Effect of carbachol on luminal release of somatostatin from isolated perfused rat duodenum

Localization of acyl ghrelin- and des-acyl ghrelin-immunoreactive cells in the rat stomach and their responses to intragastric pH

Acyl ghrelin has a 28-amino acid sequence with O-n-octanoyl acid modification at the serine 3 position, whereas des-acyl ghrelin has no octanoyl acid modification. Although these peptides exert different physiological functions, no previous studies have shown the different localization of acyl ghrelin and des-acyl ghrelin in the stomach. Here we have developed an antibody specific for des-acyl ghrelin that does not crossreact with acyl ghrelin. Both acyl ghrelin- and des-acyl ghrelin-immunoreactive cells were distributed in the oxyntic and antral mucosa of the rat stomach, with higher density in the antral mucosa than oxyntic mucosa. Immunofluorescence double staining showed that acyl ghrelin- and des-acyl ghrelin-positive reactions overlapped in closed-type round cells, whereas des-acyl ghrelin-positive reaction was found in open-type cells in which acyl ghrelin was negative. Acyl ghrelin-/des-acyl ghrelin-positive closed-type cells contain obestatin; on the other hand, des-acyl ghrelin-positive open-type cells contain somatostatin. We measured the release of acyl ghrelin and des-acyl ghrelin in vascularly perfused rat stomach by ELISA, and the effects of different intragastric pH levels on the release of each peptide were examined. The release of des-acyl ghrelin from the perfused stomach was greater at pH 2 than at pH 4; however, the release of acyl ghrelin was not affected by intragastric pH. The present study demonstrated the differential localization of acyl ghrelin and des-acyl ghrelin in the rat stomach and their different responses to the intragastric pH.

NeuroD-betacellulin gene therapy induces islet neogenesis in the liver and reverses diabetes in mice

To explore induced islet neogenesis in the liver as a strategy for the treatment of diabetes, we used helper-dependent adenovirus (HDAD) to deliver the pancreatic duodenal homeobox-1 gene (Ipf1; also known as Pdx-1) to streptozotocin (STZ)-treated diabetic mice. HDAD is relatively nontoxic as it is devoid of genes encoding viral protein. Mice treated with HDAD-Ipf1 developed fulminant hepatitis, however, because of the exocrine-differentiating activity of Ipf1. The diabetes of STZ mice was partially reversed by HDAD-mediated transfer of NeuroD (Neurod), a factor downstream of Ipf1, and completely reversed by a combination of Neurod and betacellulin (Btc), without producing hepatitis. Treated mice were healthy and normoglycemic for the duration of the experiment (>120 d). We detected in the liver insulin and other islet-specific transcripts, including proinsulin-processing enzymes, beta-cell-specific glucokinase and sulfonylurea receptor. Immunocytochemistry detected the presence of insulin, glucagon, pancreatic polypeptide and somatostatin-producing cells organized into islet clusters; immuno-electron microscopy showed typical insulin-containing granules. Our data suggest that Neurod-Btc gene therapy is a promising regimen to induce islet neogenesis for the treatment of insulin-dependent diabetes.

Differentiation of immature enterocytes into enteroendocrine cells by Pdx1 overexpression

The development of a variety of enteroendocrine cells of the gut is poorly understood. We tested whether immature intestinal stem cells were switched to multiple enteroendocrine hormone-producing cells by in vitro transfer of a homeobox gene. We transfected the pancreatic-duodenal homeobox 1 gene (Pdx1) into IEC-6 cells, an embryonic intestinal epithelial cell line derived from a normal rat, and selected the cells that overexpressed Pdx1 by 150-fold compared with control. The cells were examined for differentiation into enteroendocrine cells by immunocytochemical and electron microscopic analyses. Transfected cells cultured on micropore filters formed a trabecular network piled up on monolayer cells. These trabecular cells showed nuclear localization of Pdx1 protein and contained well-developed rough endoplasmic reticulum as well as many secretory granules of pleomorphic shape in the cytoplasm. Antibodies against chromogranin A, serotonin, cholecystokinin, gastrin, and somatostatin stained these secretory granules in the cytoplasm. Furthermore, immunofluorescence double staining analysis showed that different hormones were produced within a cell. These results provide the evidence that immature intestinal epithelial cells can differentiate into multiple hormone-producing enteroendocrine cells in response to overexpression of Pdx1.

