Ischemia/reperfusion injury in the rat colon.

This study investigated metabolic and biochemical consequences of colonic ischemia/reperfusion (I/R) in the rat and evaluated whether antioxidants prevent I/R-induced functional damage in the rat colon. The surgical preparation involved a 10 cm segment of the colon and occlusion of the superior mesenteric artery (SMA) to induce I/R. Arterial blood from the aorta and venous blood from the superior mesenteric vein (SMV) was collected to measure blood gases, lactic acid (LA) and arachidonic acid (AA) metabolites. Tissue xanthine oxidase (XO) and thiobarbituric acid (TBA) derivatives were measured before and after reperfusion. In addition, vascular and mucosal permeability, and the effect of MDL 73404 (a water soluble vitamin E analog) and 5-aminosalicylic acid on LA, AA, XO and TBA was measured. After ischemia, the colon displayed a metabolic shift from aerobic to anaerobic course by increasing lactic acid production in the colon (183% increase in SMV lactate level compared 87% in the SMA; p < 0.03). After 10 minutes of reperfusion, circulating 6-keto-prostaglandin F1 alpha increased by 3.85 fold (p < 0.001) and thromboxane B2 increased by 2 to 3 fold. An Ischemia time longer than 60 minutes was required to cause changes in tissue XO levels. Tissue TBA levels showed a good dose response corresponding with I/R time. I/R (60 minutes) caused a three and 16 fold increase (p < 0.01) in vascular and mucosal permeability, respectively. MDL 73404 and 5-aminosalicylic acid significantly inhibited the vascular permeability and decreased LA, AA, XO and TBA. These observations provide the first direct experimental evidence for I/R-induced damage in the colon and some of its effects can be reversed by conventional and novel antioxidants.

Effects of neuropeptide Y and substance P on antigen-induced ion secretion in rat jejunum.

We previously described a model of intestinal hypersensitivity in which isolated gut segments from sensitized rats demonstrated a rapid epithelial secretory response to luminal antigen that was mediated by mucosal mast cells and capsaicin-sensitive nerves. In this study, we examined the ability of the inhibitory neuropeptide, neuropeptide Y (NPY), to diminish the antigen-induced secretory response. Rats were sensitized to egg albumin (EA), and 12-14 days later, jejunal tissue was excised and mounted in Ussing chambers. NPY inhibited the short-circuit current (Isc) increase and Cl- secretion evoked by addition of EA to the luminal side of the tissue; neural blockade with tetrodotoxin (TTX) had a similar inhibitory effect. In contrast, NPY was much less effective, and TTX was completely ineffective, on the response to serosal antigen. Additional experiments examined the cell target for NPY action. NPY and TTX almost abolished the Isc response to electrical transmural stimulation of enteric nerves, suggesting a possible neural site of action. In addition, NPY significantly reduced baseline Isc; this inhibition involved both TTX-dependent and TTX-independent components. Because nerves were previously shown to facilitate antigen uptake and substance P was implicated in the response to only luminal antigen, we postulated that NPY was inhibiting nerves that facilitate antigen transport from the lumen to effector cells in the lamina propria. We therefore examined the effect of exogenous substance P added after NPY inhibition. Substance P restored the luminal antigen-induced secretory response to pretreatment values. We conclude that the neuropeptides play a significant role in immunophysiology by acting at neural and epithelial sites in the intestinal mucosa.

Role of Shiga-like toxin I in bacterial enteritis: comparison between isogenic Escherichia coli strains induced in rabbits.

BACKGROUND/AIMS: Enteroadherent Escherichia coli that produce Shiga-like toxins are important causes of human disease, including enterohemorrhagic E. coli-induced colitis (EHEC). The role of Shiga-like toxins in these illnesses is unclear. The aim of this study was to establish an animal model for human EHEC and to determine the role of Shiga-like toxin I (SLT-I) in this model. METHODS: E. coli strain RDEC-1 is an enteroadherent rabbit diarrheal pathogen. An isogenic variant of RDEC-1 (termed RDEC-H19A) producing high levels of SLT-I was obtained by infecting RDEC-1 with an SLT-I-converting bacteriophage. The effects of in vivo enteric infection produced in rabbits by RDEC-H19A were compared with those in uninfected and RDEC-1-infected animals. RESULTS: SLT-I-producing RDEC-H19A induced a severe, noninvasive, enteroadherent infection in rabbits. Clinically, infection with RDEC-H19A was more severe than infection with RDEC-1 and caused more serious histological lesions including vascular changes, edema, and more severe inflammation. Interleukin 1 and platelet-activating factor appear to be important inflammatory mediators to this infection. CONCLUSIONS: The illness induced by RDEC-H19A in rabbits resembled enterohemorrhagic E. coli-induced colitis of humans. SLT-I is an important virulence factor in the pathogenesis of EHEC.

Action of peptidoleukotrienes on ion transport in rabbit distal colon in vitro.

Serosal but not mucosal addition of the peptidoleukotrienes, leukotriene (LT) C4 (LTC4), LTD4 and LTE4 transiently (maximal response within 2 min) increased short-circuit current (Isc) and transepithelial conductance across stripped rabbit colonic mucosa mounted in Ussing chambers. All three peptidoleukotrienes elicited their responses in the presence of amiloride (10 microM) and were inhibited by serosal addition of the NaCl cotransport inhibitors bumetanide (100 microM) and furosemide (1 mM). The effects of the peptidoleukotrienes on Isc and transepithelial conductance were concentration-dependent with maximal effects occurring at 10 microM. Half-maximal effects were produced at 30 nM for LTC4, 50 nM for LTD4 and 450 nM for LTE4. The secretory responses to both LTD4 and LTE4 were antagonized in a concentration-dependent manner by the LTD4/LTE4 receptor antagonist, SK&F 104353 (2(S)-hydroxy-3-(R)-[(2-carboxyethyl)thio]-3-[2-(8 phenyloctyl)phenyl]propanoic acid). Complete inhibition of the LTD4 and LTE4 effects were observed at 0.1 microM SK&F 104353 and half-maximal effects were achieved at 0.6 nM SK&F 104353. At 10 microM SK&F 104353 only 50% inhibition of the LTC4-induced increase in Isc was observed. These results suggest the peptidoleukotrienes stimulate colonic Cl- secretion by receptor-mediated mechanisms and that receptors for LTC4 are distinct from those mediating the action of LTD4/LTE4.

