Showing papers in "American Journal of Physiology-gastrointestinal and Liver Physiology in 1983"

Protective effects of prostaglandins against gastric mucosal damage: current knowledge and proposed mechanisms.

Abstract: Recent evidence indicates that prostaglandins (PGs) possess potent gastric antiulcer properties independent of their known inhibitory effects on acid secretion. The mechanism underlying this cytoprotective property, as it has been called, has remained elusive. Although exogenously administered PGs can prevent disruption of the gastric mucosal barrier, enhance gastric mucosal blood flow, and stimulate mucus and bicarbonate secretion, as well as a number of cellular transport processes, evidence for and against each of these proposed mechanisms for cytoprotection has been demonstrated. Thus, it is doubtful whether any of these effects of PGs on gastric epithelium is the mechanism responsible for cytoprotection, if indeed a single, common mechanism exists. In addition, an association between alterations in endogenous PGs and gastric mucosal injury induced by a variety of damaging agents has also been observed, but the importance of this association in terms of mediating gastric damage needs further clarification. Finally, the phenomenon of adaptive cytoprotection in which mild irritants protect the gastric mucosa against the damaging effects of various necrotizing agents may also be PG mediated since it can be blocked by indomethacin, an inhibitor of PG synthesis, but a clear association between changes in endogenous PGs and adaptive cytoprotection remains to be demonstrated. Despite being inconclusive, these findings suggest that PGs may play a significant role in the pathogenesis of gastric ulceration and may serve an important function in maintaining normal gastric mucosal integrity.

Hyperdynamic circulation in portal-hypertensive rat model: a primary factor for maintenance of chronic portal hypertension

Abstract: Two dissimilar hemodynamic hypotheses, the “backward flow” theory and the “forward flow” theory, have been advanced to define splanchnic hemodynamics in portal hypertension. An animal model with portal hypertension and high-grade portal-systemic shunting, the portal vein-stenotic rat, was studied to determine whether a hemodynamic picture compatible with either theory would develop. Splanchnic and systemic hemodynamics and portal-systemic shunting were measured by radioactive microsphere techniques. The portal-hypertensive rats (portal pressure, 12.8 +/- 0.5 vs. 8.3 +/- 0.4 mmHg) with greater than 95% portal-systemic shunting had a 60% increase in portal venous inflow (23.46 +/- 2.54 vs. 14.97 +/- 1.61 ml/min; P less than 0.01) with a concomitant 50% decrease in splanchnic arteriolar resistance (3.86 +/- 0.43 vs. 7.60 +/- 0.80 dyn . s . cm-5 . 10(5); P less than 0.001) compared with control rats. Cardiac index (391 +/- 17 vs. 250 +/- 20 ml . min-1 . kg-1) was elevated 50% (P less than 0.001), and total peripheral resistance (7.1 +/- 0.4 vs. 11.7 +/- 0.8 dyn . s . cm-5 . 10(4)) was decreased 60% (P less than 0.001). The resistance to portal blood flow in portal vein-stenotic rats (4.77 +/- 0.57 dyn . s . cm-5 . 10(4)) was similar to the resistance to portal blood flow in control rats (4.82 +/- 0.43 dyn . s . cm-5 . 10(4)), indicating that the hyperdynamic portal venous inflow, not resistance, provided the main impetus for maintaining the elevated portal venous pressure. The splanchnic hemodynamic observations directly support the forward flow theory of portal hypertension. The relation between splanchnic arteriolar resistance and total peripheral resistance (r = 0.67; P less than 0.01) indicated that the systemic hemodynamic parameters were secondarily altered by the splanchnic hemodynamic changes. This animal model of chronic portal hypertension gave evidence for a generalized splanchnic arteriolar vasodilation occurring in the presence of high-grade portal-systemic shunting.

Ischemia-induced vascular changes: role of xanthine oxidase and hydroxyl radicals.

Abstract: The results of previous studies indicate that oxygen-derived free radicals are responsible for the increased vascular permeability produced by 1 h of intestinal ischemia. The aims of this study were 1) to test the hypothesis that the enzyme xanthine oxidase is the source of oxygen radicals in the ischemic bowel and 2) to assess the role of the hydroxyl radical in the ischemia-induced vascular injury. The capillary osmotic reflection coefficient was estimated from lymphatic protein flux data in the cat ileum for the following conditions: ischemia, ischemia plus pretreatment with allopurinol (a xanthine oxidase inhibitor), and ischemia plus pretreatment with dimethyl sulfoxide (a hydroxyl radical scavenger). The increased vascular permeability produced by ischemia was largely prevented by pretreatment with either allopurinol or dimethyl sulfoxide. These findings support the hypothesis that xanthine oxidase is the source of oxygen radicals produced during ischemia. The results also indicate that hydroxyl radicals, derived from the superoxide anion, are primarily responsible for the vascular injury associated with intestinal ischemia.

