Membrane-transport of sugars in diabetes mellitus.

The transport of sugars in the isolated small intestine of diabetic rats was examined. It was found earlier (Csaky and Fischer 1981 and 1984) that one symptom of the diabetes, hyperglycemia, sustained for at least 4 hours, causes a marked enhancement of the mucosal-to-serosal flux of glucose, galactose and 3-0-methylglucose. Based on the finding that the enhanced sugar flux was inhibited by phloretin but not by phlorizin and was completely abolished by the protein-synthesis inhibitor, cycloheximide, the theory was proposed that sustained maintenance of high blood sugar induces the synthesis of new sugar carrier sites which are mostly likely localized in the basolateral membrane. In the present study it was found that sustained hyperglycemia significantly enhances the mucosal-to-serosal flux of 2-deoxy-glucose (2DG) but does not alter the flux of alpha-methylglucoside (alpha MG). As it is known that in the small intestine alpha MG is preferentially transported in the brush border while 2DG is a preferred substrate for the basolateral membrane, the present findings corroborate the theory that the enhancement of the intestinal sugar transport produced by sustained hyperglycemia is localized in the basolateral membrane. A working hypothesis is proposed that the high blood sugar sensing receptor localized in the basolateral membrane is identical with the transport receptor (carrier).

Intestinal transport of di-isopropylidene-glucose.

Based on a previous observation, it was postulated that dimethylsulfoxide (DMSO) acts as a carrier-model in the intestinal absorption of glucose and galactose (Csáky and Ho, 1966). It was further hypothetized that DMSO forms a loosely-bonded hydrophobic complex with the hexoses. The co-valently-bonded di-isopropylidene-glucose (DAG) displays a structure similar to that of the hypothetical DMSO-glucose complex. Therefore, in the course of the present study the intestinal absorption of the DAG was examined. Three criteria were used to determine whether DAG utilizes the glucose carrier or is absorbed by simple diffusion: the absorption kinetics, inhibition of the transport of glucose by DAG, and inhibition of the DAG absorption by phlorizin. By all three criteria DAG does not utilize the glucose carrier, but is absorbed from the intestine by simple diffusion similarly to the hypothetical DMSO-glucose complex. The possibility of a DAG produced disturbance in the epithelial membrane is suggested by the enhanced absorption of glucose by DAG and by a previous observation that it produces anesthesia (Csáky, 1952).

Factors regulating the exchange of nutrients and drugs between lymph and blood in the small intestine.

An experimental design was developed to study the factors regulating the absorption of nutrients and drugs into the intestinal lymphatic system in normal and cirrhotic rats. Test compounds given via in situ jejunal luminal perfusion or systemic intravenous infusion were measured simultaneously in intestinal lymph, portal venous plasma and intestinal perfusate. Steady-state lymph plasma concentration ratios were used to compare the permeability characteristics of the intestinal mucosal-lymph and blood-lymph barriers. The absorption of compounds from the interstitial space into the intestinal lymphatic system is determined primarily by their permeability through the capillary of the portal circulation and by their lipid solubility; if a compound is too large to be absorbed into the portal circulation it is absorbed into the intestinal lymphatics.

Effects of ketohexosemia on the ketohexose transport in the small intestine of rats.

It was observed previously (Cs aky , T.Z. and Fischer, E. (1981) Diabetes 30, 568-574), that sustained hyperglycemia enhances the intestinal transport of aldohexoses ; on the other hand, hyperfructosemia affects primarily the transport of fructose. The present study examines in detail the hyperketosemia -induced intestinal ketose transport. Intravenously infused 3-O- methylfructose produces marked 3-O- methylfructosemia without concomitant hyperglycemia; in such animals the intestinal transport of both fructose and 3-O- methylfructose increased. The hyperketosemia -induced increased ketose transport was inhibited by phloretin but only if placed on the serosal compartment. Phlorizin affects neither the basal nor the induced intestinal ketohexose transport. The enhancement of the intestinal ketohexose transport is not sodium-dependent and is not inhibited by ouabain.

Intestinal sugar transport in experimental diabetes.

