Sodium-glucose interactions in the goldfish intestine

Intestinal glucose and galactose transport in the cultured gilthead bream (Sparus aurata)

1. Electrical parameters and transepithelial glucose and galactose transport were determined in vitro across anterior and posterior intestine of the culture fish Sparus aurata. 2. Electrical potential difference (PD) and short-circuit current (Isc) were serosa-positive in anterior intestine, while they were serosa-negative or near zero in posterior intestine. 3. Tissue conductance (Gt) was higher in posterior than in anterior intestine. In both parts it was decreased when the Na ion was omitted in mucosal and serosal reservoirs. 4. Addition of glucose or galactose to the mucosal side of intestine caused an increase in PD and Isc in posterior intestine but did not significantly change PD and Isc in anterior intestine. 5. Isotopic flux of glucose and galactose measurements in short-circuit conditions showed a net active glucose and galactose absorption in posterior intestine, while in anterior intestine active transport of glucose or galactose was not observed. 6. The net transport of glucose and galactose in posterior intestine was decreased to zero in the absence of Na in mucosal and serosal reservoirs or in the presence of ouabain (1 mM) in serosal solution.

A chloride requirement for Na+-dependent amino-acid transport by brush border membrane vesicles isolated from the intestine of a Mediterranean teleost (Boops salpa)

The uptake of D-glucose, 2-aminoisobutyric acid and glycine was studied with intestinal brush border membrane vesicles of a marine herbivorous fish: Boops salpa. The uptake of these three substances is stimulated by an Na+ electrochemical gradient (Cout greater than Cin). For glucose, an increase of the electrical membrane potential generated by a concentration gradient of the liposoluble anion, SCN-, increases the Na+-dependent transport. This responsiveness to the membrane potential was confirmed by valinomycin. Differently from glucose, uptake of glycine and 2-aminoisobutyric acid requires, besides the Na+ gradient, the presence of Cl- on the external side of the vesicles. In the absence of Cl-, amino acid uptake is not stimulated by the Na+ gradient and is not influenced by an electrical membrane potential generated by SCN- gradient (Cout greater than Cin) or by a K+ diffusion potential (Cin greater than Cout). This Cl- requirement differs from the Na+ requirement, since a Cl- gradient (Cout greater than Cin) does not result in an accumulation of glycine or 2-aminoisobutyric acid similar to that produced by an Na+ gradient.

Cell volume regulation in goldfish intestinal mucosa

1. Ion and water content of goldfish intestinal mucosa, stripped free from muscular layers were measured under various incubation conditions. 2. Ouabain induces an increase in cation content that is electrically compensated for by chloride. The increase in solute content is accompanied by an increase in water content. 3. When extracellular chloride is partially replaced by sulphate, ouabain does induce cell shrinkage. 4. Anoxia induces a rapid increase in cell volume that is restored by oxygenation of the incubation solution. Ouabain prevents the restoration of volume. 5. It is concluded that the classical ouabain-sensitive Na/K pump participates in the maintenance of cellular volume. We suggest that the constancy in volume after ouabain poisoning as is reported for many tissues might be due to a low chloride conductance of its membranes. 6. Anisotonic media (range: 0.6-1.2 isotonicity), made by variation on mannitol concentration, induce changes in cell water content that deviates from the simplified van't Hoff equation by about 10%. No change in water content after the initial increase was found. 7. We conclude that goldfish enterocytes do not possess a mechanism for rapid volume readjustment.

Effects of glucose and ouabain on transepithelial electrical resistance and cell volume in stripped and unstripped goldfish intestine

1. In goldfish intestine (perfused unstripped segments and mucosal strips) the serosal addition of ouabain (10(-4) M) resulted in a vanishment of the transepithelial potential difference and in a continuous increase in transepithelial resistance. 2. Incubation of mucosal strips with ouabain resulted in an increase in sodium content which was greater than the decrease in potassium content. The resulting increase in cation content was accompanied by an increase in chloride content and an increase in water content. 3. Histological examination showed that exposure to ouabain resulted in a swelling of the epithelial layer as compared to the control situation. 4. The ouabain induced resistance increase is greater in the presence of glucose, 3-OMG or fructose than in the presence of mannitol. Phlorizin (10(-4) M) inhibits the extra resistance increase induced by mucosal glucose but is without effect on the fructose induced extra resistance increase. The initial velocity and the magnitude of the glucose induced extra resistance increase depends on the glucose concentration. 5. The results suggest that in goldfish intestine ouabain induces cellular swelling with a concomitant collapse of the lateral intercellular spaces, which is the cause of the increased transepithelial resistance. The additional changes in resistance induced by sugars suggest that the cell membrane is more permeable to glucose, 3-OMG and fructose than to mannitol. The resulting changes in osmotically active material within the epithelial cell influence the cross-sectional area and consequently the conductivity of the paracellular shunt pathway. The hypothesis that these sugars do not induce a resistance change in the absence of ouabain is discussed.