Gastric mucosal barrier: hydrophobic lining to the lumen of the stomach

Abstract: The contact angle subtended between a droplet of aqueous fluid and nonwettable surfaces provides a direct estimation of their degree of hydrophobicity. The mean contact angle recorded in dogs at the oxyntic mucosal surface was 85.2 degrees, a value characteristic of acid-resistant substances such as polyethylene. This indicates that the mucosal surface of the stomach has a hydrophobic lining that may be attributed to the surface-active phospholipids known to be present in both the gastric mucosa and juice. Barrier breakers such as bile and aspirin were found virtually to eliminate the hydrophobicity. Hydrophobicity was found to be different in the esophagus, antrum, proximal and distal duodenum, and the colon but consistent with their resistance to acid attack. Endogenous surfactants are discussed for their capability to provide a cohesive and strongly adsorbed protective monolayer--a physical model for the gastric mucosal barrier compatible with the major properties of the gastric lining and many features of ulcerogenesis, including the protection afforded by prostaglandins.

Adaptive regulation of sugar and amino acid transport by vertebrate intestine.

Abstract: The adaptive regulation of sugar and amino acid transport by vertebrate intestine constitutes a neglected area. We review the patterns, signals, and mechanisms involved in adaptation. Mechanisms include changes in mucosal mass, specific transport systems, and the sodium gradient. Signals include the transported solutes themselves, hormones, and pancreaticobiliary secretions. The pattern of adaptation is examined for effects of dietary solutes, starvation, hyperphagia, dietary bulk, diabetes, intestinal position, intestinal resection, time of day, season of the year, hibernation, gestation, lactation, and aging and for differences among species. These observed patterns are compared with the patterns predicted by a simple teleologically deduced hypothesis: increased metabolic requirements should be met by increased absorption achieved through increased mucosal mass, while nutritionally essential solutes and nonessential solutes used as calorie sources should, respectively, repress and induce their own transport. We conclude with a summary of major unsolved questions in this area.

Morphology and electrophysiology of guinea pig gastric mucosal repair in vitro

Abstract: Guinea pig gastric mucosae stripped of their outer muscle layers were studied in Ussing chambers for up to 14 h. Ten minutes after the mucosae were mounted in the chamber, the electrical parameters were low but continued to rise over 90 min until steady-state potential difference (PD), resistance (R), and short-circuit current (Isc) were recorded. Morphological analysis during the first 10 min of the tissue in the chamber revealed gaps in the epithelium due to damaged cells. However, tissues examined after 20 min in the chamber showed little evidence of epithelial discontinuity. Thereafter, the initial rise in the electrical parameters was noted. After steady-state attainment, the lumen was exposed to 1.25 M NaCl for 5 min and then changed back to 150 mM NaCl. Ten minutes after washout and return to control solutions, the PD, R, and Isc had fallen to low values. At 30 min after washout of the NaCl, the PD, R, and Isc began to increase and after 2 h were back to control values. Morphological analysis of mucosae fixed up to 10 min after exposure to 1.25 M NaCl showed extensive damage and exfoliation of surface cells. However, by 30 min the epithelium was restored and had very few discontinuities, which was then followed by the return of the electrical parameters. The conclusions from these studies are 1) guinea pig gastric mucosae exposed to hypertonic NaCl on the luminal side will primarily result in surface epithelial cell destruction with an immediate drop in the transepithelial electrical values; 2) after return to isotonic saline the damaged mucosa can repair itself within minutes, which then allows the reestablishment of the transepithelial electrical parameters by 2 h; and 3) the good viability and reproducibility of this preparation present a suitable mammalian model system for the study of factors of mucosal repair.

Duodenal and ileal calcium absorption in the rat and effects of vitamin D.

Abstract: An in situ ligated loop procedure was applied to dissect transmural calcium transport in the intestine into two components, a saturable and a nonsaturable process. The existence of two such processes was confirmed in the duodenum, but ileal calcium transport was devoid of the saturable component. There was a small saturable component in the upper jejunum. The level of CaBP, the vitamin D-dependent cytosolic calcium-binding protein (Mr, approximately or equal to 9,000), corresponded to the magnitude of the saturable component. No CaBP was detected in the ileum. Vitamin D dependence of the saturable component was established by inducing it in the duodenum of vitamin D-deficient animals following intraperitoneal injection of 1,25-dihydroxyvitamin D3. In these same animals, conversely, the ileum did not respond to exogenous 1,25-dihydroxyvitamin D3. This confirms the absence in the ileum of the saturable component of transmural calcium movement and the fact that the nonsaturable component is not vitamin D dependent. Everted sac experiments also showed that duodenal sacs from vitamin D-replete or -repleted animals transported calcium against a chemical gradient, whereas ileal sacs did not. Vitamin D regulation of intestinal calcium absorption thus occurs only in the proximal intestine, even though calcium is absorbed down its chemical gradient all along the small intestine.

