Hyperglycemia and net transintestinal glucose and sodium transport in the rat

Effect of insulin induced hypoglycaemia on in vitro uptake of 3-O-methylglucose by rat jejunum

Acute hypoglycaemia (less than 2.0 mmol/l) selectively increased in vitro 3-O-methylglucose uptake by rat jejunum. In contrast, uptake of 3H-phenylalanine or 59Fe (III) was unchanged in the hypoglycaemic animals. The hypoglycaemia was accompanied by decreased tissue adenosine nucleotide energy charge. The increased uptake was phlorrhidzin-inhibitable and therefore reflected increased Na+ dependent glucose carrier activity. Cycloheximide did not block the increased 3-O-methylglucose uptake, implying that the mechanism is not the result of increased de novo carrier protein synthesis.

Effect of hyperglycaemia on sugar transport in the isolated mucosa of guinea-pig small intestine

The effect of hyperglycaemia on sugar transport was studied by comparing transepithelial permeation and tissue content of 3-O-methyl-D-glucose (3-O-MG), beta-methyl-D-glucoside (beta-MDG) and D-glucose in isolated mucosae of guinea-pig jejunum mounted in a flux chamber. Sugars were administered either to the luminal or the blood side of mucosae prepared either from normal animals or those maintained in a hyperglycaemic state by I.V. glucose infusion for 12 h. In control animals, absorptive sugar fluxes increased in the order glucose greater than beta-MDG greater than 3-O-MG. Only beta-MDG was accumulated in the tissue beyond the medium concentration. Permeation of 3-O-MG and beta-MDG in the direction blood-to-lumen was mainly paracellular as indicated by the strict correlation with the simultaneous permeation of polyethylene glycol (mol. wt. 900). Luminal addition of 10(-3) M-phlorhizin increased permeation and decreased tissue content of beta-MDG and D-glucose when administered on the blood side, suggesting that these sugars are recaptured at the brush border even from vigorously mixed solutions. For flux coefficient calculation the preparation was regarded as a three-compartment system. With all three sugars, the influx coefficient was higher at the luminal, but lower at the basolateral membrane than the corresponding efflux coefficient. 3-O-MG displayed the highest basolateral influx coefficient of all three sugars, being even higher than its luminal influx coefficient. The luminal influx coefficient of beta-MDG was 22 times greater, and its basolateral efflux coefficient 2.5 times less than the corresponding values for 3-O-MG, resulting in cellular beta-MDG accumulation. D-Glucose was suited best for transepithelial transport, having a luminal influx coefficient only 1.6 times less, and a basolateral efflux coefficient almost 10 times greater than those for beta-MDG. Prolonged hyperglycaemia increased the lumen-to-blood permeation of all three sugars 1.3-2-fold. No significant differences in tissue content to control values were observed after 45 min (3-O-MG, D-glucose) or 90 min (beta-MDG) incubation. Therefore, flux coefficients increased by the same factors in luminal and basolateral membranes, i.e. 1.7, 1.3 and 1.7 for 3-O-MG, beta-MDG and D-glucose, respectively. These results indicate that changes in both the luminal and basolateral membranes play a role in the increase of sugar transport in hyperglycaemia and that a regulatory mechanism might exist between the transport systems located in both membranes.(ABSTRACT TRUNCATED AT 400 WORDS)

Time dependent and other possible changes of the intracellular D-glucose and Na+ concentration in the cortical tissue of rabbit kidney perfused "in vitro"

The mean intracellular D-glucose concentration (Gi), the ratio between Gi and the serosal extracellular concentration (Gi/Ge) and the cell Na+ (Nai+) were calculated on cortex slices at increasing times after their excision from two groups of kidneys perfused with blood containing 18.1 +/- 0.4 and 23.1 +/- 2.0 mmol.l-1 plasma glucose concentration. The distribution ratios Gi/Ge, always lower than 1, were substantially unchanged up to at least 50s after removal of the tissue from the organ. Between these time limits the calculated Gi and Gi/Ge does not seem to be affected by a rapid glucose breakdown in the isolated tissue. Neither such low values depend on a degrading of the intraextracellular Na+ concentration gradient, since this gradient seems to remain unchanged throughout the same time. In another group of experiments, in which phloretin was added to the perfusing blood (mean 0.6 X 10(-3) mol.l-1) at different glucose levels, the mean values of Gi/Ge in the cortical tissue were higher (in many cases greater than 1) than those obtained in a group of control kidneys perfused in absence of phloretin. According to the presented data, the mean Gi and Gi/Ge values, as calculated by us, seem to correspond to those existing in the functioning tissue just before its excision. The already suggested view that the low values of Gi and Gi/Ge could depend on the possible existence of a mechanism actively extruding glucose from the basolateral membrane of tubular cells seems to be reinforced.

In vivo and in vitro sugar transport in frog intestine

In the frog intestine, both in vitro and in vivo, experiments were carried out in order to increase knowledge of the mechanism of sugar exit across the basolateral membrane of the enterocyte. The frog intestine was chosen because it lacks crypt cells and, consequently, any external fluid circuit mechanism during sugar transport can be avoided. Therefore, the sugar concentration in the absorbate collected on the serosal side is likely to be similar to that present underneath the basolateral membrane of the enterocyte. Under this condition, cell and absorbate sugar concentrations are similar; yet there is a concomitant net transintestinal sugar transport. Moreover, in in vivo experiments a net transintestinal sugar transport takes place even against a concentration difference. These results suggest that sugar exit across the basolateral membrane is not simply due to a chemically facilitated diffusion.

d-Glucose balance in the enterocyte of rat jejunum "in vitro"

An everted sac of male albino rat jejunum (Wistar strain) incubated "in vitro" in used. Net d-glucose and Na+ transport together with d-glucose concentration in the emerging fluid [5], or in ths serosal fluid, and in the enterocytes are determined. Cell d-glucose concentration does not change significantly in a range between 20-200 mumoles or between 50-500 mumoles of net d-glucose transepithelial transport, depending on the experimental conditions. As far as cellular d-glucose and Na+ concentration is concerned, the enterocyte behaves as an homeostatic system. The mechanism involved in d-glucose extrusion is extensively discussed. Two hypothesis seem to be possible. First, the mechanism is an active metabolically dependent one, just as it is for sodium transport. Second, the metabolic activity favours d-glucose facilitated permeability through the basolateral membrane in such a way as to maintain a constant relationship between d-glucose and Na-extrusion, notwithstanding the fact that d-glucose concentration gradient across the basolateral membrane lowers by increasing Na and glucose extrusion rate.