Effect of vasoactive intestinal peptide, somatostatin, neurotensin, cholecystokinin octapeptide, and secretin on intestinal absorption of amino acid in rat

Amino acids in the rat intestinal lumen regulate their own absorption from a distant intestinal site

Intestinal nutrient transport is altered in response to changes in dietary conditions and luminal substrate level. It is not clear, however, whether an amino acid in the intestinal lumen can acutely affect its own absorption from a distant site. Our aim is to study the effect of an amino acid present in rat small intestinal segment on its own absorption from a proximal or distal site and elucidate the underlying mechanisms. The effect of instillation of alanine (Ala) in either jejunum or ileum on its own absorption at ileal or jejunal level was examined in vivo. The modulation of this intestinal regulatory loop by the following interventions was studied: tetrodotoxin (TTX) added to Ala, subdiaphragmatic vagotomy, chemical ablation of capsaicin-sensitive primary afferent (CSPA) fibers, and IV administration of calcitonin gene-related peptide (CGRP) antagonist. In addition, the kinetics of jejunal Ala absorption and the importance of Na+-dependent transport were studied in vitro after instilling Ala in the ileum. Basal jejunal Ala absorption [0.198 +/- 0.018 micromol x cm(-1) x 20 min(-1) (means +/- SD)] was significantly decreased with the instillation of 20 mM Ala in the ileum or in an adjacent distal jejunal segment (0.12 +/- 0.015; P < 0.0001 and 0.138 +/- 0.014; P < 0.002, respectively). Comparable inhibition was observed in the presence of proline in the ileum. Moreover, basal Ala absorption from the ileum (0.169 +/- 0.025) was significantly decreased by the presence of 20 mM Ala in the jejunum (0.103 +/- 0.027; P < 0.01). The inhibitory effect on jejunal Ala absorption was abolished by TTX, subdiaphragmatic vagotomy, neonatal capsaicin treatment, and CGRP antagonism. In vitro studies showed that Ala in the ileum affects Na+-mediated transport and increases K(m) without affecting Vmax. Intraluminal amino acids control their own absorption from a distant part of the intestine, by affecting the affinity of the Na+-mediated Ala transporter, through a neuronal mechanism that involves CSPA and CGRP.

Effects of vasoactive intestinal polypeptide microinjected into the nucleus tractus solitarius on jejunal electrolytes absorption in rats

It has been shown that vasoactive intestinal polypeptide (VIP) injected into the nucleus tractus solitarius inhibits alanine absorption across the jejunum. The aim of the present study was to investigate the effects of VIP injection into the nucleus tractus solitarius on jejunal absorption of electrolytes in the rat. Fifty-three Wistar rats were submitted to midline laparotomy to expose and isolate 20 cm of jejunal loop and to perform a subdiaphragmatic troncular vagotomy. Saline or VIP (10 pg 100 nl(-1)) was injected into the rostral nucleus tractus solitarius using a stereotaxic instrument. Tyrode solution, pH 8, containing twice glucose, sodium and potassium concentration was infused (0.5 ml min(-1)) into the jejunal loop. Samples were taken at 10-min intervals during the 40-min-experiment. Injection of VIP into the nucleus tractus solitarius increased jejunal potassium absorption. Moreover, VIP associated with vagotomy resulted in inhibition of jejunal potassium absorption by VIP alone at 40 min after perfusion (5.99 +/- 0.74 vs. 9.83 +/- 0.57 microM). There was no change in jejunal sodium absorption in any of the experimental groups. VIP had a modulatory action on jejunal potassium absorption when injected into the nucleus tractus solitarius.

Effects of vasoactive intestinal polypeptide microinjected into the nucleus tractus solitarius on jejunal glucose absorption in rats

It has been shown that vasoactive intestinal polypeptide (VIP) injected into the nucleus tractus solitarius and into the dorsal motor nucleus of the vagus inhibits alanine absorption across the jejunum. The aim of the present study was to investigate the effects of VIP injection into the nucleus tractus solitarius on jejunal absorption of glucose in the rat. Forty Wistar rats were submitted to midline laparotomy to expose and isolate 20 cm of jejunal loop and to perform a subdiaphragmatic troncular vagotomy. Saline or VIP (10 pg 100 nl(-1)) was injected into the rostral nucleus tractus solitarius using a stereotaxic instrument. Tyrode solution, pH 8, containing twice glucose, sodium, and potassium concentrations was infused (0.5 ml min(-1)) into the jejunal loop. Samples were taken at 10-min intervals during the 40-min experiment. Injection of VIP into the nucleus tractus solitarius associated with vagotomy resulted in inhibition of jejunal glucose absorption by VIP alone at 10 and 40 min after perfusion (2.75+/-0.19 vs. 3.53+/-0.29 mg). The vagal outflow tract maintained jejunal glucose absorption even when VIP was microinjected into the nucleus tractus solitarius.

Enhancement of jejunal absorption of conjugated bile acid by neurotensin in rats

BACKGROUND & AIMS

Release of neurotensin (NT) from intestines is markedly stimulated by ingested fat, and NT may facilitate lipid digestion and absorption through various actions that are not fully understood. Our recent finding that NT stimulates hepatic output of bile acids only when bile delivery to the intestine is maintained has led us to investigate the effects of NT on bile acid absorption in the rat small intestine.