Mechanisms in regulating the release of serotonin from the perfused rat stomach

The mechanisms regulating the release of serotonin into the portal circulation as well as into the gastric lumen were studied in the isolated vascularly and luminally perfused rat stomach. Immunohistochemical study of the rat stomach showed that serotonin-containing enterochromaffin (EC) cells were densely packed in the antral mucosa, sparsely scattered in the corpus, and not found in the fundus. Such morphological findings suggest that serotonin detected in this study may have originated from antral EC cells. Luminal acidification stimulated the vascular release of serotonin but did not affect the luminal release of serotonin. The basal release of serotonin into the vasculature was 10 times higher than that into the gastric lumen at intragastric pH 2. The vascular release of serotonin is regulated by stimulation from cholinergic nicotinic mechanisms, whereas inhibitory neurotransmitters such as vasoactive intestinal peptide and NO are probably not involved. Somatostatin and peptide YY originating from endocrine cells may exert direct inhibitory effects, possibly via somatostatin and peptide YY receptors on the EC cells, and a cholinergic muscarinic mechanism may exert indirect effects on the vascular release of serotonin via the muscarinic receptor on the endocrine cells.

Effects of VIP and NO on the motor activity of vascularly perfused rat proximal colon

The effects of vasoactive intestinal polypeptide (VIP) and nitric oxide (NO) on the motor activity of the rat proximal colon were examined in an ex vivo model of vascularly perfused rat proximal colon. VIP reduced motor activity and this inhibitory effect was not altered by either atropine, hexamethonium, tetrodotoxin (TTX) nor TTX plus acetylcholine (ACh), but was completely antagonized by NO synthase inhibitor N(G)-nitro-L-arginine (L-NA) and by VIP receptor antagonist, VIP(10-28). These results suggest that VIP may exert a direct inhibitory effect on the motor activity of the rat proximal colon via a VIP receptor located on the smooth muscle and this effect is mediated by NO but not by cholinergic pathways. Atropine and hexamethonium reduced but ACh stimulated motor activity and the effect of ACh was not changed by TTX, suggesting that the cholinergic pathway may exert a direct stimulatory effect on motor activity. Single injection of TTX, VIP(10-28) or L-NA induced a marked increase in motor activity, suggesting that the motor activity of rat proximal colon is tonically suppressed by VIP and NO generating pathways, and elimination of inhibitory neurotransmission by TTX may induce an abnormal increase of the motor activity. The interaction between VIP and NO in regulation of motor activity was further examined by a measurement of NO release from vascularly perfused rat proximal colon. Results showed that NO release was significantly increased during infusion of VIP and this response was reversed by L-NA. These results suggest that VIP generating neurons may inhibit colonic motility by stimulating endogenous NO production in either smooth muscle cells or nerve terminals.

Peptidergic regulation of gastrointestinal motility in rodents

Peptides involved in the endocrine and enteric nervous systems as well as in the central nervous system exert concerted action on gastrointestinal motility. Mechanical and chemical stimuli which induce peptide release from the epithelial endocrine cells are the earliest step in the initiation of peristaltic activities. Gut peptides exert hormonal effects, but peptide-containing stimulatory (Ach/substance P/tachykinin) and inhibitory (VIP/PACAP/NO) neurons are also involved in the induction of ascending contraction and descending relaxation, respectively. The dorsal vagal complex (DVC), located in the medulla of the brainstem, constitutes the basic neural circuitry of vago-vagal reflex control of gastrointestinal motility. Several gut peptides act on the DVC to modify vagal cholinergic reflexes directly (PYY and PP) or indirectly via afferent fibers in the periphery (CCK and GLP-1). The DVC is also a primary site of action of many neuropeptides (such as TRH and NPY) in mediating gastrointestinal motor activities. The identification over the last few years of a number of neuropeptide systems has greatly changed the field of feeding and body weight regulation. By exploring the brain and gut systems that employ recently identified peptidergic molecules, it will be possible to elaborate on the central and peripheral pathways involved in the regulation of gastrointestinal motility.

