Antisecretory effects of somatostatin and vasopressin in the rat colon descendens in vitro

Role of enteroendocrine L-cells in arginine vasopressin-mediated inhibition of colonic anion secretion

Key points

Arginine vasopressin (AVP) stimulates the release of enteroendocrine L‐cell derived hormones glucagon‐like peptide‐1 (GLP‐1) and peptide YY (PYY) in vitro from mouse and human colons.

This is mediated by the AVP receptor 1B, which is highly enriched in colonic L‐cells and linked to the elevation of L‐cell calcium and cAMP concentrations.

By means of Ussing chambers, we show that AVP reduced colonic anion secretion, although this was blocked by a specific neuropeptide Y receptor Y1 receptor antagonist, suggesting that L‐cell‐released PYY acts locally on the epithelium to modulate fluid balance.

In human serum samples, PYY concentrations were higher in samples with raised osmolality and copeptin (surrogate marker for AVP).

These findings describe, for the first time, the role of L‐cells in AVP regulated intestinal fluid secretion, potentially linking together hormonal control of blood volume and blood glucose levels, and thus adding to our understanding of the complex pathways involved in the gut hormonal response to different stimuli.

Abstract

Arginine vasopressin (AVP) regulates fluid balance and blood pressure via AVP receptor (AVPR)2 in the kidney and AVP receptor 1A in vascular smooth muscle. Its role in intestinal function has received less attention. We hypothesized that enteroendocrine L‐cells producing glucagon‐like peptide 1 (GLP‐1) and peptide YY (PYY) may be a target of AVP and contribute to the control of fluid balance. Avpr1b expression was assessed by quantitative RT‐PCR on flourescence‐activated cell sorting‐isolated L‐ and control cells and was enriched in colonic L‐cells. AVP stimulated GLP‐1 and PYY release from primary cultured murine and human colonic cells and was associated with elevated calcium and cAMP concentrations in L‐cells as measured in cultures from GLU‐Cre/ROSA26‐GCaMP3 and GLU‐Epac2camps mice. An antagonist of AVPR1B reduced AVP‐triggered hormone secretion from murine and human cells. In Ussing chambers, basolaterally applied AVP reduced colonic anion secretion and this effect was blocked by a specific neuropeptide Y receptor Y1 (NPY1R) antagonist. In human serum, PYY concentrations were higher in samples with raised osmolality or copeptin (a surrogate marker for AVP). In conclusion, we propose that AVP activates L‐cell AVPR1B, causing GLP‐1 and PYY secretion. PYY in turn reduces colonic anion secretion via epithelial NPY1R. Our data suggest L‐cells are active players in the hypothalamic control of intestinal fluid homeostasis, providing a potential link between the regulation of blood volume/pressure/osmolality and blood glucose.

Arginine vasopressin (AVP) stimulates the release of enteroendocrine L‐cell derived hormones glucagon‐like peptide‐1 (GLP‐1) and peptide YY (PYY) in vitro from mouse and human colons.

This is mediated by the AVP receptor 1B, which is highly enriched in colonic L‐cells and linked to the elevation of L‐cell calcium and cAMP concentrations.

By means of Ussing chambers, we show that AVP reduced colonic anion secretion, although this was blocked by a specific neuropeptide Y receptor Y1 receptor antagonist, suggesting that L‐cell‐released PYY acts locally on the epithelium to modulate fluid balance.

In human serum samples, PYY concentrations were higher in samples with raised osmolality and copeptin (surrogate marker for AVP).

These findings describe, for the first time, the role of L‐cells in AVP regulated intestinal fluid secretion, potentially linking together hormonal control of blood volume and blood glucose levels, and thus adding to our understanding of the complex pathways involved in the gut hormonal response to different stimuli.

Vasopressin receptor mRNA expression in the human gastrointestinal tract

BACKGROUND/AIM

Vasopressin and oxytocin are closely related peptides, and both exert effects on the gastrointestinal function. In the present study, we wanted to map the expression of vasopressin receptor mRNAs (V1a, V1b/V3, and V2) in nontumorous tissue biopsy specimens of human gastrointestinal tract and surrounding tissues.

