Modulation of intestinal urea cycle by dietary spermine in suckling rat

Argininosuccinate synthetase, an ubiquitous enzyme in mammals, catalyses the formation of argininosuccinate, the precursor of arginine. Arginine is recognised as an essential amino acid in foetuses and neonates, but also as a conditionally essential amino acid in adults. Argininosuccinate synthetase is initially expressed in enterocytes during the developmental period, it disappeared from this organ then appeared in the kidneys. Although the importance of both intestinal and renal argininosuccinate synthetases has been recognised for a long time, nutrients have not yet been identified as inducers of the gene expression. In the context of a proteomic screening of intestinal modifications induced by dietary spermine in suckling rats, we showed that argininosuccinate synthetase and carbamoyl phosphate synthase disappeared from enterocytes after this treatment. The disappearance of argininosuccinate synthetase in small intestine was confirmed by immunodetection. Expression of carbamoyl phosphate synthase and argininosuccinate synthetase coding genes decreased also after spermine administration. Expression of other urea cycle enzyme coding genes was modulated by spermine administration: argininosuccinate lyase decreased and arginase increased. Our results fit with the developmental variation of argininosuccinate synthetase and carbamoyl phosphate synthase. Modulation of the gene expression for several urea cycle enzymes suggests a coordination between all the pathway steps and switch toward polyamine (or proline and glutamate) biosynthesis from ornithine.     

Epidermal growth factor accelerates the intestinal cessation of macromolecular transmission in the suckling rat

1. The effects of EGF administered subcutaneously on the intestinal cessation of macromolecular transmission and sucrase development were investigated in suckling rats and compared with those on hydrocortisone-treated pups. 2. In the EGF-treated pups, intestinal absorptive response of IgG was suppressed 50% whereas, the sucrase activity was not affected. In the hydrocortisone-treated pups, the absorptive response was inhibited completely, while sucrase activity was induced precociously. 3. The characteristics of intestinal cessation was morphologically observed at the jejunal epithelial cells in EGF and hydrocortisone-treated pups. 4. These results suggest that EGF affects the maturation of gastrointestinal function in a manner different from that of glucocorticoid hormones.     

Electron microscope autoradiographic study of intestinal absorption of decanoic and octanoic acids in the rat

Intestinal absorption of [3H]octanoic acid and [3H]decanoic acid was investigated in the rat by electron microscope autoradiography. The common duct (bile and pancreatic common duct) of the rats was diverted and a loop of the duodenum was cannulated 24 h later. The lipid mixture to be investigated was introduced into each experimental loop, and after 15 min or less the loop was removed. One part of each loop was used to determine the distribution of radioactivity in different lipid fractions, and an autoradiographic study was performed on the other part of the loop. Radioactivity distribution studies confirmed that medium chain fatty acids are absorbed in their nonesterified form and established that these fatty acids are absorbed much more rapidly than oleic acid. Autoradiographic studies indicated that the medium chain fatty acids are taken up in a molecular or aggregate molecular form, leave the epithelial cells by way of the lateral plasma membrane, and are next found in the blood capillaries. Our results suggest that the Golgi complex does not play an important role in the absorption of unesterified fatty acids.     

Effect of ethanol on intestinal lipid absorption in the rat

The effect of ethanol infusion on intestinal lipid absorption was studied in rats with a duodenal cannula. Rats were infused with ethanol overnight and ethanol was included in a trioleoylglycerol emulsion infusion given for 3 hr the next day. These rats were compared to control animals infused with glucose (isocalorically). The ethanol-infused rats had a greatly impaired lipid absorptive capacity. The monoacylglycerol and free fatty acid contents in the intestinal lumen in the ethanol-infused rats were 4- and 7-fold greater, respectively, than controls. The inhibition of absorption was not due to an effect of ethanol on lipolytic activity. The lipase content of the ethanol-infused rats was greater than controls and the separate infusion of monoacylglycerol and fatty acids demonstrated impaired absorption of these end products of lipolysis as compared to controls. To observe if these changes were due to an effect of ethanol on the enterocyte brush border membrane, the membrane lipids were analyzed. The phosphatidylcholine, lysophosphatidylcholine, and phosphatidylethanolanine content was reduced but not the neutral lipids, sphingomyelin, or phosphatidylserine. The uptake of fatty acid into intestinal rings was also shown to be impaired by ethanol infusion. Lastly, the specific activity of the neutral lipids remaining in the intestinal lumen after [3H]glycerol-labeled trioleoylglycerol-infusion was similar to controls even though the mass was much greater. It is concluded that ethanol impairs neutral lipid absorption due to an effect on the enterocyte brush border membrane and by increasing the efflux of low specific activity lipid from the enterocyte back out into the intestinal lumen. A potential pathway for this efflux is the recently described increased porosity of the apical junctional complex in response to ethanol infusion.     

