Functional activities of the colon of the desert gerbil (Gerbillus cheesmani)

The effects of starvation and undernourishment on the potential differences (pd in mV), basal short-circuit current (Isc in microA/cm(2)), resistance (R in omega) and glucose-dependent short-circuit current (Isc in microA/cm(2)) across stripped sheets of proximal, mid and distal colon of the gerbil (Gerbillus cheesmani) were investigated. The effects of replacing sodium chloride by lithium chloride, replacing chloride in Krebs buffer by gluconate and removing bicarbonate from bathing buffers were also investigated. Starvation (4 days, water ad lib) and undernourishment (50% control food intake for 21 days) had no significant changes on pd and R of the three regions of stripped colon. Starvation increased the basal Isc in the proximal and the mid-colon only while undernourishment increased the basal Isc of three regions of the colon. In addition, starvation and undernourishment increased the glucose-dependent Isc in the three regions. Replacing sodium chloride by lithium chloride caused a slight decrease in the basal Isc of proximal and mid colon taken from starved animals. In undernourished gerbils, although there was a slight decrease in basal Isc of proximal and mid colon the big decrease was observed in Isc of the distal colon. Replacing chloride by gluconate had no effect on the Isc of the different regions of colon taken from fed and starved animals but decreased the Isc of the three regions of undernourished animals. The absence of bicarbonate reduced the Isc of proximal and mid colon taken from starved gerbils and those of three regions taken from undernourished animals. The results of the present study suggest that the Isc of proximal and mid colon from starved gerbils could result from active sodium transport together with bicarbonate secretion while the Isc of the three regions taken from undernourished gerbils results from active sodium absorption together with both chloride and bicarbonate secretion.     

Apical adenosine regulates basolateral Ca2+-activated potassium channels in human airway Calu-3 epithelial cells

In airway epithelial cells, apical adenosine regulates transepithelial anion secretion by activation of apical cystic fibrosis transmembrane conductance regulator (CFTR) via adenosine receptors and cAMP/PKA signaling. However, the potent stimulation of anion secretion by adenosine is not correlated with its modest intracellular cAMP elevation, and these uncorrelated efficacies have led to the speculation that additional signaling pathways may be involved. Here, we showed that mucosal adenosine-induced anion secretion, measured by short-circuit current (Isc), was inhibited by the PLC-specific inhibitor U-73122 in the human airway submucosal cell line Calu-3. In addition, the Isc was suppressed by BAPTA-AM (a Ca2+ chelator) and 2-aminoethoxydiphenyl borate (2-APB; an inositol 1,4,5-trisphosphate receptor blocker), but not by PKC inhibitors, suggesting the involvement of PKC-independent PLC/Ca2+ signaling. Ussing chamber and patch-clamp studies indicated that the adenosine-induced PLC/Ca2+ signaling stimulated basolateral Ca2+-activated potassium (KCa) channels predominantly via A2B adenosine receptors and contributed substantially to the anion secretion. Thus, our data suggest that apical adenosine activates contralateral K+ channels via PLC/Ca2+ and thereby increases the driving force for transepithelial anion secretion, synergizing with its modulation of ipsilateral CFTR via cAMP/PKA. Furthermore, the dual activation of CFTR and KCa channels by apical adenosine resulted in a mixed secretion of chloride and bicarbonate, which may alter the anion composition in the secretion induced by secretagogues that elicit extracellular ATP/adenosine release. Our findings provide novel mechanistic insights into the regulation of anion section by adenosine, a key player in the airway surface liquid homeostasis and mucociliary clearance.     