METHODS

Total and polyA+ RNAs were isolated from human tissue biopsy specimens using an automated nucleic acid extractor and, subsequently, converted into single-stranded cDNA. Semi-nested PCR amplifications were carried out, using gene-specific V1a, V1b/V3, and V2 receptor primers. The PCR amplicons were partially sequenced to confirm their identity.

RESULTS

The present study demonstrated the expression of vasopressin receptor mRNAs in human gastrointestinal tract, pancreas, kidney, lung, brain, and ovary. The expression pattern varied between different parts of the gastrointestinal tract. In the colon ascendens, V1a receptor mRNA expression could not be detected in 3 out of 4 analyzed tissue biopsy specimens. On the other hand, all the vasopressin receptor mRNAs were expressed in all colon transversum biopsy samples.

CONCLUSIONS

V1a, V1b/V3, and V2 receptor mRNAs are widely expressed throughout human gastrointestinal tract and surrounding tissues. The data obtained provide information for further mapping and determination of the physiological role of the vasopressin receptor mRNA expression in normal and tumorous tissues.

A functional comparison of recombinant and native somatostatin sst2 receptor variants in epithelia

BACKGROUND AND PURPOSE

Somatostatin (SRIF-14) exerts broad spectrum antisecretory effects by activating the somatostatin 2 (sst(2)) receptor. The rat (r) sst(2) receptor exists in 'long' (sst(2a)) and 'short' (sst(2b)) forms that differ in their C termini, while a single human (h) sst(2a) exists. This study compares the characteristics of recombinant rsst(2a), rsst(2b) and hsst(2a) activation in human epithelia, and with native sst(2) responses in rat colon.

EXPERIMENTAL APPROACH

Epithelial layers of each clone or rat colon were placed in Ussing chambers and short-circuit current (I (SC)) measured in response to SRIF-14 and chosen analogues. The relative potencies and ability to cause desensitization to SRIF-14 were assessed, and the affinities of the sst(2) antagonist, D-Tyr(8) CYN154806 for hsst(2a), rsst(2a) and native rat colon sst(2) receptors were established.

KEY RESULTS

Basolateral SRIF-14 responses were transient in hsst(2a) and rsst(2a) epithelia, but prolonged in rsst(2b)-expressing cells. Activation of rsst(2a) resulted in significant desensitization to SRIF-14 and receptor phosphorylation, whereas the rsst(2b) receptor did neither. Sst(2)-preferred agonists (BIM23190C and BIM23027) reduced I (sc) with similar potency and both caused complete desensitization to SRIF-14. CYN154806 antagonized hsst(2a) and rsst(2a) receptors with pK (B) values of 7.9 and 7.8, respectively. In rat colon mucosa, CYN154806 blocked SRIF-14 responses with a pA (2) value of 8.2, and BIM23190C responses with a pK (B) of 8.4.

CONCLUSIONS AND IMPLICATIONS

SRIF-14 caused rapid rsst(2a) receptor phosphorylation and desensitization of epithelial antisecretory responses, neither of which occurred with the rsst(2b) receptor. These mechanisms are most likely to be a prerequisite for sensitivity to sst(2)-analogues with radiotherapeutic potential.

Effect of somatostatin on electrogenic ion transport in the duodenum and colon of the mouse, Mus domesticus