METHODS

We measured the effects of intravenous infusion of NT (3-10 pmol x kg(-1) x min(-1)) on biliary recovery of (3)H-taurocholate ((3)H-TC) and (3)H-cholate administered into proximal and distal intestines or into isolated intestinal segments in situ in biliary fistula rats. To further understand the underlying mechanisms involved, the effects of NT on intestinal absorption of (3)H-D-glucose, (3)H-leucine, (14)C-antipyrine, and (51)Cr-EDTA were investigated by monitoring the absorption of radioactivity into superior mesenteric venous blood.

RESULTS

Infusion of NT, at doses that caused near physiologic increases in blood NT levels, increased biliary recovery of (3)H-TC from the jejunum (3.4-fold) and ileum (1.7-fold), but did not enhance absorption of (3)H-cholate. NT also facilitated transcellular uptake of (3)H-glucose and (3)H-leucine and increased paracellular uptake to (51)Cr-EDTA and (3)H-mannitol, but did not alter the absorption rate for (14)C-antipyrine.

CONCLUSIONS

These results indicate that NT can exert a facilitative effect on intestinal bile acid absorption and return to liver. This effect of NT may involve increases in paracellular absorption and carrier-mediated transport by mechanisms not yet identified.

Luminal and systemic signals trigger intestinal adaptation in the juvenile python

Juvenile pythons undergo large rapid upregulation of intestinal mass and intestinal transporter activities upon feeding. Because it is also easy to do surgery on pythons and to maintain them in the laboratory, we used a python model to examine signals and agents for intestinal adaptation. We surgically isolated the middle third of the small intestine from enteric continuity, leaving its mesenteric nerve and vascular supply intact. Intestinal continuity was restored by an end-to-end anastomosis between the proximal and distal thirds. Within 24 h of the snake's feeding, the reanastomosed proximal and distal segments (receiving luminal nutrients) had upregulated amino acid and glucose uptakes by up to 15-fold, had doubled intestinal mass, and thereby soon achieved total nutrient uptake capacities equal to those of the normal fed full-length intestine. At this time, however, the isolated middle segment, receiving no luminal nutrients, experienced no changes from the fasted state in either nutrient uptakes or in morphology. By 3 days postfeeding, the isolated middle segment had upregulated nutrient uptakes to the same levels as the reanastomosed proximal and distal segments, but it still lacked any appreciable morphological response. These contrasting results for the reanastomosed intestine and for the isolated middle segment suggest that luminal nutrients and/or pancreatic biliary secretions are the agents triggering rapid upregulation of transporters and of intestinal mass and that systemic nerve or hormonal signals later trigger transporter regulation but no trophic response.

Neural mediation of vasoactive intestinal polypeptide inhibitory effect on jejunal alanine absorption

It was recently shown that vasoactive intestinal polypeptide (VIP) inhibits rat jejunal alanine absorption, an effect that was significantly reduced by vagotomy. This study assesses the role of capsaicin-sensitive primary afferents (CSPA) and the myenteric plexus in the inhibition of rat jejunal alanine absorption by VIP. Continuous intravenous infusion of VIP (11.2 ng . kg-1 . min-1) reduced alanine absorption by 60% in sham control rats and by 20% in rats neonatally treated with capsaicin (P < 0.01). In in vitro experiments, VIP decreased alanine uptake by jejunal strips isolated from sham control rats in a dose-dependent manner. In the presence of 40 nM VIP, alanine uptake by full-thickness jejunal strips was reduced by 54% in sham control rats and by 25% in rats neonatally treated with capsaicin (P < 0.001). On the other hand, VIP reduced alanine uptake by mucosal scrapings by 25% in sham rats compared with 9% reduction in neonatally treated rats. Chemical ablation of the extrinsic innervation and jejunal myenteric plexuses by pretreatment with benzalkonium chloride significantly (P < 0.001) reduced basal alanine absorption and the inhibitory effect of VIP. Moreover, incubation of intestinal strips with tetrodotoxin and atropine reduced significantly (P < 0.05) the inhibitory effect of VIP on alanine absorption. These data suggest that VIP exerts its inhibitory effect on alanine absorption through the CSPA fibers and the myenteric plexus. The neuronal circuitry of this inhibitory process may involve cholinergic muscarinic mechanisms.

Serotonin-induced changes in L-leucine transport across rabbit jejunum

The aim of the present study has been to determine the effect of serotonin (5-HT) on the absorption of L-leucine across the rabbit jejunum. The results show that serotonin significantly diminishes the uptake and steady-state tissue accumulation and the mucosal to serosal flux of L-leucine. This effect does not change with previous intestinal exposure of the mucosa to the 5-HT. Serotonin does not seem to modify amino acid simple diffusion across the intestinal epithelium. The effect on the amino acid uptake is due to an inhibition of the Na(+)-dependent system of transport, mainly through a reduction of the apparent Vmax. Moreover, this hormone, added to the incubation solution, does not affect the L-leucine uptake across brush border membrane vesicles. In presence of trifluoroperazine, TMB-8 and staurosporine, the serotonin effect disappears. These results suggest an effect mediated by intracellular processes related to protein kinase C which inhibit the intestinal absorption of L-leucine.