Cell site and time course of DNA synthesis in pancreas after caerulein and secretin

Abstract: Pancreatic weight, [3H]-thymidine incorporation into DNA, labeling indices, and total DNA and RNA content were measured in rats treated with vehicle or 1 microgram/kg caerulein, 100 micrograms/kg secretin, or a combination of these peptides injected every 8 h for 1-5 days Incorporation of [3H]thymidine into DNA increased 12-fold after 2 days of treatment with the combination of peptides DNA content increased after 3 days and reached a level 18 times control after 5 days Autoradiography showed that two cell types, acinar and an unidentified type, were the sites of increased DNA synthesis Different patterns of labeling were seen in the two populations: acinar cell labeling indices were increased at 1 and 2 days (20-fold) and then fell; nonacinar cells showed an increase only after 2 days and maintained this increase after 5 days Potentiation (greater than additive effects) was found when caerulein and secretin were injected together for all measurements except RNA content These data indicate that DNA

Sodium and chloride transport across rabbit ileal brush border. I: Evidence for Na-H exchange

Abstract: A series of experiments were performed to demonstrate the presence of and characterize the Na-H exchanger on rabbit ileal brush border with a vesicle preparation. An outwardly directed proton gradient (pH 5.5 inside, pH 7.5 outside) stimulated Na uptake, and a fourfold "overshoot" was observed. Conversely, an inwardly directed proton gradient (pH 7.5 inside, pH 5.5 outside) inhibited Na uptake. This stimulation/inhibition of Na uptake could not be accounted for by a proton diffusion potential, because Na uptake was found to be potential insensitive. Amiloride and harmaline inhibited pH-stimulated Na uptake, but other transport inhibitors (acetazolamide, DIDS, SITS, furosemide, and bumetanide) had no effect. Amiloride also inhibited Na efflux in the presence and absence of a pH gradient. Proton gradient-stimulated Na uptake was saturable with a Km of 16.2 mM and a Vmax of 129 nmol X min-1 X mg protein-1. Tetramethylammonium did not affect pH-stimulated Na uptake, but other cations tested inhibited Na uptake, with NH4+ and Li+ causing greater inhibition than K+ or Cs+. Using the fluorescent probe acridine orange, an inwardly directed Na gradient was shown to stimulate proton efflux from the vesicles and an outwardly directed Na gradient stimulated proton influx.

Effect of rabbit antimotilin serum on myoelectric activity and plasma motilin concentration in fasting dog.

Abstract: It is known that a cyclic increase in plasma motilin concentration occurs during the interdigestive state of dog and the increase coincides with migrating myoelectric complexes (MMCs) of the antrum as well as proximal duodenum. The purpose of the present study is to determine the role of endogenous motilin in the occurrence of MMCs in 10 dogs prepared with a gastric cannula and platinum monopolar electrodes in the gastric antrum, duodenum, jejunum, and ileum. After recording at least two consecutive cycles of MMCs from the proximal duodenum, each dog received an intravenous infusion of highly specific rabbit antimotilin sera in varying doses ranging from 3.5 to 15 ml for a period of 60 or 90 min. During the motility recording period ranging from 6 to 30 h following the administration of the antimotilin serum, several changes in the motility were observed. 1) The occurrence of MMCs in the antrum, duodenum, jejunum, and ileum was temporarily interrupted for varying periods depending on the individual dog studied and the amount of antiserum administered. When the higher dose of antimotilin was administered, a more profound and prolonged inhibition occurred. 2) Phase I activity rarely occurred. Instead, a phase II-like activity continued throughout the recording period. 3) MMCs in the jejunum or ileum occurred at irregular intervals without aboral propagation of MMCs from the duodenum or jejunum. 4) The plasma motilin concentration decreased to levels lower than that observed during phase I of the duodenum and exhibited no cyclic increase until the MMCs reappeared in the proximal duodenum.(ABSTRACT TRUNCATED AT 250 WORDS)

Polyamines and intestinal growth ― increased polyamine biosynthesis after jejunectomy