Diet-induced adaptation of intestinal fructose absorption in the rat.

The effect of dietary sucrose, fructose and glucose on the intestinal absorption of fructose and glucose was investigated in adult rats in vivo: Glucose absorption was not affected by the type of dietary carbohydrate, while the absorption of fructose was increased by the ingestion of the sucrose or fructose diet, as compared with the glucose diet. An almost maximal increase of fructose absorption was already observed when the quarter of the total dietary carbohydrates was replaced by fructose. Faecal fructose elimination declined during the feeding experiment. The enhanced intestinal absorption of the fructose load in rats fed the fructose diet was manifested by higher concentrations of fructose, but also of glucose and lactate in the hepatic portal blood.     

Non-catalytic role of carbonic anhydrase in rat intestinal absorption

Carbonic anhydrase (CA) inhibition reduces NaCl absorption in rat distal ileum, a pH-sensitive, low CA activity tissue, and in distal colon, a CO(2)-sensitive, high CA activity tissue. We hypothesized that CA plays a non-catalytic role in NaCl absorption in these segments. Unidirectional fluxes of Na(+) and Cl(-), and total HCO(3)(-) generation (estimated as the sum of radiolabeled HCO(3)(-) and CO(2) produced from glucose) were measured in Ussing chambers in nominally CO(2), HCO(3)(-)-free HEPES Ringer. Measurements were made in the presence and absence of 0.1 mM methazolamide, a membrane-permeant CA inhibitor. Ringer pH reduction from 7.6 to 7.1 stimulated ileal but not colonic Na(+) and Cl(-) absorption. In the ileum, methazolamide reduced J(ms)(Na) and J(ms)(Cl) and caused net Cl(-) secretion at pH 7.6, and prevented the stimulatory effect of lowering pH. In the colon, methazolamide reduced Na(+) and Cl(-) absorption at pH 7.6. Total HCO(3)(-) generation was minimal in HEPES at pH 7.6 and 7.1 in both segments, was minimally affected by methazolamide, and did not account for the changes in Cl(-) absorption caused by pH or methazolamide. We conclude that CA plays a role in ileal and colonic NaCl absorption independent of its catalytic function.     

The kinetics of intestinal calcium absorption in the rat: an analytical and model building study

The experimental data obtained from in vivo single pass perfusion of duodenal, jejunal, and ileal intestinal segments of 33- and 50-day-old rats have been used to test a series of models for calcium absorption. Each model was checked for the statistical validity and goodness-of-fit with the experimental data. The model adopted for the duodenum and jejunum had two major components, one saturable and the other nonsaturable, and a minor secretory component. This model was not applicable to ileal calcium absorption. Here the secretory component appeared to be much more important, and the absorption parameters varied in such a manner as to suggest that this intestinal segment was capable of short term autoregulation of dietary calcium absorption.     