A mechanistic explanation of the effect of potassium on goldfish intestinal transport

Partial replacement of sodium by potassium or rubidium in the solution used to perfuse isolated intestinal segments of goldfish causes an increase in transmural electrical resistance. Serosal replacements have a stronger effect than mucosal replacements. A 70% inhibition of the glucose-evoked transmural electrical current is brought about by serosal replacement of 40 mM sodium by potassium. Transmural mucosal to serosal flux of 3-O-methyl-D-glucose is also strongly inhibited by serosal potassium. These inhibitory effects of potassium do not occur when the intestinal mucosa is stripped free from the intestinal muscular layers. It is concluded that potassium-induced muscular contractures cause a decrease in transport area by pressing the mucosal folds closer against each other. Certain effects of high potassium concentrations that have been reported in mammalian intestinal preparations may involve a similar mechanism.

REgional functional differentiation in the gut of the grasscarp, Ctenopharyngodon idella (Val.)

A regional differentiation--reflecting structural differences--of the intestine of larval and juvenile grasscarps can be illustrated by studying the activity of alkaline phosphatase and the uptake of orally administered horseradish peroxidase. Pinocytosis takes place in a welldefined area of about 23% of the length of the gut (segment II). Neither the rostral +/- 68% (segment I) nor the caudal +/- 9% (segment III) shows absorption of the enzyme. Alkaline phosphatase activity, mainly localized at the microvilli of the enterocytes is high in the first segment of the gut and low in the second segment. In larvae, the activity decreases sharply at the transition from segment I to segment II. The activity is weak or absent in the caudal third segment. Quantitative histochemical data are confirmed by biochemical analyses. Alkaline phosphatase activity is found all over the mucosal folds of the first segment, with relatively weak activity at the base and at the tip of the folds. This may be related to a renewal of the epithelium. Our results suggest that active absorption of digested food takes place mainly in the rostral first segment, while the uptake of macromolecules by pinocytosis is a function of the second segment. Comparison of the results with information available in literature leads to a rejection of the hypothesis that the uptake of protein macromolecules in Cyprinids is to be attributed to the absence of a stomach and therefore to an inefficient digestion of proteins.

Characteristics of changes in the intracellular potential associated with transport of neutral, dibasic and acidic amino acids in Triturus proximal tubule

(1)Introduction of L-alanine and L-lysine into the lumen of the proximal tubule of Triturus kidney evoked an immediate and sustained depolarization of the peritubular membrane potential (Epm) and a small increase in the transtubular potential (Ett). L-Aspartate had no effect. (2) The alanine-induced depolarization was absolutely dependent on the presence of Na+, whereas the lysine-induced one was partially dependent on Na+. In the absence of Na+, alanine usually evoked a transient hyperpolarization of the Epm, while lysine evoked a diffusion potential-like PD change. (3) Addition of alanine or lysine to the peritubular fluid did not cause any immediate change in the Epm, but the cells depolarized with a marked time delay. The delayed depolarization could be ascribed to the entrance of amino acids into the lumen through the nephrostromes and the paracellular pathways. (4) Cellular uptake of alanine and lysine was partially dependent on Na+, while that of aspartate was completely dependent on Na+. (5) Characteristics of the observed electrical events were explained in terms of the differences in the charge transfer associated with transport of these amino acids across the luminal membrane.