In this study, we have used the mouse intestine and the Ussing short circuit technique to compare the effects and mechanism of action of somatostatin (SST, 0.1 microM) on cAMP- and Ca(2+)-mediated ion secretion in the duodenum and colon of the Swiss-Webster mouse. The cAMP-dependent secretagogues, prostaglandin E(2) (1 microM) and dibutyryl-cAMP (150 microM) increased short circuit current (I(sc)) in both regions, but only the colonic response was inhibited by SST. This inhibition was independent of enteric nerves, suggesting a direct action on the epithelial cells. The Ca(2+)-dependent secretagogue carbachol (10 microM) stimulated a transient increase in I(sc) in both intestinal segments. In the duodenum, SST partially inhibited this increase in I(sc) and both the responses to carbachol and SST were independent of enteric nerves. In the colon, while SST inhibited the carbachol induced increase in I(sc), pre-treatment with tetrodotoxin (750 nM) profoundly inhibited the carbachol induced increase in I(sc), thus markedly reducing the inhibitory effect of SST. This indicates an involvement of the enteric nervous system in the response to carbachol and the action of SST in the colon. These data indicate marked regional differences within the mouse intestine of the effects of SST on ion secretion and demonstrate different mechanisms of action of SST in the duodenum and colon.

Somatostatin peptides inhibit basolateral potassium channels in human colonic crypts

Somatostatin is a powerful inhibitor of intestinal Cl(-) secretion. We used patch-clamp recording techniques to investigate the effects of somatostatin on low-conductance (23-pS) K(+) channels in the basolateral membrane of human colonic crypts, which are an important component of the Cl(-) secretory process. Somatostatin (2 microM) elicited a >80% decrease in "spontaneous" K(+) channel activity in cell-attached patches in nonstimulated crypts (50% inhibition = approximately 8 min), which was voltage-independent and was prevented by pretreating crypts for 18 h with pertussis toxin (200 ng/ml), implicating a G protein-dependent mechanism. In crypts stimulated with 100-200 microM dibutyryl cAMP, 2 microM somatostatin and its synthetic analog octreotide (2 microM) both produced similar degrees of K(+) channel inhibition to that seen in nonstimulated crypts, which was also present under low-Cl(-) (5 mM) conditions. In addition, 2 microM somatostatin abolished the increase in K(+) channel activity stimulated by 2 microM thapsigargin but had no effect on the thapsigargin-stimulated rise in intracellular Ca(2+). These results indicate that somatostatin peptides inhibit 23-pS basolateral K(+) channels in human colonic crypt cells via a G protein-dependent mechanism, which may result in loss of the channel's inherent Ca(2+) sensitivity.

Role of the vasopressin V(1) receptor in regulating the epithelial functions of the guinea pig distal colon

Vasopressin has a wide spectrum of biological action. In this study, the role of vasopressin in regulating electrolyte transport in the colon was elucidated by measuring the short-circuit current (I(sc)) as well as the Na(+), K(+), and Cl(-) flux in a chamber-mounted mucosal sheet. The cytosolic Ca(2+) concentration ([Ca(2+)](i)) was also measured in fura 2-loaded cells by fluorescence imaging. Serosal vasopressin decreased I(sc) at 10(-9) M and increased I(sc) at 10(-7)-10(-6) M. The decrease in I(sc) was accompanied by two effects: one was a decrease in the amiloride-sensitive Na(+) absorption, whereas the other was an increase in the bumetanide-sensitive K(+) secretion. The increase in I(sc) was accompanied by an increase in the Cl(-) secretion that can be inhibited by serosal bumetanide or mucosal diphenylamine-2-carboxylate. Vasopressin caused an increase in [Ca(2+)](i) in crypt cells. These responses of I(sc) and the [Ca(2+)](i) increase in crypt cells were all more potently inhibited by the vasopressin V(1) receptor antagonist than by the V(2) receptor antagonist. These results suggest that vasopressin inhibits electrogenic Na(+) absorption and stimulates electrogenic K(+) and Cl(-) secretion. In all of these responses, the V(1) receptor is involved, and the [Ca(2+)](i) increase may play an important role.