The increased sugar transport was examined in the isolated small intestine of streptozotocin-diabetic rats. In the small intestine of these animals, the rate of glucose absorption in vivo is slightly increased, but not that of galactose of 3-O-MG (3-O-Methylglucose); however, in the isolated small intestine, the mucosal-to-serosal but not the serosal-to-mucosal flux of glucose, galactose, and 3-O-MG is increased. The enhanced sugar transport is due neither to the direct toxic effect of streptozotocin nor to a lack of circulating insulin. It is not the result of an increased intraepithelial sugar metabolism. Hyperglycemia, produced gy i.v. glucose infusion, generates the same increase of the intestinal sugar transport as experimental diabetes but the high blood sugar has to be maintained for 4 h before the intestinal effect appears. Hyperglycemia and hypergalactosemia enhance the intestinal transport of glucose, galactose, and 3-O-MG, but not that of fructose; the transport of the latter is increased by hyperfructosemia. The enhanced intestinal sugar transport produced by high blood sugar inhibited by phloretin but not by phlorizin and is completely estimated in cycloheximide-treated animals. It is proposed that sustained high blood sugar induces the synthesis of new carrier sites which are most likely located in the basolateral membrane.

Seasonal variation in the active transporting ability and in the membrane ATPase activity of the frog intestinal epithelium.

The active sugar and amino acid transport in the small intestine of the American leopard frog (Rana pipiens) and a species of European frog (Rana esculenta) decreases during the winter months. Parallel with this the (Na+, K+)-stimulated ("pump") ATPase activity is markedly depressed. No seasonal changes are observed in the intestine of the tropical bullfrog (Rana catesbeiana). It is assumed that the low pump-ATPase activity is caused by the hibernation of the frogs living in moderate or subtropical areas and is connected to a biological clock. The decreased active transport of non-electrolytes appears to be a consequence of the change of the ATPase activity.

Water absorption and swelling in the rat small intestine in vitro.

Net mucosal-to-serosal water transport and water retention within the tissue were simultaneously measured in the isolated, everted rat jejunum and ileum along with time. The swelling of the epithelium (delta w/DW-delta w/DW-delta v/DW) precedes the absorption of fluid and then it remains contant throughout the experiment. The water uptake, determined as a weight increase, (delta w/DW) and the net water transport determined as a serosal volume increase (delta v/DW) were higher in Krebs-Ringer-bicarbonate than in a Krebs-Ringer-phosphate medium. In the jejunum glucose markedly stimulated both the tissue retention and net transport of water, but such effect was not seen in the ileum. Replacement of chloride with sulphate in the medium diminishes both water transport and retention, but both were markedly stimulated by the addition of glucose to such medium. As a conclusion it seems that during water transport the epithelial layer swells and the swelling increases when the amount of water transported is increased.

Induction of an intestinal epithelial sugar transport system by high blood sugar.

Intravenous infusion of glucose produces a marked increase of the mucosal-to-serosal sugar flux in the isolated everted small intestine of rats. The phenomenon is partially substrate specific, is inhibited by phloretin but not by phlorizin and is completely abolished by cycloheximide. The results suggest that sustained hyperglycemia may stimulate the synthesis of new transport receptors (carriers).

Changes in the blood supply of the gastrointestinal tract in rats with age.

Regional blood flow of the gastrointestinal (GI) tract and cardiac output have been determined in male Sprague-Dawley rats weighing from 30-726 g. The cardiac output (ml/min per kg) was highest in rats weighing 80-100 g. In heavier rats the cardiac output decreased proportionally with the body weight. The gradient of blood flow to the different parts of the GI tract develops step by step. In the weaning period the blood flow (ml/min per g tissue) through the stomach was less than that through the distal parts of the GI tract. However, the blood flow through the small intestine, cecum and large intestine was uniform at this age. In rats weighing 80-100 g the blood flow through both the cecum and large intestine was less than that through the small intestine. The gradient in blood flow through the various segments of small intestine developed last.

Subepithelial capillary blood flow estimated from blood-to-lumen flux of barbital in ileum of rats.

The outflux of barbital from the blood into the small intestine perfused with an isosmotic buffer of pH 9.5 in anesthetized rats was measured to determine the subepithelial capillary blood flow in the gut. It was shown that barbital clearance is blood flow limited when total blood flow to the small intestine varied between 0.3 and 1.3 ml/min per g wet gut. The barbital clearance amounted to an average of 50.3% of the total blood flow. The total mucosal blood flow determined by the use of the distribution of microspheres in the layers of gut wall was 62.76% of the total blood flow. It is concluded that, because of anatomical reasons, a subepithelial blood flow available for the transport process is somewhat less than the measured total mucosal blood flow.