by affecting a specific phase involved in the mechanism of active transport of sugars. The presence of L-glutamine stimulates active transport of glucose, whereas that of L-glutamate tends to diminish it.

Read full abstract