Effects of pituitary adenylate cyclase activating polypeptide-27 (PACAP) and vasoactive intestinal polypeptide (VIP) on chloride in HT29 cells studied by X-ray microanalysis

The colon cancer cell line HT29 is a useful model to study intestinal chloride secretion. These cells have both cAMP-activated and calcium-activated chloride channels. Changes in elemental content of the cells after stimulation with agonists were determined by X-ray microanalysis in the scanning or scanning transmission electron microscope. Exposure of HT29 cells to pituitary adenylate cyclase activating polypeptide-27 (PACAP) caused a transient decrease in the cellular Cl and K concentrations, indicating (net) efflux of chloride. The effect of PACAP is inhibited by somatostatin, which is known to inhibit cAMP-activated as well as calcium-activated chloride secretion and by U-73122, an inhibitor of phospholipase C. Alloxan, an inhibitor of adenylate cyclase, did not significantly affect the PACAP-induced loss of chloride. The calcium-chelating agent EGTA inhibited the PACAP-induced loss of chloride, indicating the need for extracellular calcium ions. Also vasointestinal polypeptide (VIP) caused a decrease of the cellular chloride concentration in HT29 cells. VIP-induced loss of chloride could be inhibited by pre-treating the cells with somatostatin or UK14,304, an alpha-2 adrenergic agonist that has been shown previously to inhibit purinergically activated chloride efflux. Our results indicate that there is cross-talk between the cAMP- and the calcium-activated pathways for chloride secretion in HT29 cells.

Somatostatin receptors mediating inhibition of basal and stimulated electrogenic ion transport in rat isolated distal colonic mucosa

The aim of this study was to examine the potencies of several recently identified selective somatostatin (SRIF)-receptor ligands as inhibitors of electrogenic ion transport in the rat distal colonic mucosa with the view to identifying the SRIF receptor type involved. Under basal conditions, cumulative administration of SRIF and SRIF28 decreased short circuit current (SCC), a measure of electrogenic ion transport, with EC50 values of 4 nM and 9 nM respectively. The peptidase inhibitors, phosphoramidon (1 microM) and amastatin (10 microM), has no effect on the potencies of either SRIF or SRIF28. The inhibitory action of SRIF on basal SCC was suppressed by piretanide and diphenylamine-2-carboxylate, compatible with the assumption that the Na+K+2Cl- co-transporter and Cl- channels, respectively, may be involved in this antisecretory action of SRIF. Tetrodotoxin (1 microM) had no effect on the antisecretory action of SRIF, suggesting that the process was not neuronally mediated. All of the SRIF analogues examined, with the exception of BIM-23056, maximally inhibited basal SCC to a similar extent as SRIF. Seglitide and octreotide were both more potent antisecretory agents than SRIF (respective EC50 values, 0.4 nM and 1.5 nM) suggesting that this effect was mediated by a receptor belonging to the SRIF1 receptor group. The most distinguishing feature of the rank order of agonist potencies was the high potency of the selective sst2 receptor ligand, BIM-23027 (EC50 value 0.32 nM), the weaker potency exhibited by the selective sst5 receptor ligand, L-362855 (EC50 value 21 nM), and the lack of agonist activity displayed by the selective sst3 receptor ligand, BIM-23056 (EC50 value > 1000 nM). This profile is comparable with that observed in binding studies on the recombinant sst2 receptor. Forskolin-stimulated secretion was suppressed by SRIF analogues with the rank order of agonist potencies BIM-23027 > SRIF > L-362855 >> BIM-23056 which resembled that exhibited under basal conditions. However, the absolute potencies of these agonists were lower (respective EC50 values 2 nM, 14 nM< 38 nM and > 1000 nM) whilst the magnitude of inhibition was about three fold greater. BIM-23027 and SRIF (both 30 nM) also inhibited carbachol-stimulated increases in basal SCC by 60-70%, while a similar concentration of L-362855 inhibited these responses by 11%. BIM-23056 (1 microM) had no effect on carbachol-simulated secretion. Radioligand binding studies on rat colonic mucosal membranes using [125I]-Tyr11-SRIF suggested heterogeneity of SRIF binding sites. Thus, SRIF and SRIF28 competed for binding (IC50 values, 0.32 and 0.63 nM, respectively) with Hill slopes less than unity; while seglitide and BIM-23027 both maximally displaced only 30-40% of specific binding with apparent high affinity (respective pIC50 values, 10.1 nM and 10.0). In conclusion, SRIF decreases basal as well as both cAMP and Ca(2+)-dependent Cl- secretion in rat colonic mucosa. The rank order of agonist potencies suggests that receptors resembling the recombinant sst2 receptor mediate inhibition of basal and forskolin-stimulated secretion. Radioligand binding studies suggest that BIM-23027 interacts with a sub-population of [125I]Tyr11-SRIF binding sites in rat colonic mucosal membranes which probably corresponds to the receptors mediating the antisecretory effects described here.

