Metabolism and transport of galactose by rat intestine

Post-oral appetite stimulation by sugars and nonmetabolizable sugar analogs

Post-oral sugar actions enhance the intake of and preference for sugar-rich foods, a process referred to as appetition. Here, we investigated the role of intestinal sodium glucose cotransporters (SGLTs) in sugar appetition in C57BL/6J mice using sugars and nonmetabolizable sugar analogs that differ in their affinity for SGLT1 and SGLT3. In experiments 1 and 2, food-restricted mice were trained (1 h/day) to consume a flavored saccharin solution [conditioned stimulus (CS-)] paired with intragastric (IG) self-infusions of water and a different flavored solution (CS+) paired with infusions of 8 or 12% sugars (glucose, fructose, and galactose) or sugar analogs (α-methyl-D-glucopyranoside, MDG; 3-O-methyl-D-glucopyranoside, OMG). Subsequent two-bottle CS+ vs. CS- choice tests were conducted without coinfusions. Infusions of the SGLT1 ligands glucose, galactose, MDG, and OMG stimulated CS+ licking above CS- levels. However, only glucose, MDG, and galactose conditioned significant CS+ preferences, with the SGLT3 ligands (glucose, MDG) producing the strongest preferences. Fructose, which is not a ligand for SGLTs, failed to stimulate CS+ intake or preference. Experiment 3 revealed that IG infusion of MDG+phloridzin (an SGLT1/3 antagonist) blocked MDG appetition, whereas phloridzin had minimal effects on glucose-induced appetition. However, adding phloretin (a GLUT2 antagonist) to the glucose+phloridzin infusion blocked glucose appetition. Taken together, these findings suggest that humoral signals generated by intestinal SGLT1 and SGLT3, and to a lesser degree, GLUT2, mediate post-oral sugar appetition in mice. The MDG results indicate that sugar metabolism is not essential for the post-oral intake-stimulating and preference-conditioning actions of sugars in mice.

Naphthalenemethyl ester derivative of dihydroxyhydrocinnamic acid, a component of cinnamon, increases glucose disposal by enhancing translocation of glucose transporter 4

AIMS/HYPOTHESIS

Cinnamon extracts have anti-diabetic effects. Phenolic acids, including hydrocinnamic acids, were identified as major components of cinnamon extracts. Against this background we sought to develop a new anti-diabetic compound using derivatives of hydroxycinnamic acids purified from cinnamon.

METHODS

We purified hydroxycinnamic acids from cinnamon, synthesised a series of derivatives, and screened them for glucose transport activity in vitro. We then selected the compound with the highest glucose transport activity in epididymal adipocytes isolated from male Sprague-Dawley rats in vitro, tested it for glucose-lowering activity in vivo, and studied the mechanisms involved.

RESULTS

A naphthalenemethyl ester of 3,4-dihydroxyhydrocinnamic acid (DHH105) showed the highest glucose transport activity in vitro. Treatment of streptozotocin-induced diabetic C57BL/6 mice and spontaneously diabetic ob/ob mice with DHH105 decreased blood glucose levels to near normoglycaemia. Further studies revealed that DHH105 increased the maximum speed of glucose transport and the translocation of glucose transporter 4 (GLUT4, now known as solute carrier family 2 [facilitated glucose transporter], member 4 [SLC2A4]) in adipocytes, resulting in increased glucose uptake. In addition, DHH105 enhanced phosphorylation of the insulin receptor-beta subunit and insulin receptor substrate-1 in adipocytes, both in vitro and in vivo. This resulted in the activation of phosphatidylinositol 3-kinase and Akt/protein kinase B, contributing to the translocation of GLUT4 to the plasma membrane.

CONCLUSIONS/INTERPRETATION

We conclude that DHH105 lowers blood glucose levels through the enhancement of glucose transport, mediated by an increase in insulin-receptor signalling. DHH105 may be a valuable candidate for a new anti-diabetic drug.

Interactions of cephradine and cefaclor with the intestinal absorption of D-galactose

Oral cephalosporins are frequently prescribed beta-lactam antibiotics. Although it has been well established that cephalosporins compete with dipeptides for absorption in the intestine, using the same transport mechanism, little is known about the action of the drugs on the absorption of other nutrients. In this work the effect of cephradine and cefaclor on the absorption of D-galactose has been studied. Intestinal sugar uptake was measured in-vitro in pieces of intestine (50 mg) and brush-border membrane vesicles, and in-vivo in intestinal loops. Galactose uptake was inhibited by cephalosporins in a dose-related, time-dependent manner. In-vivo the inhibition appeared when the antibiotics were on the luminal side of the enterocyte and when they reached the gut from the basolateral side. Only the active transport of the sugar was modified; passive transfer did not change in the presence of cephalosporins. In brush-border membrane vesicles, cephradine and cefaclor did not alter sugar uptake in either sodium or potassium gradients. Both antibiotics non-competitively inhibited basolateral Na+,K(+)-ATPase activity. These findings show that cephradine and cefaclor inhibit the active-transport component of galactose absorption because they reduce the activity of the basolateral Na+,K(+)-ATPase.