Secretin, VIP, and PHI stimulate rat proximal duodenal surface epithelial bicarbonate secretion in vivo

The surface epithelial cells of the stomach and duodenum secrete bicarbonate at rest and in response to a number of agonists including the gastrointestinal hormones, glucagon, and GIP. Since those hormones with structural homology may have similar effects, the purpose of the present study was to examine the effect of graded doses (6, 24, and 96 nmol/kg) of pure porcine secretin, VIP, and PHI on bicarbonate secretion by the proximal duodenum containing Brunner's glands. Experiments were performed in vivo on unanesthetized Sprague-Dawley rats with chronic Thiry-Vella type loops of the proximal 2 cm of duodenum. The order of testing was random and only one hormone was tested on a single day. Compared to the saline control, each dose of VIP produced a significant increase in duodenal bicarbonate secretion in a dose-response manner. The two higher doses of secretin and only the 96 nmol/kg dose of PHI significantly increased bicarbonate output. The responses to 96 nmol/kg dose of secretin and VIP were similar, and each was significantly greater than observed with PHI. It is concluded that secretin and VIP stimulate proximal duodenal bicarbonate secretion and are more potent than PHI.     

Shark rectal gland vasoactive intestinal peptide receptor: cloning, functional expression, and regulation of CFTR chloride channels

Vasoactive intestinal peptide (VIP) is a secretagogue that mediates chloride secretion in intestinal epithelia. We determined the relative potency of VIP and related peptides in the rectal gland of the elasmobranch dogfish shark and cloned and expressed the VIP receptor (sVIP-R) from this species. In the perfused rectal gland, VIP (5 nM) stimulated chloride secretion from 250 +/- 66 to 2,604 +/- 286 microeq x h(-1) x g(-1); the relative potency of peptide agonists was VIP > PHI = GHRH > PACAP > secretin, where PHI is peptide histidine isoleucine amide, GHRH is growth hormone-releasing hormone, and PACAP is pituitary adenylate cylase activating peptide. The cloned sVIP-R from shark rectal gland (SRG) is only 61% identical to the human VIP-R1. It maintains a long, extracellular NH2 terminus with seven cysteine residues, and has three N-glycosylation sites and eight other residues implicated in VIP binding. Two amino acids considered important for peptide binding in mammals are not present in the shark orthologue. When sVIP-R and the CFTR chloride channel were coexpressed in Xenopus oocytes, VIP increased chloride conductance from 11.3 +/- 2 to 127 +/- 34 microS. The agonist affinity for activating chloride conductance by the cloned receptor was VIP > GHRH = PHI > PACAP > secretin, a profile mirroring that in the perfused gland. The receptor differs from previously cloned VIP-Rs in having a low affinity for PACAP. Expression of both sVIP-R and CFTR mRNA was detected by quantitative PCR in shark rectal gland, intestine, and brain. These studies characterize a unique G protein-coupled receptor from the shark rectal gland that is the oldest cloned VIP-R.     

Epididymal phenotype in luteinizing hormone receptor knockout animals and its response to testosterone replacement therapy

Previous studies reported that epididymis contains functional LH receptors. The LH receptor knockout mice, which have epididymal phenotypes, gave us an opportunity to test the hypothesis that testosterone replacement alone may not be sufficient to reverse phenotypes to wild-type epididymis. The morphological phenotype in knockout animals includes a decrease in luminal diameter of the proximal and distal caput and cauda epididymis, the absence of clear and halo cells in the epithelial lining, a decrease in the height of principal cells and the number of cells containing cilia, a decrease in cilia length, and a change from basal to central location of nuclei in the principal cells. The biochemical phenotype includes a decrease in periodic acid-Schiff reaction product, reflecting the glycogen and glycoprotein synthesis and secretion, a decrease in androgen receptor (AR) and estrogen receptor (ER)beta, and an increase in ERalpha levels. Twenty-one-day testosterone replacement therapy in 30-day-old knockout animals reversed some, but not all, morphological and biochemical phenotypes. Those that did not reverse include luminal diameters of proximal and distal caput and cauda epididymis, the percentage of ciliated principal cells in caput epididymis, and nuclear AR localization. In summary, while our results reaffirm that androgens are important for normal epididymal morphology and function, they indicate that LH could be required for certain facets of epididymal morphology and/or function.     