Abstract: Transient increases in the activities of ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (SAM-DC), key enzymes in polyamine biosynthesis, may be critical to initiation of cell growth. We now report that such increases in ODC (X170) and SAM-DC (X83) activities, and their synthetic products putrescine (X4) and spermidine (X2), occur in rat ileal mucosa between days 1 and 4 after 50% intestinal resection. This is the time period of initiation of mucosal cell hyperplasia in intestinal adaptation after resection and is characterized by increased mucosal cell proliferation, as measured morphologically and biochemically. Intestinal weight increased by 76% and mucosal thickness by 48%. Mucosal DNA content increased by 67% and mucosal DNA synthesis by 104%. Increased intestinal crypt cell proliferation was manifested by a 120% increase in labeling per crypt and a 152% increase in crypt cell production rate (CCPR). The increase in ODC activity was closely associated with the increases in CCPR and rate of villus lengthening. Rates of mucosal cell proliferation, as measured by CCPR, and villus and crypt lengthening were significantly correlated with ODC activity (r = 0.97, 0.98, and 0.94, respectively; P less than 0.01 for all). Our results indicate that the increase in ODC activity, SAM-DC activity, and polyamine biosynthesis is closely associated with the process of adaptive postresectional crypt cell proliferation.

Monoclonal antibodies against gastric H+ + K+ ATPase

Abstract: Monoclonal antibodies were prepared against a purified membrane fraction from hog gastric mucosa containing the H+ + K+ ATPase. On sodium dodecyl sulfate gels the molecular weight of this fraction corresponds to a single band of about 95,000. In contrast, on isoelectric focusing gels three groups of peptides are resolved with isoelectric points of 5.7, 6.2, and 8.5. One of the monoclonal antibodies (HK111) was shown to react selectively with the acidic peptide, whereas another antibody (HK113) reacted with the alkaline peptide, showing that the three peptides were antigenically distinct. Both monoclonal antibodies selectively labeled the parietal cell, and antibody HK111 labeled the tubulovesicles of the resting parietal cell and the microvilli of the secretory canaliculus of the secreting cell. This finding suggests translocation of membrane from the tubulovesicles to the secretory surface on stimulation.

Distribution of surfactants in the canine gastrointestinal tract and their ability to lubricate.

Abstract: Surface-active phospholipids, chemically similar to pulmonary surfactants, have been identified by thin-layer chromatography in gastric juice and on the mucosal lining of five tissues along the canine gastrointestinal tract. These included esophagus, oxyntic tissue, midduodenum, midjenunum, and midcolon. The mucosal phospholipid composition did not change appreciably along the length of the gastrointestinal tract, with the most prominent species being phosphatidylcholine (PC) (34-45%) and phosphatidylethanolamine (PE) (18-32%). The gastrointestinal mucosal surface also contained lesser amounts (5-10% each) of sphingomyelin (Sp), lysophosphatidylcholine, phosphatidylinositol (PI), and phosphatidylserine (PS) and small but detectable quantities of phosphatidylglycerol (PG) and phosphatidic acid. Both the lipid extracts of all six gastrointestinal sources as well as commercially available phospholipids identified in these samples (PC, Sp, PE, PI, PS, and PG) proved to be good boundary lubricants. Each reduced the coefficient of friction by greater than 75% between glass and carboxylated yarn when tested by a standard method for evaluating textile "sizes." The results are discussed in relation to the possible role of the surface-active molecules in imparting properties conducive to lubrication and acid protection onto the gastrointestinal epithelium.

Hepatic uptake of albumin-bound substances: albumin receptor concept.

Abstract: The single-pass hepatic uptake of long-chain fatty acids and other substances bound tightly to albumin in plasma is surprisingly efficient. Recent kinetic studies for several of these substances suggest that uptake is mediated primarily by direct interaction of the albumin-ligand complex with the hepatocyte surface rather than by the small fraction of unbound ligand, as has been generally believed. Furthermore, 125I-albumin has been found to bind specifically, saturably, and reversibly to isolated hepatocytes, adipocytes, and erythrocytes. Although the nature and possible regulation of these binding sites remain to be fully elucidated, the putative albumin receptor may play an important role in the bidirectional transfer of many classes of endogenous and exogenous substances between albumin and cells.

Sodium-dependent chloride secretion across rabbit descending colon.