Effects of serosally added sugars on the transepithelial electrical properties of the perfused goldfish intestine

1. A study has been made of the effect of serosally added sugars on the transmural potential difference and electrical resistance of the perfused goldfish intestine. 2. Addition of glucose at the serosal side resulted in a decrease of the transmural potential difference independent of the presence or absence of glucose at the mucosal side. The transepithelial resistance did not change. 3. The serosal glucose effect persisted in the presence of phlorizin at the mucosal side. 4. With the activity transported non-metabolized glucose analogue 3-oxy-methylglucose the same effects were observed as with glucose. 5. Replacement of NaCl by cholineCl, RbCl or LiCl at both sides of the intestine had a diminishing effect on the glucose evoked potentials and on the transepithelial conductance. 6. Phlorizin in concentrations lower than 10(-4) M, at the serosal side did not influence neither the mucosal nor the serosal glucose effects. 7. Ouabain at the serosal side inhibited the serosal glucose effect and decreased the transepithelial conductance. 8. The results support the concept that sugar transport at the serosal side of the epithelial cell has features in common with the sodium-dependent sugar transport mechanism at the mucosal side.

The effect of salt adaptation on the permeability and cation selectivity of the goldfish intestinal epithelium

1. The short-term uptake of Na by the goldfish mucosa was compared using both inulin and choline as markers of extracellular space. The results were virtually identical, the distribution of both choline and inulin increasing rapidly to measure a space at 1 min which then remained nearly constant during a following 4 min incubation.2. Using inulin as space marker, the uptake of various alkali metal cations was determined from a 1 min contact with the mucosa. The relative rates of uptake were Tl > K > Rb > Cs > Na > Li, with a low selectivity ratio, the range of permeabilities being no greater than 5.3. The selectivity sequence was the same in both salt and fresh-water adapted fish. Of the alkali metal cations tested, only Na showed a significantly decreased uptake on adaptation to salt.4. Isolated intestinal preparations from salt-adapted fish showed a reduced short-circuit current compared with fish adapted to fresh water, the values being 12.3 +/- 1.2 and 35.7 +/- 1.5 muA cm(-2) respectively. In both cases the short-circuit current was equivalent to the net transport of Na measured isotopically.5. In Krebs-Henseleit medium, the measured tissue resistance was approximately 100 Omega cm(-2) for both salt and fresh-water adapted fish.6. It is concluded that regulation of cation transport in goldfish intestinal epithelium is specific for Na and mediated primarily through cellular rather than extracellular pathways.

Changes in the intestinal transport of sodium induced by exposure of goldfish to a saline environment

1. The uptake and transflux of sodium by goldfish intestines has been compared under different experimental conditions. Uptake was measured from a 1 min contact of the intestinal mucosa with radioactive sodium chloride solution. Transflux was measured over a period of 2 hr using the everted sac technique.2. Keeping goldfish in saline reduced the transflux of sodium to one quarter the value found for fresh-water fish. The uptake of sodium was halved by this treatment. Cortisol injected previously into saline-adapted fish changed neither the transflux nor the uptake of sodium measured subsequently.3. In fresh-water fish hypophysectomy reduced sodium transflux while leaving the uptake of sodium unchanged. Injection of cortisol restored sodium transflux to control levels without producing any additional effects on the uptake of sodium.4. It is suggested that adaptation to saline involves regulation of sodium movement across the microvillar membrane of the mucosal cell. Cortisol would appear to play no part in this type of regulation.5. The presence of cortisol, or possibly other steroids with similar actions, has however been shown to be essential for the normal operation of sodium transport in this tissue. It is not clear exactly how cortisol exerts this effect. What evidence there is suggests that cortisol exerts a metabolic control rather than changing directly the membrane permeability of the cell.

Sodium and sugar fluxes across the mucosal border of rabbit ileum

Unidirectional influxes of sugars and Na from muscosal solution into the cells of rabbit ileum have been examined. The influxes of glucose, galactose, and 3-0-methyl glucose (3 MG) follow Michaelis-Menten type kinetics and are markedly dependent on the presence ofNa in the mucosal solution. For 3 MG, reduction of Na concentration causes a decrease in maximal rate of influx and little change in the "apparent Michaelis constant." There appeared to be little mediated entry of 3 MG into the cells from Na-free solution. The influx of Na was increased by the presence of 3 MG in the mucosal solution and at all Na concentrations tested, there was a 1:1 ratio between sugar influx and the sugar-dependent Na influx. On the basis of these observations, a model has been developed for the sugar transport system involving a transport site that combines with both sugar and Na.