Effects of tetrodotoxin and ion replacements on the short-circuit current induced by Escherichia coli enterotoxin STa across the colon of the gerbil (Gerbillu cheesmani) in different dietary states

The effects of mucosally added Escherichia coli heat stable enterotoxin (STa, 30 ng mL(-1)) on the basal short-circuit current (Isc in microA cm(-2)) across stripped and unstripped sheets of proximal, mid and distal colon were investigated. Samples were taken from fed, starved (4 days, water ad lib) and undernourished (50% of control food intake for 21 days) gerbils (Gerbillus cheesmani). The effect of neurotoxin tetrodotoxin (TTX 10 microM) and the effect of replacing chloride by gluconate or the effect of removing bicarbonate from bathing buffer on the maximum increase in Isc induced by E. coli heat stable enterotoxin STa were also investigated. The results showed that there is a segmental difference both in the basal and STa-induced Isc. Also, STa is more effective in the proximal than distal colon in the three feeding conditions. Undernourishment raised the STa-induced Isc in the three regions of the colon. In fed and starved gerbils part of STa-induced Isc in the proximal colon was chloride-dependent, while the other was bicarbonate-dependent; in the mid colon, the STa-induced Isc was bicarbonate-dependent only. In the three regions of the colon taken from undernourished gerbil, the STa-induced Isc was both chloride-and bicarbonate-dependent. The increase in STa-induced Isc as a results of undernourishment in proximal and mid colon was chloride-dependent, while in the distal colon, it was both chloride- and bicarbonate-dependent.     

Immature rat epididymal epithelial cells grown in static primary monolayer culture on permeable supports

A cell culture system is characterized for monolayers of immature rat epididymal epithelial cells grown on permeable supports. Cover of the filters was achieved by days 4-5 and was maintained for 9-12 days. The secretion of acid phosphatase (ACP), alkaline phosphatase (AKP) and N-acetylglucosaminidase (NAG) into apical and basal compartments of culture chambers was monitored with time in culture for cells from the proximal and distal epididymis of 37-day-old animals. There was independent secretion of the three enzymes: secretion of NAG and AKP was mainly apical, that of ACP basal; daily secretion of ACP and AKP was constant throughout culture, that of NAG declined; there was greater secretion of NAG and AKP by cells from the proximal than the distal region. The initial high apical secretion of NAG is thought to reflect loss of enzyme from unattached cells, whereas the later AKP secretion is truly directional. Secretion was not influenced by the enzymes used in cell preparation. The cytotoxic agent Thimerosal inhibited secretion of all enzymes when placed beneath the cultures, indicating that secretion depended on viable cells, but initially stimulated release of AKP when applied above the cells possibly reflecting release from the cell membrane.     

Morphometric analysis of coated pits and vesicles in the proximal and distal caput epididymidis

A morphometric analysis of coated and uncoated structures found in the apical portion of principal cells from both the proximal and distal caput epididymidis has been carried out. Almost all endocytic, coated vesicles are found within 1 micron of the luminal surface of principal cells and the volume fraction of these and of uncoated vesicles is much greater in the proximal caput epididymidis. A serial section analysis indicated that many coated "vesicles" are tangentially sectioned coated pits and that a complex network of interconnected vesicular and tubular structures exists in the apical cytoplasm. Efferent duct ligation has no effect on the number of size of large coated and uncoated vesicles in either the proximal or distal caput epididymidis, indicating that substances delivered to principal cells from the lumen are not required to maintain the endocytic machinery. However, this treatment does result in a considerable increase in the number of large coated vesicles associated with the basal surface of principal cells from the proximal but not the distal caput epididymidis. The volume fraction of small, presumably exocytic, coated vesicles is significantly greater in the apical cytoplasm of cells from the distal caput epididymidis in control animals. Efferent duct ligation results in a significant increase in the volume fraction of these vesicles in the proximal but not distal caput epididymidis. These results show that there are marked differences in structure among principal cells from these two regions of the epididymis and that this may reflect differences in control and function.     