Mutual inhibition of sugars and amino acid intestinal absorption

1. Intestinal absorption of sugars shows interaction with amino acids and vice versa. 2. The percentage of inhibition induced by a substrate depends on the type of substrate and the substance affected. 3. The mutual inhibition of monosaccharides and amino acids can be explained by five different hypotheses. 4. The most accepted hypotheses are the "allosteric interaction" and the "accelerated efflux". 5. From the results in the literature it is difficult to decide which of the two hypotheses is the more accurate.

Absorption kinetics of some carbohydrates in conscious pigs. 2. Quantitative aspects

Concentrations of reducing sugars, glucose, fructose and lactic acid in blood obtained from arterial and portal catheters were measured together with the portal hepatic blood flow-rate for periods of 8-24 h in twenty-six unanaesthetized pigs (mean body-weight 51 kg). The animals received experimental meals containing different amounts (100-1600 g) of different sugars (glucose fifteen meals, sucrose twenty-four meals, lactose fourteen meals, maize starch nineteen meals) together with a protein-mineral-vitamin mixture (150 g) 6-8 after implantation of the catheters and an electromagnetic flow probe. Because the portal blood flow pattern did not differ between test meals, net absorption followed the same trends as for porto-arterial concentration differences (R erat et al. 1984). Apart from lactose, the amounts of reducing sugars appearing in the portal vein correlated with the intake of the test meal, but the absorption pattern was different for each sugar. The appearance of reducing sugars was faster and earlier after intake of glucose than after sucrose and the same was the case for sucrose relative to maize starch. The differences between the three carbohydrates tended to increase with the level of intake. With a test meal containing 1 kg carbohydrate, i.e. a normal meal in a 50 kg pig, digestion of sucrose and maize starch was not finished 8 h after the meal since only 60 and 52% respectively of their hydrolysis products were recovered in the portal blood. In the case of lactose, the amounts of reducing sugars appearing in the portal blood were always very small and constant (113-118 g within 8 h) whatever the level of intake, i.e. 30 and 15% of their hydrolysis products for intakes of 400 and 800 g respectively. Depending on the carbohydrate ingested, the uptake of glucose by the gut cell wall ranged from 14 to 21 g/h and the production of lactic acid from 2.5 to 3.5 g/h.

Effect of age-dependent changes on jejunal and ileal amino acid-sugar interaction in vivo

The effects of L-leucine on jejunal and ileal D-galactose absorption have been studied at three different ages in rats using a perfusion system in vivo. Both, D-galactose and L-leucine have been perfused through the jejunum and the ileum at two concentrations, 5mM (low) and 20 mM (high). L-leucine induced a reversible inhibition of D-galactose absorption at the three ages studied and in both intestinal segments. The inhibition was higher at 20 mM when carriers were saturated. A lineal correlation exits between the inhibition percentage and the age of animals, increasing with growth in the jejunum and decreasing in the ileum. Attention is called to the possibility of different transport mechanisms at low concentration (5 mM), one of low affinity in the jejunum and the other of high affinity in the ileum.

Influence of viscous incubation media on the resistance to diffusion of the intestinal unstirred water layer in vitro

Everted sacs of rat intestine were incubated in media made viscous with Na-carboxymethylcellulose (CMC). This increased the resistance to diffusion of the mucosal unstirred water layer, as measured by the half-time for the establishment of osmotic streaming potentials. The increase ws not however linearly related to viscosity, but reached a maximum, equivalent to an apparent thickness of 500 mu, at a viscosity of about 100 cP (shear rate = 50 s-1). The inhibition of maltose hydrolysis by CMC conformed to theoretical expectations, and was characterised by an increased apparent Km, with no change in Vmax. This was not true however of galactose transport by everted sacs which showed a decline in maximum transport rate, with no apparent increase in Km. It appears that increased unstirred layer resistance may induce partial anoxia in the everted sac preparation and thus reduce the energy available for active transport. We conclude that the effect of enhanced viscosity is to reduce turbulent flow, and hence stirring, in the intervillous compartment. Such an effect may be brought about by viscous components of the diet and may influence rates of absorption and digestion in the intact intestine in vivo.