Involvement of 5-HT3 and 5-HT4 receptors in the motor activity of isolated vascularly perfused rat duodenum

The involvement of serotonin (5-HT) receptor subtypes in motor activity of the ex vivo vascularly perfused rat duodenum was investigated. Clusters of phasic contractions (CPCs), migrating in an oral to anal direction, were obtained without any stimulation. Drug effects were evaluated by changes in different components of the pressure waves, such as motor index (MI), frequency, amplitude and duration of the CPC. The effect of 5-HT depletion on motor activity was examined in animals treated with p-chlorophenylalanine (PCPA). The MI, frequency and duration of CPC were decreased by PCPA, but the amplitude was not affected, suggesting that endogenous 5-HT may play an important role in regulation of the motor activity of the rat intestine. The importance of the 5-HT receptor subtypes in the regulation of motor activity was examined. Neither the nonselective 5-HT1 and 5-HT2 receptor antagonist, methysergide, nor the 5-HT2 receptor antagonist, ketanserin, affected motor activity. However, the 5-HT3 receptor antagonists, granisetron and azasetron, decreased percentage MI, frequency, percentage amplitude and percentage duration of CPC. The 5-HT4 receptor antagonist, SB204070, exerted both excitatory and inhibitory actions, with a higher dose (10 nM) stimulating percentage MI, frequency, percentage amplitude and percentage duration, and a lower dose (0.1 nM or 1 nM) decreasing percentage MI and percentage duration of CPC. These results suggest that endogenous 5-HT regulates the motor activity of the rat duodenum through 5-HT3 and 5-HT4 receptors, with the former mediating the stimulatory influence and the latter mediating both stimulatory and inhibitory influences.

Motor activity of vascularly perfused rat duodenum. 1. Characteristics of spontaneous movement

We developed an ex vivo model of arterially perfused rat duodenum to examine the motor activity of intestine. In this preparation, spontaneously occurring pressure waves with regular rhythm were observed. The oxygen consumption and motor activity of the intestine were compared at different arterial perfusion rates to determine the degree of oxygenation required to elicit spontaneous motility. Pressure waves with regular rhythm occurred at a frequency of 1 min-1 when the arterial perfusion was 3-5 mL min-1, and stopped when the perfusion rate fell below 2 mL min-1. Atropine and hexamethonium reduced the percentage motor index/10 min of pressure waves in a dose-dependent manner, and tetrodotoxin completely blocked motor activity. Acetylcholine stimulated motor activity, and this effect was not antagonized by TTX. These findings suggest that spontaneous contraction in the ex vivo perfused rat duodenum might be mediated by a cholinergic mechanism via muscarinic receptors on smooth muscle, but that noncholinergic mechanisms may also participate in this response.

Effect of VIP and PACAP on vascular and luminal release of serotonin from isolated perfused rat duodenum

Effects of CCK, VIP, PACAP38, and PACAP27 on the release of 5HT into the intestinal lumen and into the portal circulation were examined in in vivo experiments of isolated rat duodenum perfused vascularly and luminally. VIP, PACAP 38 and 27 reduced the release of 5HT into the lumen but did not affect the vascular release of 5HT. These effects were not affected by the presence of atropine, hexamethonium, or TTX, suggesting that VIP, PACAP 38 and 27 exert a direct inhibitory effect on the luminal release of 5HT from the EC cells. Nitric oxide synthase inhibitor, NG-nitro-L-arginine, antagonized the inhibitory effects of VIP, PACAP 38 and 27, suggesting that nitric oxide seems to be essential to exert the inhibitory action of VIP and PACAPs on the release of 5HT into the intestinal lumen from the EC cells.