The effect of neuropeptide Y on sodium, chloride and potassium transport across the rat distal colon

1. Neuropeptide Y (NPY; 10(-10)-10(-7) mol l-1) reduced basal short-circuit current (Isc) in a concentration-dependent manner in the rat distal colon but was ineffective in the proximal colon. 2. The action of NPY was dependent upon the presence of Cl- and HCO3- anions and was blocked by prior treatment of the tissue with a Cl- channel blocker. The decrease in Isc was associated with an increase in mucosa-to-serosa fluxes of Na+, Rb+ (K+) and Cl-, whereas the serosa-to-mucosa flux of Cl- was decreased. 3. The size of the inhibitory NPY effect was linearly correlated with the height of the basal Isc, i.e. it inhibited 55% of basal secretory Isc. 4. The action of NPY was unaffected by indomethacin and tetrodotoxin, when given alone, but was abolished, when the basal Isc was decreased to values near zero by a combination of both inhibitors. This inhibition could be overcome by restoring basal Isc with prostaglandin E2, indicating that the effect of NPY is not mediated by nerves or prostaglandins, but that NPY is only effective, when anion secretion is stimulated by the spontaneous release of neurotransmitters and prostaglandins. 5. NPY inhibited the increase in Isc induced by veratridine and prostaglandin E2, but it had no effect on the Isc induced by direct stimulation of the adenylate cyclase with forskolin, or on Isc induced by stimulation of the Ca(2+)-pathway with carbachol. Inhibition of the response to veratridine or prostaglandin E2 by NPY showed the same dependence on the height of the ISC just prior to addition of NPY as seen in control conditions, i.e. NPY inhibited 55% of cyclic AMP-mediated secretion.6. These results suggest that the effect of NPY is mediated by an inhibition of cyclic AMP-stimulated secretion, which is stimulated in the rat distal colon by a continuous release of prostaglandins and neurotransmitters.

Colonic secretory effect in response to enteral feeding in humans

Diarrhoea complicating enteral feeding is a common clinical problem affecting up to 25% of patients. Its pathogenesis remains unknown. A new technique of human in vivo segmental colonic perfusion was used to investigate colonic water and electrolyte movement in response to enteral feeding. Four groups of studies were performed in which low and high load polymeric enteral diet infusions were undertaken, either intragastrically or intraduodenally (n = 6 each group). Net absorption of sodium, chloride, and water occurred during fasting throughout the colon in all groups. There was a significant net secretion of sodium, chloride, and water in the ascending colon during low load (sodium: -42 mmol/h; 95% confidence limits -52 to -19, Chloride: -18 mmol/h; -50 to +16, water: -174 ml/h; -348 to -30) and high load (sodium: -24 mmol/h; -60 to +8, chloride: -18 mmol/h; -31 to +16, water: -120 ml/h; -246 to +6) gastric feeding, and during high load duodenal feeding (sodium: -12 mmol/h; -22 to -6, chloride; -6 mmol/h; -16 to +3, water: -72 ml/h; -144 to -6). Net secretion persisted in the distal colon only during high load gastric feeding. In the other three groups there was a net absorption in the distal colon. This study identified a significant colonic secretory response to enteral feeding, which is related to the site and load of the diet infusion. This response may play an important part in the pathogenesis of enteral feeding related diarrhoea.