Effects of intravenous vasoactive intestinal peptide injection on jejunal alanine absorption and gastric acid secretion in rats

The effect of intravenous vasoactive intestinal polypeptide (VIP) injection on jejunal L-alanine absorption and gastric acid secretion in the rat was investigated. Continuous intravenous VIP infusion (11.2 ng/kg per min) throughout the experimental period (160 min) produced 60% decrease in alanine absorption and 40% decrease in gastric acid secretion during the second hour of the experiment. Subdiaphragmatic vagotomy reduced alanine absorption to 91% (P > 0.05) and 71.3% (P < 0.05) of control value during the first and second hours of perfusion, respectively. VIP infusion following vagotomy elicited a reduced effect when compared to that produced by similar injections in normal rats. Gastric secretion in vagotomized rats was reduced by 40% (P < 0.05) below control. VIP infusion in vagotomized rats exerted a significant decrease (P < 0.05) of gastric acid secretion. Moreover, water absorption was decreased by almost 10% (P < 0.05) after i.v. injection of VIP and was increased by 20-24% above control value following vagotomy. However, i.v. administration of VIP following vagotomy did not elicit any further change in water absorption. It can be concluded that VIP inhibits alanine absorption and gastric acid secretion in the rat and that these inhibitory effects might be partially mediated by the vagus nerve.

Effect of immunization against somatostatin in the pregnant ewe on growth and endocrine status of the neonatal lamb

Absorption of somatostatin (SRIF) specific antibodies from colostrum of ewes actively immunized against SRIF may improve growth rate of the neonatal lamb by neutralizing the inhibitory effects of SRIF on pituitary and thyroid function. Growth and endocrine parameters in the offspring of SRIF immunized (SI) and control (C) crossbred ewes were examined. Lamb weight was recorded at birth and twice each week to 24 days of age. Blood samples were collected prior to first suckle and twice each week. At 21 to 24 days of age, in separate experiments, lambs were infused with glucose (0.29 g/kg), arginine (0.25 g/kg) or thyrotropin-releasing hormone (TRH; 0.33 microgram/kg). A strong correlation (R = 0.88; P less than .01) was observed between anti-SRIF titre in the ewe at parturition and in the lamb at 3 days of age. No effect on lamb birth weight (SI 4.28 +/- 0.27 kg; C 4.35 +/- 0.23 kg) was observed. At 24 days of age cumulative gain in SI lambs (5.4 +/- 0.32 kg) was greater (P less than .05) than in C lambs (4.5 +/- 0.32 kg). The growth hormone secretory responses to glucose or arginine were not affected by treatment. Plasma IGF-I, plasma thyroxine (T4) and the plasma thyrotropin and T4 responses to TRH were not different between treatments. Plasma triiodothyronine (T3) was higher (P less than .05) in SI (2.46 +/- .10 ng/ml) than in C (2.01 +/- .05 ng/ml) lambs, however, the plasma T3 response to TRH was lower in SI lambs. Plasma glucose (mg/dl) was higher (P less than .05) in SI (118.4 +/- 1.7) than in C (106.0 +/- 4.0) lambs. Plasma insulin was not affected by treatment. Increased plasma T3 and glucose concentrations during SRIF immunoneutralization in the neonate lamb may be important factors contributing to the growth response observed.

beta-Casomorphins alter the intestinal accumulation of L-leucine

Everted sacs of the rat jejunum change the accumulation of [3H]leucine when beta-casomorphins (BCMs) or synthetic analogs, in a concentration range of 10(-8) mol/l, are coincubated with the amino acid. BCM5 (BCM fragment 1-5, Tyr-Pro-Phe-Pro-Gly) and [D-Ala2]-BCM5-NH2 (Tyr-D-Ala-Phe-Pro-Gly) increase, whereas [D-Pro4]-BCM5 (Tyr-Pro-Phe-D-Pro-Gly) decreases the leucine accumulation and [Arg8]-vasopressin has no effect. No effect of BCM5 could be observed on the accumulation of the space marker [14C]inulin. Specific binding sites for casomorphins were detected microautoradiographically, exclusively at the epithelial cell layer using [3H][D-Pro4]-BCM5 in competition studies as a model. HPLC analysis revealed that under the experimental conditions about 50% of the studied [D-Pro4]-BCM5 was enzymatically degraded and no intact peptide is accumulated within the samples of everted sacs. From the results we postulate a brush-border receptor contact of the BCMs which induces an alteration of the amino acid uptake. A contraluminal binding of the chemical signals is not likely, because there is no evidence for a transepithelial transport of intact BCMs. The observed effects of the BCMs demonstrate as yet unknown peptide-receptor interactions, probably at the brush-border membrane, with subsequent effects on the nutrient supply. Furthermore, the results support the general hypothesis of distinct peptide-receptor interactions in those types of epithelia in which the cells are connected by tight junctions.