Abstract: Electrogenic, cAMP-mediated Cl secretion across rabbit descending colon in vitro is independent of the rate or presence of active Na absorption. Yet, several observations indicate that this process is Na dependent: a) Cl secretion requires the presence of Na in the serosal solution alone, b) the kinetics of Cl transport as a function of external Na concentration are virtually identical to the Cl concentration dependence, and c) exchange of cell Cl with isotopic Cl added to the serosal solution is inhibited by Na-free media and by addition of furosemide to the serosal solution; the diuretic also inhibits Cl secretion. These findings suggest that Cl entry into the secretory cells across the basolateral membrane is mediated by NaCl cotransport. Addition of ouabain to, or removal of K from, the serosal solution inhibits Cl secretion so that Na entering the secretory cell across the basolateral membrane may be returned to the serosal solution by the Na-K pump. Finally, increasing the K concentration of the serosal solution inhibits Cl secretion under short-circuit conditions. This appears to result from K-induced depolarization of the electrical potential difference across the apical membrane so that diffusional Cl exit from cell to mucosal solution is reduced.

Ca2+ in the control of active intestinal Na and Cl transport: involvement in neurohumoral action

Abstract: Intracellular Ca2+ is a regulator of active intestinal Na and Cl transport. Most studies have been done with rabbit ileum. Increasing intracellular Ca2+ decreases active Na and Cl absorption and/or stimulates active Cl secretion; lowering intracellular Ca2+ stimulates Na and Cl absorption. Based on studies with microvillus membrane vesicles from rabbit ileum, a direct effect of Ca2+ and calmodulin on linked Na and Cl uptake is established. Intracellular Ca2+ and cAMP affect the same transport processes and act in a nonadditive manner. Intracellular Ca2+ does not act by changing intestinal cAMP or cGMP contents, and increasing cAMP mobilizes intracellular Ca2+. Whether this Ca2+ is involved in regulation of ion transport is not known. The aspects of Ca2+ handling identified as involved in regulation of active intestinal Na and Cl transport include entry of Ca2+ across the basolateral membrane, mobilization of Ca2+ from intracellular stores, and involvement of the Ca2+-binding protein calmodulin. Several neurohumoral substances alter intestinal transport by Ca2+-dependent mechanisms and appear to act primarily by increasing (serotonin, carbachol, substance P, and neurotensin) or decreasing (dopamine) Ca2+ entry across the basolateral membrane of intestinal epithelial cells.

Corticosteroid alteration of active electrolyte transport in rat distal colon

Abstract: To determine the effect of corticosteroids on active transport processes, unidirectional fluxes of 22Na, 36Cl, and 42K were measured under short-circuit conditions across isolated stripped distal colonic mucosa of the rat in control, secondary hyperaldosterone, and dexamethasone-treated animals. In controls net sodium and chloride fluxes (JNanet and JClnet) and short-circuit current (Isc) were 6.6 +/- 2.2, 7.6 +/- 1.6, and 1.3 +/- 0.2 mu eq X h-1 X cm-2, respectively. Although aldosterone increased Isc to 7.3 +/- 0.5 mu eq X h-1 X cm-2, JNanet (6.9 +/- 0.7 mu eq X h-1 X cm-2) was not altered and JClnet was reduced to 0 compared with controls. Dexamethasone also stimulated Isc but did not inhibit JClnet. In Cl-free Ringer both aldosterone and dexamethasone produced significant and equal increases in JNanet and Isc. Theophylline abolished JNanet in control animals but not in the aldosterone group. Aldosterone reversed net potassium absorption (0.58 +/- 0.11 mu eq X h-1 X cm-2) to net potassium secretion (-0.94 +/- 0.08 mu eq X h-1 X cm-2). Dexamethasone reduced net potassium movement to 0 (-0.04 +/- 0.12 mu eq X h-1 X cm-2). These studies demonstrate that 1) corticosteroids stimulate electrogenic sodium absorption and 2) aldosterone, but not dexamethasone, inhibits neutral NaCl absorption and stimulates active potassium secretion. The effects of mineralocorticoids and glucocorticoids on electrolyte transport are not identical and may be mediated by separate and distinct mechanisms.

Enterocyte alpha 2-adrenergic receptors: yohimbine and p-aminoclonidine binding relative to ion transport

Abstract: We previously reported that alpha 2-adrenergic agonists enhance absorption and inhibit secretion of electrolytes in small intestine. The present study was undertaken to characterize and localize the relevant receptors. Plasma membranes derived from isolated rabbit ileal epithelial cells were incubated with either [3H]yohimbine (Yo), an alpha 2-antagonist, or p-[3H]aminoclonidine (PAC), an alpha 2-agonist. Scatchard analysis of [3H]Yo binding suggests a single receptor. Competitive displacement of Yo from this receptor by other ligands had a potency order characteristic for alpha 2-receptors in other tissue systems. A Scatchard plot of [3H]PAC binding was curvilinear and best fit by assuming two independent site. Competitive displacement of [3H]PAC by PAC in the presence of 140 mM Na+ or 0.1 mM GTP increased the IC50 for PAC binding from 10 nM to 100 and 105 nM, respectively, and the Hill coefficient from 0.7 to 1.2 and 1.0, respectively. The ED50 for PAC effect on short-circuit current (200 nM) does not differ significantly from these values. We conclude that alpha 2-receptors are present on ileal enterocytes and that these receptors mediate enterocyte fluid and electrolyte transport function.