Effect of somatostatin on intestinal absorption of nutrients the rat

Effects of somatostatin on absorption of D-glucose, L-leucine and triacylglycerols by the small intestine were studied in rats after treatment with the peptide in vivo and in everted jejunal segments in vitro.Absorption of glucose was not affected in vitro by somatostatin or the analogue [D-Trp⁸, D-Cys¹⁴]somatostatin at concentrations up to 0.006 mM. Addition of various peptidase inhibitors had no influence, suggesting that failure of somatostatins to inhibit absorption was not due to inactivation by peptidases. Glucose absorption in vitro by jejunum from rats treated with high doses of somatostatin in vivo was not different from that of untreated rats. The biguanide phenformin inhibited glucose absorption, whether added in vitro (IC₅₀ ≈ 1 mM) of after treatment in vivo (3–100 mg/kg per os). The blood glucose increase following oral glucose administration in fasted rats was not affected by somatostatin, but significantly suppressed by phenformin.Absorption of leucine in vitro was not affected by somatostatin (up to 0.03 mM) or [D-Trp⁸, D-Cys¹⁴]somatostatin (0.01 mM), but inhibited by phenformin (IC₅₀ = 2 mM).Absorption of acylglycerols (glycerol tri[1-¹⁴C]oleate) administered orally was significantly inhibited by somatostatin (twice 5 mg/kg subcutaneously) and phenformin (100 mg/kg per os).In rats — apparently in contrast to man — somatostatin does not decrease role of somatostatin in carbohydrate absorption remains controversial. Investigations in healthy [9] and diabetic [20] human subjects suggest that the peptide inhibits (directly or indirectly) the intestinal absorption of glucose in man. On the other hand, our results and those of others obtained in experiments in rats [4,11,21] and Rhesus monkeys [7] clearly do not support such a role in these species. Further studies are therefore needed to resolve this problem.     

Effect of undernutrition and hormone treatments on the absorption of proteins in suckling rat intestine.

The absorption of 125I-labeled BSA and gamma-globulin was significantly (P less than 0.01) elevated in UN pups compared to the controls. Administration of pharmacological doses of cortisone, thyroxine, and insulin markedly (P less than 0.001) reduced the absorption of BSA and gamma-globulin in UN pups. There was no significant difference in the binding of 125I-labeled BSA and gamma-globulin to microvillus membrane in the control and experimental animals. However, the degradation of labeled BSA and gamma-globulin by luminal content was considerably higher (55-70%) in controls compared to UN pups. This suggested that observed increase in the absorption of proteins in nutritionally deprived pups was unrelated to their binding to the microvillus surface but presumably it is a consequence of reduced luminal degradation together with delayed maturational development as suggested by the pattern of brush border enzymes in the UN intestinal tissue.     

Transport of glycyl-L-proline in intestinal brush-border membrane vesicles of the suckling rat: characteristics and maturation

Transport of the dipeptide glycine-L-proline (Gly-L-Pro) in the developing intestine of suckling rats and its subsequent maturation in adult rats was examined using the brush-border membrane vesicles (BBMV) technique. Uptake of Gly-L-Pro by BBMV was mainly the result of transport into the intravesicular space with little binding to membrane surfaces. Transport of Gly-L-Pro in BBMV of suckling rats was: (1) Na+ independent; (2) pH dependent with maximum uptake at an incubation buffer pH of 5.0; (3) saturable as a function of concentration (apparent Km = 21.5 +/- 7.9 mM, Vmax = 8.6 +/- 1.5 nmol/mg protein per 10 s); (4) inhibited by other di- and tripeptides; and (5) stimulated and inhibited by inducing a negative and positive intravesicular membrane electrical potential, respectively. Similarly, transport of Gly-L-Pro in intestinal BBMV of adult rats was saturable as a function of concentration (apparent Km = 17.4 +/- 8.6 mM, Vmax = 9.1 +/- 2.1 nmol/mg protein per 10 s) and was stimulated and inhibited by inducing a relatively negative and positive intravesicular membrane potential, respectively. No difference in the transport kinetic parameters of Gly-L-Pro was observed in suckling and adult rats, indicating a similar activity (and/or number) and affinity of the transport carrier in the two age groups. These results demonstrate that the transport of Gly-L-Pro is by a carrier-mediated process which is fully developed at the suckling period. Furthermore, the process is H+-dependent but not Na+-dependent, electrogenic and most probably occurs by a Gly-L-Pro/H+ cotransport mechanism.