Regulation of amino acid transport across intestines of goldfish acclimatized to different environmental temperatures

1. Serosal transfers of valine and threonine were measured using everted sacs of anterior intestine taken from goldfish acclimatized to different temperatures.2. Both valine and threonine were actively transported at incubation temperatures equal to or greater than the previous environmental temperature of the fish. There was also a positive serosal transfer of valine, but not threonine, at incubation temperatures below the previous environmental temperature of the fish.3. The mean stable transmural potentials and amino-acid-evoked potentials depended both on the temperature to which the fish had been acclimatized and on the temperature at which the sacs were incubated.4. There was a linear relation between the transmural potential and the serosal transfer of amino acid, one additional mumole of valine or threonine being transferred/2 hr incubation period for each 3 mV rise in potential. There was a less obvious correlation between the amino-acid-evoked potential and on serosal transfer of amino acid.5. Acclimatization of the goldfish intestine from 8 to 25 degrees C, assessed by changes occurring in the transmural potential and serosal transfer of amino acids, tended to stabilize both parameters, but the compensation in each case was only partial.6. It is possible that the imbalance in transfer of valine-like and threonine-like amino acids, seen at incubation temperatures below the previous acclimatization temperature of the fish, has a special function in initiating the process of acclimatization to the new environmental temperature.

Time course and nature of temperature-induced changes in sodium-glucose interactions of the goldfish intestine

1. Glucose-evoked potentials measured at low incubation temperatures were found to be highly temperature dependent (phase 1), but less so at high incubation temperatures (phase 2) and acclimatization of an 8 degrees C fish to 25 degrees C resulted in the extension of phase 1 up to the environmental temperature of the fish. This change was only part of the mechanism controlling the acclimatization of sodium transport across the intestine.2. The temperature at which the glucose-evoked potential changed from phase 1 to phase 2 was approximately equal to the temperature at which glucose began to raise the steady transmural potential of the intestine.3. No changes in intestinal electrical parameters could be detected when fish, acclimatized to 8 degrees C, were heated at 25 degrees C for 15 hr, but after 20 hr at the higher temperature, acclimatization to the new temperature was complete.4. Intestines from fish acclimatized to 8 degrees C, but which had first spent 15 hr at 25 degrees C and then 10 hr at 8 degrees C, still behaved qualitatively like 8 degrees C - intestines but the magnitude of the glucose-evoked potentials was slightly reduced.5. It is suggested as a working hypothesis that acclimatization of the sodium-glucose interaction to different environmental temperatures involves the synthesis of new carrier molecules, qualitatively different from the old ones.

Influence of temperature acclimatization on sodium--glucose interactions in the goldfish intestine

1. Transmural potentials across goldfish intestines in vitro were found to depend on the acclimatization temperature of the fish. At any incubation temperature potentials were lower in fish kept previously at a high temperature, and if the transmural potentials were recorded at incubation temperatures equal to the previous acclimatization temperatures the values remained constant from 8 to 30 degrees C. The glucose-evoked potential was also reduced by previous acclimatization of the fish to a high temperature.2. As the sodium concentration was reduced the steady transmural potential increased and later fell in proportion to the low external sodium concentration, but the glucose-evoked potential fell as soon as the sodium concentration was reduced below 140 mM. Similar changes were seen with intestines taken from fish acclimatized to a high temperature but both the steady-state potential and the transitory glucose-evoked potential were more dependent on the external sodium concentration.3. The maximum glucose-evoked potential depended on the concentration of glucose used and temperature acclimatization had no significant effect on this relation. The steady potential was lower in the presence of glucose at low incubation temperatures but higher at higher incubation temperatures, and the temperature at which glucose ceased to inhibit depended on the previous acclimatization temperature. Glucose also lowered the steady potential, whatever the previous acclimatization temperature, when the external sodium concentration was low.4. The inhibitory effect of glucose on the steady potential of an intestine taken from a 30 degrees -acclimatized fish could be abolished by lowering the external concentration of glucose from 27 to 16 mM.5. Intestines taken from fish acclimatized to 3 degrees C gave variable results.6. It is concluded that sodium moves across the luminal membrane of the goldfish mucosa attached to a carrier which can exist in one of two forms. It is changes in this postulated carrier which serve to stabilize sodium transport at different acclimatization temperatures. Changes in the concentration of this postulated carrier may also occur and function in the regulation of sodium transport, particularly at acclimatization temperatures below 15 degrees C, where the switching of the carrier does not operate.