Differentiation among inhibitory actions of omeprazole, cimetidine, and SCN- on gastric acid secretion

Abstract: The action of the substituted benzimidazole omeprazole (H 168/68) was studied in three different in vitro preparations: the isolated guinea pig gastric mucosa, isolated intact and permeable rabbit gastric glands, and hog fundic microsomal membrane vesicles containing H+-K+-ATPase. The effects of omeprazole were compared with those of cimetidine and thiocyanate (SCN-). Under all the conditions studied, cimetidine only counteracted histamine-induced acid secretion, consonant with its H2-receptor antagonism. In contrast, omeprazole and SCN- were found not only to inhibit histamine-induced secretion but also basal acid formation and acid formation induced by dibutyryl cAMP and a high cell medium concentration of K+. Moreover, acid production induced by ATP in permeable gastric glands was antagonized by omeprazole and SCN-, whereas cimetidine was without effect. The interaction pattern of omeprazole and SCN- was differentiated by studies using the weak base antipyrine in the isolated mucosal preparation, where it was found that antipyrine could reverse the inhibition induced by SCN- but not that of omeprazole. Furthermore, omeprazole was found to inhibit the isolated H+-K+-ATPase, whereas cimetidine or SCN- was without effect. In the isolated mucosal preparation omeprazole caused an increase in K+ secretion rates in parallel with the inhibition of acid formation. This was in contrast to what was observed for cimetidine and SCN-, which exhibited no such increased K+ secretion. The results obtained from intact mucosa and isolated glands are in agreement with the ability of omeprazole to inhibit the isolated H+-K+-ATPase and thus provide evidence of a novel mechanism of action for this inhibitor.

Arachidonic acid stimulates mucin secretion in prairie dog gallbladder

Abstract: Mucin glycoprotein secretion from prairie dog gallbladder explants was studied in 24-h organ culture using [3H]glucosamine as a precursor. Indomethacin caused a reversible dose-dependent inhibition of both mucin release (50% inhibition between 10(-6) and 10(-5) M indomethacin) and 6-ketoprostaglandin F1 alpha (6-keto-PGF1 alpha) release (66% inhibition at 10(-7) M). Sepharose 4B chromatography of secreted [3H]-glucosamine-labeled glycoproteins revealed that indomethacin inhibited release of a high-molecular (greater than 10(6) daltons) mucin-type glycoprotein. Addition of sodium arachidonate (10(-4) M) to organ culture medium caused an approximate two- to fivefold increase (P less than 0.02) in mucin release compared with control and an increase in the secretory component of total glycoprotein synthesis from 37.5% in control explants to 75.2% (P less than 0.01). The stimulatory effect of arachidonate on mucin secretion was blocked by indomethacin. We incubated explants for 1 h with [14C]arachidonate and studied prostaglandin and thromboxane products in medium by high-performance liquid chromatography. The only conversion product identified was 6-keto-PGF1 alpha, the stable breakdown product of prostacyclin. Release of gallbladder mucin is significantly inhibited by indomethacin and increased by arachidonate. Mucin secretion may be regulated in part by endogenous levels of prostacyclin, the major cyclooxygenase product in prairie dog gallbladder epithelium.

Regulation of cytosolic free Ca2+ concentration in acinar cells of rat pancreas.

Abstract: Ca2+ uptake into isolated exocrine pancreatic cells with highly permeable plasma membrane was determined by measuring the decrease in free Ca2+ concentration of the surrounding incubation medium with a Ca2+-specific electrode. In the presence of Mg-ATP and respiratory substrates the free Ca2+ concentration of the incubation medium decreased rapidly after addition of leaky cells until a stable medium free Ca2+ concentration of 4.2 +/- 0.1 X 10(-7) mol/l was obtained. Changes in the medium free Ca2+ concentration at steady state by addition of Ca2+ or EGTA were buffered by cellular uptake or release, respectively, until the steady-state free Ca2+ concentration was reestablished. When nonmitochondrial Ca2+ uptake was determined in the presence of a combination of mitochondrial inhibitors (10(-5) mol/l antimycin, 5 X 10(-6) mol/l oligomycin, and 10(-2) mol/l azide), the rate of uptake was considerably reduced, while the steady-state concentration was unaltered. In contrast, mitochondrial uptake that could be observed in the presence of the ATPase inhibitor vanadate (2 X 10(-3) mol/l) proceeded at the same rate as the control, but the minimal medium free Ca2+ concentration reached was 2.4 +/- 0.1 X 10(-7) mol/l higher than the control. Addition of secretagogues at steady-state free Ca2+ concentration resulted in a Ca2+ release of 0.73 +/- 0.08 nmol/mg protein. The increase in medium free Ca2+ concentration was entirely transient and followed by reuptake to the prestimulation level. The data indicate that a cytosolic free Ca2+ concentration of 4 X 10(-7) mol/l can be regulated in pancreatic acinar cells by a nonmitochondrial Mg2+-dependent Ca2+ pool.