Electrogenic ion secretion in proximal,mid and distal colon from fed and starved mice

1. Electrogenic ion transport was monitored in vitro as the short-circuit current (Isc in μA/cm²) across proximal, mid and distal colon removed from fed and 48 hr-starved Swiss albino mice (Mus muscaris).2. Electrogenic secretion was induced either with serosal bethanechol (muscarinic agonist), DMPP (nicotinic agonist) or dibutyryl-cyclic AMP (DbcAMP). Proximal and distal colon from starved mice showed greater electrogenic secretion in response to bethanechol than those from the fed controls while DMPP and DbcAMP did not activate the hypersecretion.3. In the distal colon, starvation induced a large increase in the basal Isc that was unaffected by mucosal amiloride but was inhibited by tetrodotoxin (TTX) and by diphenylamine-2-carboxylic acid (DPC) unlike the fed basal Isc. Bethanechol activated a biphasic response consisting of a transient decrease in the Isc followed by a sustained increase both of which were significantly greater in the starved than the fed tissue and were inhibited by TTX, DPC and atropine but not hexamethonium.4. Starvation enhances the secretory response to muscarinic activation in proximal and distal colon and induces an increased basal electrogenic (Cl ⁻) secretion in the distal colon stimulated by an augmented neural tone.     

Immunolocalization of androgen and vitamin D receptors in the epididymis of mature ram (Ovis aries)

This study illustrated the immunohistochemical distribution of androgen and vitamin D receptors of epididymis in 20 sexually mature ram (Rahmani breed) with average age ranged from (2^_4) years and average weight ranged from (50^_65kg). Androgen receptor was localized in the cytoplasm of both ciliated and non ciliated cells of efferent ductules, besides the principal cells via the entire epididymal duct. The principal cells of both corpus and proximal cauda epididymis showed the highest immunoreactivity to androgen receptors. Furthermore, vitamin D receptor was localized in the cytoplasm of all epithelium of the efferent ductules besides principal cells of all epididymal regions, however the immunoreaction was significantly higher in the efferent ductules, distal caput and distal cauda epididymis. In conclusion, these results suggest that the function of ram epididymis is regulated by both androgen and Vitamin D.     

Localization of sodium bicarbonate cotransporter (NBC) protein and messenger ribonucleic acid in rat epididymis

An acidic environment is important for sperm maturation in the epididymis and also helps to maintain mature sperm in an immotile state during storage in this organ. Both an Na+/H+ exchanger and an H+ATPase have been implicated in this process. The H+ATPase is concentrated in specialized apical (and/or narrow) and clear cells of the epididymis, while the Na+/H+ exchanger has not yet been localized in situ. As in other proton-secreting epithelia, bicarbonate transport occurs in the epididymis, where it is implicated in luminal acidification. In this study we used an antibody raised against a fusion protein (maltose-binding protein: MBP-NBC-5) from the C-terminus of the recently cloned rat kidney Na+/HCO3- cotransporter (NBC) to localize this protein in the epididymis and vas deferens of the rat. The distribution of the respective mRNA was mapped by in situ hybridization. NBC message was strongly expressed in the initial segment and the intermediate zone of the epididymis, and the NBC-5 antibody gave a strong basolateral staining in both principal cells and apical/narrow cells in this region. Western blotting revealed a single band at about 160 kDa in the epididymis. The intensity of staining as well as mRNA levels decreased in the cauda epididymidis and in the vas deferens, where only weak staining was seen. Basolateral NBC may function in parallel with apical proton secretion to regulate luminal acidification and/or bicarbonate reabsorption in the excurrent duct system.     