Effect of VIP and PACAP on basal release of serotonin from isolated vascularly and luminally perfused rat duodenum

The effect of vasoactive intestinal polypeptide (VIP), pituitary adenylate cyclase-activating peptide-38 (PACAP-38), and PACAP-27 on the release of serotonin (5-HT) into the intestinal lumen and the portal circulation was studied by using in vivo isolated vascularly and luminally perfused rat duodenum. 5-HT levels were determined by HPLC. VIP, PACAP-38, and PACAP-27 reduced the luminal release of 5-HT but did not affect the vascular release of 5-HT. The inhibitory effect caused by VIP, PACAP-38, and PACAP-27 was not affected by either atropine, hexamethonium, TTX, or TTX plus ACh, but it was completely antagonized by the nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine (L-NNA). The VIP receptor antagonist VIP-(10-28) blocked the effects of VIP, PACAP-38, and PACAP-27. These results suggest that VIP and PACAP exert a direct inhibitory effect on the luminal release of 5-HT from the enterochromaffin (EC) cells via a common receptor site on the EC cells and that this effect is mediated by NO but not by cholinergic pathways. A single injection of TTX, atropine, or hexamethonium reduced the luminal release of 5-HT, whereas a single injection of VIP-(10-28) stimulated the luminal release of 5-HT and this effect was antagonized by atropine, hexamethonium, or TTX. These results suggest that EC cells may receive the direct innervation of cholinergic neurons as well as VIP and/or PACAP neurons, with the former exerting a tonic stimulatory influence and the latter exerting a tonic inhibitory influence on 5-HT release into the intestinal lumen.

Change in subcellular localization of gastrin-like immunoreactivity in epithelial cells of rat duodenum induced by carbachol

The present study provides morphological evidence to support the contention of exocrine secretion from duodenal gastrin-containing cells. The isolated vascularly perfused duodenal preparation with or without carbachol stimulation was used. At the end of perfusion, tissue was fixed and prepared for electron microscopic examination. For immunoelectron microscopic study for gastrin, postembedding immunogold reaction combined with preembedding DAB staining was used. In saline-treated controls, DAB reaction was restricted to the basal cytoplasm and immunogold labeling was concentrated over electron-dense cores of secretory granules packed at the basal cytoplasm. However, in carbachol-stimulated animals, immunogold labeling as well as DAB reactions were accumulated at the apical portion of the cytoplasm, suggesting that a high concentration of gastrin was involved in the apical cytoplasm. In carbachol-stimulated cells, aggregation of small vesicles was observed beneath the microvilli, and most of these vesicles had no cores but were similar in size to the basal secretory granules. Immunogold particles were diffusely scattered at the cytoplasm outside these vesicles. These findings suggest that the gastrin-like immunoreactivity was pooled at the matrix of apical cytoplasm in carbachol-stimulated cells, which might be derived from the secretory granules migrated from the basal cytoplasm into apical portion of the cells. In conclusion, the present study demonstrated the change in subcellular localization of gastrin-like immunoreactivity in intestinal gastrin cells after stimulation with carbachol. Aggregation of immunoreactivity at the apical portion of the cells suggests that gastrin may be released into the intestinal lumen.