Quantitative determination of macromolecular transport rate across intestinal Peyer's patches.

Abstract: We used horseradish peroxidase (HRP) (mol wt, 40,000) to compare in vitro, in Ussing chambers, the rates of protein transport across segments of piglet jejunum with and without Peyer's patches. The...

Binding of Escherichia coli heat-stable enterotoxin to receptors on rat intestinal cells.

Abstract: This study was performed to determine whether receptors for Escherichia coli heat-stable enterotoxin (ST) exist on intestinal epithelial cells. Binding sites for 125I-ST were found on rat jejunal and ileal villus cells. Binding was rapid, reversible, linear with cell number, saturable, and temperature dependent. Significant degradation of 125I-ST occurred when incubated with cells at 37 degrees C but not at 25 degrees C. Binding was specific to ST since binding of 125I-ST was competitively inhibited by increasing concentrations of human or porcine ST but not by E. coli heat-labile, cholera, or staphylococcal enterotoxins. Addition of excess unlabeled ST to cells preincubated with 125I-ST resulted in dissociation of much but not all of the bound 125I-ST. Binding of 125I-ST to jejunal and ileal cells occurs with two affinities, and this is due to the phenomenon of negative cooperativity. The potency of ST for inhibiting the binding of 125I-ST was identical to the potency of ST in stimulating cGMP production. These data support the existence of receptors for ST on intestinal cells, and these receptors may be involved in the action of ST.

Developmental changes in the mechanisms of duodenal calcium transport in the rat.

Abstract: Duodenal calcium transport was resolved into a saturable and a nonsaturable process by means of an in situ ligated loop procedure applied to Wistar rats at 3, 12, 19, 24, 30, 40, 60, 110, and 150 days of age. All postweaning animals were males that had been placed on a 1.5% calcium, 1.5% phosphorus semisynthetic diet. Duodenal calcium-binding protein (CaBP) levels were determined at all ages. The newborn rat had no saturable transport component and no CaBP. Its nonsaturable component was very high. With increasing age the saturable component and CaBP varied biphasically, increasing steeply until the animals were about 35 days old; thereafter, each decreased to low but detectable values. The nonsaturable component, on the other hand, decreased in near-linear fashion in the first 35 days; in animals beyond that age it remained invariant. The difference in age dependence between the saturable and nonsaturable components may be considered to constitute additional evidence for the existence of the two transport processes. CaBP and the saturable transport process were highly correlated, further proof that both are vitamin D dependent. Histological studies have revealed the presence of many vacuoles in the intestinal cells of the very young rats; these vacuoles were absent in rats older than 35 days. It is suggested that these vacuoles may be implicated in a pinocytosislike nonsaturable transport that is superimposed on the nonsaturable, non-vitamin D-dependent calcium transport found in all enterocytes.

Vasopressin modulates liver regeneration in the Brattleboro rat

Abstract: Liver regeneration following partial hepatectomy is significantly impaired in rats with hereditary vasopressin deficiency (Brattleboro strain), both in rate of DNA synthesis and in return of liver DNA content to normal. Vasopressin treatment at physiological doses ameliorates the defect and thus appears to be an important modulator of liver regeneration in response to partial hepatectomy in the rat.

Local modulation of intestinal ion transport by enteric neurons.