Multiple G-protein-dependent pathways mediate the antisecretory effects of somatostatin and clonidine in the HT29-19A colonic cell line

Using the functionally differentiated colonic cell line, HT29-19A, we have examined sites at which inhibitory G-proteins mediate the antisecretory actions of somatostatin (SST) and the alpha 2-adrenergic agonist, clonidine (CLON) at the epithelial level. Both agents caused a dose-dependent inhibition (EC50:SST 35 nM; CLON 225 nM) of Cl- secretion (assessed by changes in short circuit current) activated by cAMP-mediated agonists, PGE2 and cholera toxin. Inhibition was accompanied by a reduction in intracellular cAMP accumulation and could be blocked by pretreatment with pertussis toxin at a concentration (200 ng/ml) which activated ADP-ribosylation of a 41-kD inhibitory G protein in HT29-19A membranes. Secretion stimulated by the permeant cAMP analogue, dibutyryl cAMP, was also inhibited by SST and CLON (30-50%; P < 0.005), indicating additional inhibitory sites located distal to cAMP production. Both agents were effective inhibitors of secretion mediated through the Ca2+ signaling pathway. SST (1 microM) and CLON (10 microM) reduced the Isc response to the muscarinic agonist, carbachol, by 60-70%; inhibition was reversed in pertussis toxin-treated cells. These effects did not, however, involve inhibition of the carbachol-induced increase in cellular inositol 1,4,5-trisphosphate levels or the rise in cytosolic calcium, [Ca]i. Inhibition by SST of secretion induced by phorbol 12,13 dibutyrate but not by the calcium agonist, thapsigargin, suggests that SST may act at a distal inhibitory site in the Ca(2+)-dependent secretory process activated by protein kinase C. We conclude that SST and alpha 2-adrenergic agonists can act directly on intestinal epithelial cells to exert a comprehensive inhibition of Cl- secretion mediated through both cAMP and Ca2+/protein kinase C signaling pathways. Inhibition is mediated via pertussis toxin-sensitive G-proteins at sites located both proximal and distal to the production of second messengers.

In vivo inhibitory effect of lanreotide (BIM 23014), a new somatostatin analog, on prostaglandin- and cholera toxin-stimulated intestinal fluid in the rat

The antisecretory action of subcutaneously (SC) administered somatostatin(1-14), octreotide, and lanreotide on jejunal net flux of water under basal, prostaglandin E1 (PGE1)- and cholera toxin (CT)-stimulated secretory conditions was determined in vivo on isolated intestinal loop in anesthetized rats. Both PGE1 and CT induced intestinal hypersecretion in the rats. This secretory effect was not affected by SC administration of saline. Lanreotide (1, 10, and 100 micrograms/kg) reduced the maximal PGE1-induced secretion, while 200 micrograms/kg had no effect. Similarly, octreotide (1 and 10 micrograms/kg) and somatostatin (1-14) (0.1 and 1 microgram/kg) reduced the increase of net water flux induced by PGE1. However, higher doses of octreotide (100 and 200 micrograms/kg) and somatostatin(1-14) (10 and 100 micrograms/kg) had no effect on PGE1-induced secretion. Lanreotide, octreotide, and somatostatin(1-14) (1 and 10 micrograms/kg) abolished the maximal secretion induced by cholera toxin. However, 100 micrograms/kg of lanreotide, octreotide, and somatostatin(1-14) had no effect on cholera toxin-induced secretion. The present study shows that lanreotide, octreotide, and somatostatin(1-14) reduce the secretion induced by PGE1 and abolish that induced by CT. These effects were obtained with doses of less than 100 micrograms/kg of the products, higher doses being ineffective. The higher efficacy against CT-induced hypersecretion as compared to PGE1-induced hypersecretion suggests a direct antisecretory effect at the enterocyte level and indicates the usefulness of these products as antidiarrheal agents in nonhormonally mediated diarrhea.