Effect of somatostatin-14 on duodenal mucosal bicarbonate secretion in guinea pigs

The role of somatostatin-14 in duodenal mucosal HCO3- secretion was investigated in anesthetized, indomethacin-treated guinea pigs. Net HCO3- output from the isolated, perfused (24 mM NaHCO3 + 130 mM NaCl) proximal duodenum was measured during intravenous infusion (alone or in combination) of somatostatin-14, carbachol, vasoactive intestinal peptide (VIP), and prostaglandin E2 (PGE2). In homogenates of duodenal enterocytes, the effect of these agents on adenylate cyclase activity was studied. Basal duodenal HCO3- secretion (3.5 +/- 0.2 mumol/cm/10 min) was reduced dose dependently by somatostatin-14 (10(-11) mol/kg, 10(-9) mol/kg, and 10(-7) mol/kg). Carbachol, VIP, and PGE2 (all 10(-8) mol/kg) increased basal duodenal HCO3- secretion two- to threefold. Somatostatin-14 (10(-7) mol/kg) abolished the stimulatory effect of carbachol and VIP, but not that of PGE2. Basal adenylate cyclase activity in isolated duodenal enterocytes (9.4 +/- 1.0 pmol cAMP/mg protein/min) was unaltered by somatostatin (10(-6) mol/liter) or carbachol (10(-3) mol/liter). VIP (10(-8) mol/liter) and PGE2 (10(-7) mol/liter) increased adenylate cyclase activity two- to threefold, and these effects were unchanged by somatostatin-14 (10(-6) mol/liter). In conclusion, somatostatin-14 inhibits basal and carbachol- and VIP-stimulated duodenal HCO3- secretion, and its mechanism of action is not via inhibition of adenylate cyclase activity in duodenal enterocytes.

Effect of capsaicin on release of substance P-like immunoreactivity from vascularly perfused rat duodenum

The release of substance P-like immunoreactivity (SPLI) from the rat duodenum was investigated using an in situ vascularly perfused preparation. Results show that the basal release of SPLI was measurable and significantly enhanced by the administration of both 1,3, and 10 microM of capsaicin, indicating that SPLI is present in capsaicin-sensitive afferent nerve fibers. It is concluded that the present model may be useful for the study of the control of duodenal release of SPLI.

Effect of carbachol on the release of peptide YY from isolated vascularly and luminally perfused rat ileum

Possible cholinergic control on the release of PYY from intestine into the lumen or blood vessel was studied by radioimmunoassay in the isolated perfused rat ileum. The basal release of PYY into the lumen was 43.1 +/- 8.9 pg/min, which was comparable with that into the vasculature (35.2 +/- 2.6 pg/min). The administration of 1 microM carbachol into the vascular perfusate resulted in a more than 40-fold increase of the luminal release but only a twofold increase of the vascular release. Carbachol-induced release of PYY into both lumen and vasculature was completely blocked by atropine, but not by hexamethonium. Tetrodotoxin abolished carbachol-induced release of PYY into lumen and vasculature. These data suggest that the ileal PYY release, into either lumen or vasculature, is under the control of postganglionic cholinergic neurons via muscarinic receptors.

Demonstration of [D-Ala2]deltorphin I-like immunoreactivity in mucosal epithelial cells of the rat gastrointestinal tract

Using a specific antiserum to [D-Ala2]deltorphin I (DADTI), a delta-opioid receptor ligand, the localization of positive structures was studied in rat gastrointestinal tract by immunocytochemistry. Immunoreactive staining was not detected in the stomach, colon, or neuronal elements of any gastrointestinal tissue. However, positive cells were distributed in the mucosal epithelium of the small intestine, including the duodenum, jejunum, and ileum. The density of positive cells was highest at a proximal part of the jejunum and was gradually decreased toward the duodenum or the distal end of the intestine. These positive cells had spindle-like somata that tended to locate more closely to the lumen compared with nonimmunoreactive cells. Some of the positive cells extended cytoplasmic basal processes toward the lamina propria. Immunoelectron microscopy revealed that positive reaction products occurred within the secretory granules as well as in the cytoplasm. Because these positive granules were frequently observed in the apical cytoplasm beneath the microvilli, it is suggested that the DADTI-like molecule(s) may be secreted to the lumen.