Abstract: Principles of autonomic nervous system control of intestinal ion transport need to include the newer concepts of the enteric nervous system (ENS). Based on studies of nervous control of the myenteric plexus, it is likely that ENS control of intestinal transport occurs through local mechanisms. In vitro transport studies and a limited number of radioreceptor-binding studies in mucosal cells support the notion that putative neurotransmitters alter transport by acting directly with mucosal receptors. In vivo and in vitro studies cannot alone uncover the indirect transport effects that neurotransmitters may have when they interact with enteric neurons. Studies focused on uptake and release of neurotransmitters suggest that norepinephrine (NE)-induced absorption may be modulated by local NE presynaptic neuronal mechanisms. Endogenous NE release may be enhanced by nicotinic and angiotensin II agents but decreased by muscarinic and alpha-adrenergic agents or prostaglandins. Presynaptic neuronal mechanisms that modulate endogenous acetylcholine (ACh) release and ACh-induced secretion are less well defined. Intestinal transport may be controlled by negative feedback, interneuronal, or transsynaptic presynaptic mechanisms. We propose that transport is controlled by a balance between the principal neurotransmitters NE and ACh. These neurotransmitters may be modulated by neuroactive peptides located either in neurons or in enteroendocrine cells. Efferent neurons may modulate release of neuropeptides from enteroendocrine cells into the luminal or antiluminal sides of mucosal cells. Intestinal transport also may be controlled by luminal factors that cause neuropeptide release from enteroendocrine cells or by specialized luminal receptors acting on sensory afferent neurons and intrinsic neuronal reflexes. Therefore, local modulation of intestinal transport by the ENS represents a finely tuned neuronal system with complex interrelations similar to many found in the central nervous system.

Cause-and-effect relationship between motilin and migrating myoelectric complexes.

Abstract: We investigated the cause-and-effect relationship between plasma motilin levels and migrating myoelectric complexes (MMCs). Each dog was implanted with a set of eight bipolar electrodes on the small intestine. Premature phase IIIs were initiated by morphine bolus injections. Plasma samples were assayed for motilin and gastrin. All spontaneous and morphine-initiated phase IIIs were associated with peaks of plasma motilin, which always occurred after phase IIIs had started in the proximal duodenum. The plasma motilin level decreased consistently during phase I and started to increase again only after phase II had started in the duodenum. Either a meal or somatostatin infusion disrupted MMC cycling, but morphine boluses overcame this disruption and initiated phase IIIs that propagated distally. The phase IIIs thus initiated were associated with peaks in plasma motilin levels. In contrast, bolus injections of motilin did not initiate phase IIIs during the fed state or during somatostatin infusion. Our findings suggest that endogenous motilin does not initiate spontaneous MMCs. Instead, MMC contractions release motilin. The physiological role of motilin, thus released, may be to act as an endocrine agent to coordinate secretory and motor events with the start of phase III activity in the upper small intestine.

Effect of vagus, gastric inhibitory polypeptide, and HCl on gastrin and somatostatin release from perfused pig antrum

Abstract: The porcine antrum was isolated with the pancreas and perfused in vitro with an artificial medium supplemented with erythrocytes. The vagal innervation was preserved. Effluent was collected from the portal vein as well as from a vein directly draining the antrum. Electrical vagal stimulation increased gastrin output and inhibited somatostatin output. Intraluminal hydrochloric acid had the opposite effect. Gastric inhibitory polypeptide (GIP) in physiological concentrations (90 and 450 pmol/l) increased somatostatin output, inhibited gastrin output, and potentiated the effect of HCl on somatostatin release. Vagal stimulation, however, abolished the GIP effect on somatostatin output. Thus gastrin and somatostatin outputs were always inversely affected by the applied stimuli, suggestive of somatostatin-mediated control of gastrin secretion. GIP may exert its effects via local somatostatin release.

Role of tight-junctional pathways in bile salt-induced increases in colonic permeability

Abstract: The effects of a dihydroxy bile salt, taurochenodeoxycholate (TCDC), on the permeability and conductance of isolated, short-circuited segments of the rabbit descending colon were examined using conventional Ussing chamber techniques. Increasing concentrations of TCDC (1-4 mM) produced dose-dependent increases in sodium backflux (JNas leads to m) and tissue conductance (Gt) when applied to either the mucosal or serosal salines. However, mucosal addition was twice as potent in increasing JNas leads to m and Gt at 4 mM. Tracer experiments indicated that the transepithelial serosal-to-mucosal fluxes of sodium and mannitol are via an aqueous, unrestricted, free-solution pathway, while albumin movements are restricted through this pathway both in the absence and presence of mucosal TCDC. The changes in JNas leads to m, JMans leads to m, and Gt caused by 4 mM mucosal TCDC were largely reversed by rinsing the mucosal chamber with fresh buffer. It was also observed that osmotically induced volume flows in the serosal-to-mucosal direction could offset or reverse the changes in Gt produced by 2 mM mucosal TCDC, suggesting that the enhanced conductance pathway is in series with the lateral intercellular spaces. Taken together, these results suggest that low concentrations of TCDC alter the integrity of tight-junctional complexes between the epithelial cells of the rabbit colon.