Difference between the antisecretory mechanisms of opioids and the somatostatin analogue octreotide in cholera toxin-induced small intestinal secretion in the rat

The antisecretory effect of morphine and the somatostatin analogue octreotide was studied on cholera toxin-induced secretion in anaesthetized rats. Small intestinal secretion was induced with cholera toxin. Morphine (6 mg/kg b.wt.) and the somatostatin analogue octreotide (3 micrograms/kg b.wt.) reduced the cholera secretion in rats whose intestines had been subjected to sympathetic denervation. This was in contrast to the secretion elicited by helodermin which was unaffected by octreotide and morphine in the presence of nicotinic ganglionic blockade. The alpha-adrenergic receptor blocker phentolamine (1-2 mg/kg b.wt. i.v.) and the inhibitor of sympathetic transmitter release guanethidine (5 mg/kg b.wt. i.v.) abolished the antisecretory effect of morphine on the cholera secretion in contrast to the antisecretory effect of somatostatin which was unaffected by the alpha-blockade. It is proposed that the antisecretory effect of morphine and octreotide on cholera toxin-induced secretion was conducted at a step prior to the activation of the secretory epithelium and that the antisecretory effect of morphine was mediated indirectly by interaction with sympathetic nerve terminals in the intestine. The findings are consistent with a model where octreotide and morphine inhibit the nervous secreto-motor reflex activated by the cholera toxin.

Segmental heterogeneity of the rat colon in the response to activators of secretion on the cAMP-, the cGMP- and the Ca(2+)-pathway

The electrolyte transport was compared in proximal and distal segments of the rat colon under control conditions and after induction of secretion on the cAMP-, the cGMP- and the Ca(2+)-pathway. Baseline short-circuit current was decreased by indomethacin and tetrodotoxin in the distal colon, indicating a spontaneous production of neuronally acting prostaglandins. In contrast, baseline short-circuit current in the proximal colon was decreased only by indomethacin, but not by tetrodotoxin. Unidirectional flux measurements revealed that in the distal colon sodium and chloride were absorbed, while the proximal colon secreted chloride. A morphological comparison between the distal and proximal epithelium revealed that the zonulae occludentes and the microvilli were longer in the distal colon. The size of the Golgi apparatus was several times larger in the crypt than in the surface region without differences between proximal and distal colon. Distal segments were more sensitive to an activator of the Ca(2+)-pathway, carbachol, or activators of the cAMP-pathway such as forskolin and a cAMP-analogue. In contrast, the activation of the cGMP-pathway by a cGMP-analogue or by the heat-stable enterotoxin of E. coli (STa) was more effective in the proximal colon. The results give evidence for a segmental specificity with regard to the intracellular pathways responsible for the activation of secretion.

Treatment of endocrine and nonendocrine secretory diarrheal states with Sandostatin

Endocrine tumors of the gastroenteropancreatic (GEP) axis elaborate excessive amounts of peptides that are potent intestinal secretagogues. The actions of these peptides on intestinal transport of water and electrolytes lead to the accumulation of fluid in the intestinal lumen and diarrhea. One of the most clinically relevant secretagogues is vasoactive intestinal polypeptide (VIP). Other relevant secretagogues elaborated from tumors are serotonin, prostaglandins, and kinins. Sandostatin (octreotide, Sandoz, Basle, Switzerland), a long-acting octapeptide analog of somatostatin, inhibits experimentally induced intestinal secretion and has been used successfully to treat patients with secretory diarrhea refractory to other pharmacotherapy. The effective dose is in the range of 50 to 200 micrograms, given subcutaneously two or three times daily. The mechanism for the inhibitory effect on secretion is not clearly understood but it appears to involve inhibition of the adenylate cyclase-cyclic adenosine monophosphate system as well as interference with calcium as an intercellular mediator of enterocyte secretion. A particularly interesting use of this drug has been to treat the watery diarrhea seen in patients with acquired immunodeficiency syndrome. It is also effective in other types of secretory diarrhea not associated with endocrine tumors. These include diabetic diarrhea, idiopathic secretory diarrhea of infancy, and high output ileostomy diarrhea.