Segmental differences in short-chain fatty acid transport in rabbit colon: Effect of pH and Na

Short-chain fatty acids (SCFAs) are the predominant luminal anion in the mammalian colon. Although they are rapidly absorbed in vivo, little is known about the mechanisms of transepithelial transport in vitro. Previous studies have suggested that SCFA transport may be linked to Na absorption or an anion exchange mechanism. We compared the transport of propionate under short-circuit conditions in rabbit proximal and distal colon to determine whether there were segmental differences, how SCFAs may be linked to either Na absorption or anion transport, and whether SCFAs, as weak electrolytes, may be affected by transepithelial pH gradients. In distal colon, propionate transport was not significantly altered by stimulation of electrogenic Na absorption, epinephrine or Cl removal. However, a modest transepithelial pH gradient (luminal 6.8/serosal 7.4) stimulated propionate absorption. In proximal colon, propionate transport was significantly altered by manuevers that either stimulated (lowered [Na] in the bathing media) or inhibited (theophylline) apical Na−H exchange. Neither Cl removal, nor the anion exchange inhibitor DIDS, nor a transepithelial bicarbonate gradient, altered propionate transport. A transepithelial pH gradient inhibited propionate secretion, but not in a manner entirely consistent with the effect of pH on the distribution of a weak electrolyte. These results suggest that there is significant segmental heterogeneity in colonic SCFA transport; that transepithelial propionate fluxes are altered by changes in pH or electroneutral Na absorption (Na−H exchange), but not by chloride removal, bicarbonate gradients or electrogenic Na absorption. Regulation of SCFA transport may be an important factor in the physiology of colonic fluid balance.     

Effect of chloride on pH microclimate and electrogenic Na+ absorption across the rumen epithelium of goat and sheep

Active Na+ absorption across rumen epithelium comprises Na+/H+ exchange and a nonselective cation conductance (NSCC). Luminal chloride is able to stimulate Na+ absorption, which has been attributed to an interaction between Cl-/HCO3- and Na+/H+ exchangers. However, isolated rumen epithelial cells also express a Cl- conductance. We investigated whether Cl- has an additional effect on electrogenic Na+ absorption via NSCC. NSCC was estimated from short-circuit current (Isc) across epithelia of goat and sheep rumen in Ussing chambers. Epithelial surface pH (pHs) was measured with 5-N-hexadecanoyl-aminofluorescence. Membrane potentials were measured with microelelectrodes. Luminal, but not serosal, Cl- stimulated the Ca2+ and Mg2+ sensitive Isc. This effect was independent of the replacing anion (gluconate or acetate) and of the presence of bicarbonate. The mean pHs of rumen epithelium amounted to 7.47 +/- 0.03 in a low-Cl- solution. It was increased by 0.21 pH units when luminal Cl- was increased from 10 to 68 mM. Increasing mucosal pH from 7.5 to 8.0 also increased the Ca2+ and Mg2+ sensitive Isc and transepithelial conductance and reduced the fractional resistance of the apical membrane. Luminal Cl- depolarized the apical membrane of rumen epithelium. 5-Nitro-2-(3-phenylpropylamino)-benzoate reduced the divalent cation sensitive Isc, but only in low-Cl- solutions. The results show that luminal Cl- can increase the microclimate pH via apical Cl-/HCO3- or Cl-/OH- exchangers. Electrogenic Na+ absorption via NSCC increases with pH, explaining part of the Cl- effects on Na+ absorption. The data further show that the Cl- conductance of rumen epithelium must be located at the basolateral membrane.     

Electrogenic ion transport in the intestine of the Australian common brushtail possum, <Emphasis Type="Italic">Trichosurus vulpecula</Emphasis>: indications of novel transport patterns in a marsupial

In this study, electrogenic ion transport in the intestine of the Australian common brushtail possum, Trichosurus vulpecula was investigated. In the ileum, a Na(+)-dependent, phloridzin- and amiloride-insensitive short-circuit current ( Isc) was present. Mucosal glucose stimulated a further phloridzin-sensitive, dose-dependent increase in Isc. A Na(+)-dependent, ouabain-sensitive Isc was also present in the caecum and colon. In the proximal and distal colon, amiloride (100 micro mol l(-1), mucosal) inhibited this Isc by 81+/-4% and 65+/-3%, respectively and the Ki for amiloride (approximately 1 micro mol l(-1)) was consistent with the inhibition of a classical epithelial Na(+) channel. In the caecum, 50% of the Isc was inhibited by amiloride (100 micro mol l(-1), mucosal). The amiloride-insensitive Isc in the colon was not due to electrogenic Cl(-) secretion, as serosal bumetanide (100 micro mol l(-1)) had no effect on the Isc. Furthermore, the secretagogues forskolin (10 micro mol l(-1)), carbachol (100 micro mol l(-1)) and dibutyryl-cAMP or dibutyryl-cGMP (100 micro mol l(-1)) did not stimulate electrogenic Cl(-) secretion by the colon. These results indicate that the transport properties of the hindgut of the possum differ significantly from those of eutherian mammals and may be associated with different functions of the hindgut of possums when compared to eutherian mammals.     

A secretin releasing peptide exists in dog pancreatic juice

Canine pancreatic juice has been shown to stimulate exocrine pancreatic secretion in the dog. In the present study we investigated whether there is a secretin-releasing peptide in canine pancreatic juice. Pancreatic juice was collected from the dogs with Thomas gastric and duodenal cannulas while pancreatic secretion was stimulated by intravenous administration of secretin at 0.5 microg/kg/h and CCK-8 at 0.2 microg/kg/h, respectively. The pancreatic juice was separated into three different molecular weight (MW) fractions (Fr) by ultrafiltration (Fr 1; MW > 10,000, Fr 2; MW=10,000-4,000 and Fr 3; MW < 4,000), respectively. All the fractions were bioassayed in anesthetized rats. Fraction 3 dose-dependently and significantly stimulated pancreatic juice flow volume from 78.0% to 99.4% (p<0.05) and bicarbonate output from 128.9% to 202.1% (p<0.01), respectively. Plasma secretin concentration also increased from 1.2 +/- 0.5 pM to 5.0 +/- 0.8 pM and 6.0 +/- 1.0 pM (p<0.05). None of these fractions increased pancreatic protein secretion or plasma CCK level. The stimulatory effect of Fraction 3 on pancreatic secretion and the release of secretin was completely abolished by treatment with trypsin (1 mg/ml for 60 min at 37 degrees C) but not by heating (100 degrees C, 10 min). Intravenous injection of a rabbit anti-secretin serum, which rendered plasma secretin almost undetectable in rat plasma, also abolished Fr 3-stimulated pancreatic secretion of fluid and bicarbonate secretion. These observations suggest that a secretin-releasing peptide exists in the canine pancreatic juice. It is trypsin-sensitive and heat-resistant. This peptide may play a significant physiological role on the release of secretin and regulation of exocrine pancreatic secretion.     

Effect of acute hyperglycemia on basal, secretin and secretin + cholecystokinin stimulated exocrine pancreatic secretion in humans

Pancreatico-biliary secretion is reduced during acute hyperglycemia. We investigated whether alterations in pancreatico-biliary flow or volume output are responsible for the observed reduction in duodenal output of pancreatic enzymes and bilirubin during hyperglycemia. Eight healthy subjects were studied on two occasions during normoglycemia and hyperglycemia (15 mmol/l). Pancreatico-biliary output was measured by aspiration using a recovery marker under basal conditions (60 min), during secretin infusion (0.1 CU/kg.h) for 60 min and during secretin + CCK (0.5 IDU/kg.h) infusion for 60 min. Secretin was infused to stimulate pancreatico-biliary flow and volume output. Secretin significantly (P<0.005-P<0.05) increased volume and bicarbonate output and CCK significantly (P<0.01) increased the output of bilirubin, pancreatic enzymes, bicarbonate and volume, both during normoglycemia and hyperglycemia. During hyperglycemia basal, secretin stimulated and secretin + CCK stimulated total pancreatico-biliary output were significantly (P<0.005-P<0.05) reduced compared to normoglycemia. The incremental outputs, however, were not significantly different between hyper- and normoglycemia. Pancreatic volume output was significantly (P<0.05) reduced during hyperglycemia compared to normoglycemia under basal conditions (31+/-16 m/h versus 132+/-33 m/h) during secretin infusion (130+/-17 ml/h versus 200+/-34 m/h) and during secretin + CCK infusion (370+/-39 ml/h versus 573+/-82 ml/h). Plasma PP levels were significantly (P<0.05) reduced during hyperglycemia. It is concluded that 1) hyperglycemia significantly reduces basal pancreatico-biliary output 2) the incremental pancreaticobiliary output in response to secretin or secretin + CCK infusion is not significantly affected during hyperglycemia, 3) a reduction in volume output contributes to the inhibitory effect of hyperglycemia on pancreatico-biliary secretion, 4) hyperglycemia reduces PP secretion suggesting vagal-cholinergic inhibition of pancreatico-biliary secretion and volume during hyperglycemia.     