The antisecretory effects of somatostatin and analogues in rat descending colon mucosa

Somatostatin-14 (SS-14) and somatostatin-28 (SS-28) produce concentration dependent reductions in short-circuit current in rat colonic mucosa. EC50 values of 15.0 and 13.3 nM were obtained for SS-14 and SS-28 respectively while the N-terminal fragments of SS-28, namely somatostatin-(1-12) (SS1-12) and somatostatin-(1-14) (SS1-14) were inactive. Cyclo(Pro-Phe-D-Trp-Lys-Thr-Phe) and cyclo(Pro-Tyr-D-Trp-Lys-Thr-Phe) were potent antisecretory peptides, like SS-14 and SS-28; while the putative somatostatin antagonist, cyclo(7-aminoheptanoyl-Phe-D-Trp-Lys-Thr[Bzl]) exhibited neither agonist nor antagonist effects. Responses to SS-14 could be regulated by agents which affected the secretory state of the epithelium. Antisecretory effects of SS-14 were markedly attenuated by piroxicam and were restored following piroxicam plus either forskolin or vasoactive intestinal polypeptide (VIP). SS-14 also attenuated secretory responses produced by carbachol, substance P (SP), VIP and alpha- and beta-calcitonin gene related peptide (alpha-, beta-CGRP). Therefore, SS-14 exhibits broad spectrum antisecretory effects in rat descending colon mucosa.

Octreotide inhibits increases in short-circuit current induced in rat colon by VIP, substance P, serotonin and aminophylline

We investigated the effect of octreotide (OCT), a stable somatostatin analog, (OCT) on changes in short-circuit current (Isc) induced by vasoactive intestinal peptide (VIP), aminophylline, serotonin (5-HT) and substance P. OCT significantly decreased basal Isc at a concentration of 10(-9) M; the maximum decrease in Isc was observed at 10(-6) M. OCT (10(-7) M) significantly inhibited the intestinal secretory response to all the secretagogues studied. The maximum Isc response was reduced when tissues were stimulated with VIP (184.9 +/- 18.0 vs. 119.7 +/- 14.1, P less than 0.05), 5-HT (135.1 +/- 14.4 vs. 79.5 +/- 13.4, P less than 0.05) and substance P (156.0 +/- 19.2 vs. 30.7 +/- 5.4, P less than 0.01). In the case of aminophylline, the concentration-response curve was shifted to the right but the maximum response was not reduced. Because VIP and aminophylline increase cAMP while 5-HT and substance P stimulate intestinal secretion principally by a calcium linked mechanism, we conclude that OCT inhibits Isc in rat colon by more than one mechanism.

Cholinergic-mediated secretion in the rat colon: neuronal and epithelial muscarinic responses

The acetylcholine receptor agonists, acetylcholine (10(-5)-10(-4 M), carbachol (5 x 10(-6)-5 x 10(-5) M), bethanechol (5 x 10(-5)-5 x 10(-4) M) and dimethylphenylpiperazinium (DMPP, 10(-5) M) increased the short-circuit current (Isc) in the rat colon descendens by a tetrodotoxin (TTX)-sensitive mechanism. Blockade by TTX was still observed after removal of the submucosa, indicating the involvement of neurons of the mucosal plexus. Hexamethonium (10(-5) M) and atropine (10(-6) M) were used to distinguish between nicotinic and muscarinic neuronally mediated effects. The inhibitor of choline uptake, hemicholinium-3 (1 mM), reversibly inhibited the effect of repeated electric field stimulation (EFS). The EFS response was only inhibited by high concentrations of atropine (greater than or equal to 10(-5) M). In mucosa-submucosa preparations 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP) was more effective than telenzepine whereas pirenzepine was ineffective. Pirenzepine inhibited the EFS response in mucosa preparations as did telenzepine and 4-DAMP. It was not possible to differentiate between the muscarinic receptors involved in the different parts of the enteric nervous system on the basis of our results.