Regulation of atrial natriuretic peptide secretion by cholinergic and PACAP neurons of the gastric antrum

Atrial natriuretic peptide (ANP) released from enterochromaffin cells helps regulate antral somatostatin secretion, but the mechanisms regulating ANP secretion are not known. We superfused rat antral segments with selective neural agonists/antagonists to identify the neural pathways regulating ANP secretion. The nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium (DMPP) stimulated ANP secretion; the effect was abolished by hexamethonium but doubled by atropine. Atropine's effect implied that DMPP activated concomitantly cholinergic neurons that inhibit and noncholinergic neurons that stimulate ANP secretion, the latter effect predominating. Methacholine inhibited ANP secretion. Neither bombesin nor vasoactive intestinal polypeptide stimulated ANP secretion, whereas pituitary adenylate cyclase-activating polypeptide (PACAP)-27, PACAP-38, and maxadilan [PACAP type 1 (PAC1) agonist] each stimulated ANP secretion. The PAC1 antagonist M65 1) abolished PACAP-27/38-stimulated ANP secretion; 2) inhibited basal ANP secretion by 28 +/- 5%, implying that endogenous PACAP stimulates ANP secretion; and 3) converted the ANP response to DMPP from 109 +/- 21% above to 40 +/- 5% below basal, unmasking the cholinergic component and indicating that DMPP activated PACAP neurons that stimulate ANP secretion. Combined atropine and M65 restored DMPP-stimulated ANP secretion to basal levels. ANP secretion in the antrum is thus regulated by intramural cholinergic and PACAP neurons; cholinergic neurons inhibit and PACAP neurons stimulate ANP secretion.     

Roles of 5-HT receptors in the release and action of secretin on pancreatic secretion in rats

5-Hydroxytryptamine (serotonin, 5-HT) is a hormone and neurotransmitter regulating gastrointestinal functions. 5-HT receptors are widely distributed in gastrointestinal mucosa and the enteric nervous system. Duodenal acidification stimulates not only the release of both 5-HT and secretin but also pancreatic exocrine secretion. We investigated the effect of 5-HT receptor antagonists on the release of secretin and pancreatic secretion of water and bicarbonate induced by duodenal acidification in anesthetized rats. Both the 5-HT(2) receptor antagonist ketanserin and the 5-HT(3) receptor antagonist ondansetron at 1-100 microg/kg dose-dependently inhibited acid-induced increases in plasma secretin concentration and pancreatic exocrine secretion. Neither the 5-HT(1) receptor antagonists pindolol and 5-HTP-DP nor the 5-HT(4) receptor antagonist SDZ-205,557 affected acid-evoked release of secretin or pancreatic secretion. None of the 5-HT receptor antagonists affected basal pancreatic secretion or plasma secretin concentration. Ketanserin or ondansetron at 10 microg/kg or a combination of both suppressed the pancreatic secretion in response to intravenous secretin at 2.5 and 5 pmol x kg(-1) x h(-1) by 55-75%, but not at 10 pmol x kg(-1) x h(-1). Atropine (50 microg/kg) significantly attenuated the inhibitory effect of ketanserin on pancreatic secretion but not on the release of secretin. These observations suggest that 5-HT(2) and 5-HT(3) receptors mediate duodenal acidification-induced release of secretin and pancreatic secretion of fluid and bicarbonate. Also, regulation of pancreatic exocrine secretion through 5-HT(2) receptors may involve a cholinergic pathway in the rat.