Cecectomized rat. A model of experimental secretory diarrhea in conscious animals.

Evaluation of antisecretory antidiarrheal agents in animal models is limited primarily to extrapolations of efficacy from enteropooling studies in vivo, isolated intestinal loops in situ, and Ussing flux chamber preparations in vitro. While these standard techniques are useful, they do not mimic secretory diarrhea. Our studies indicate that in rats, the cecum may serve a "reservoir" function in response to secretagogue administration. Thus, diarrhea is not observed consistently and reliably in this species to allow valid evaluation of potential antidiarrheal agents. Therefore, we have developed a reproducible model of secretory diarrhea utilizing conscious cecectomized rats by surgical resection of the cecum, without compromising ileocecal patency, and by the use of potent intestinal secretagogues. Animals quickly recover and maintain normal growth and other physiologic parameters for as long as 60 days. After 48 hr on standard chow, secretory diarrhea can be induced by oral administration of standard intestinal secretagogues (dimethyl prostaglandin E2, cholera toxin, or carbachol). Dimethyl prostaglandin E2 (300 micrograms/kg, p.o.) induces diarrhea within 1 hr that continues for approximately 3.5 hr. Oral administration of known antidiarrheal agents chlorpromazine (10 mg/kg), clonidine (1 mg/kg), or morphine (10 mg/kg) all significantly reduce fecal output within 30-60 min following administration. These studies indicate that in the rat, the cecum may serve as a fluid reservoir during periods of small intestinal hypersecretion and that the cecectomized rat serves as a useful, accurate, and reliable tool for evaluating new compounds with proposed antidiarrheal activity.

Cold-restraint stress increases rat fecal pellet output and colonic transit.

Increased fecal pellet output that occurs during cold-restraint stress (CRS) was evaluated systematically. Free-feeding rats, which exhibit a reduced occurrence of gastric ulcers under these conditions, were studied. CRS significantly increased fecal pellet production and fluid content. However, the fecal output produced during CRS was not associated with increased gut secretory activity or somatic motor activity associated with cold restraint and did not occur in anesthetized animals. Cold and restraint stress were additive in producing increased fecal output. Significant dose-related decreases in fecal output were produced by drugs that decrease gut transit (i.e., B-HT 920, clonidine, Lomotil, loperamide, and lidamidine). Anticholinergic-antisecretory drugs, antidepressants, and tranquilizers had little effect on fecal output or fluid content. Changes in gastrointestinal transit did not contribute to the increased fecal output during CRS. Transit in the lower small intestine was not altered, but the cecum tended to empty more contents into the large intestine during CRS. Colonic transit was dramatically affected by CRS, which eliminated retrograde transit and produced the evacuation of the majority of colonic contents. The increased colonic transit produced by CRS was decreased in a dose-related fashion by hexamethonium, nifedipine, loperamide, and B-HT 920. In several time-response and drug-inhibition studies during CRS, both fecal pellet output and colonic transit were affected similarly. These data indicate that CRS appears to change central nervous system output to the colon and that it alters colonic smooth muscle motility in a manner that facilitates colonic transit and evacuation. Small intestinal transit is not involved in this phenomenon and is regulated differently during CRS.

Inflammatory mediators of experimental colitis in rats.

Colonic inflammation was induced in rats by intracolonic administration of 0.25 ml of 50% ethanol containing 30 mg of trinitrobenzene sulfonic acid (TNB). Control rats were treated with 0.25 ml of 50% ethanol or with 30 mg of TNB in 0.25 ml of saline. After 24 h, mucosal ulceration and hemorrhage were observed in TNB/ethanol-, 50% ethanol-, and to a lesser extent, in TNB/saline-treated rats. After 1 wk, mucosal damage was completely resolved in the 50% ethanol and TNB/saline-treated rats but the lesions in the TNB/ethanol-treated rats persisted and progressed to a chronic active inflammatory process after 3 wk. Myeloperoxidase activity was significantly elevated in mucosal scrapings from all treatment groups at all time intervals when macroscopic and microscopic mucosal injury was evident. Interleukin-1 was found to be the most sensitive indicator of mucosal inflammation, and its mucosal values correlated with myeloperoxidase activity. Leukotriene B4 was increased in control rats at 1 wk and in TNB/ethanol-treated rats at all time intervals. The maximal increase in leukotriene B4 was observed at 1 wk. Thromboxane B2 generation was reduced while platelet activating factor generation was not increased in TNB/ethanol-treated rats. These results indicate that in this TNB/ethanol model of gut inflammation, myeloperoxidase activity and interleukin-1 are reliable and sensitive indicators of colonic inflammation, and that thromboxane B2 is not involved in the acute lesions, whereas leukotriene B4 appears in the chronic active inflammatory response.

Secretagogue-induced protein phosphorylation and chloride transport in Caco-2 cells.

The effects of vasoactive intestinal polypeptide (VIP), 16,16-dimethyl prostaglandin E2 (DMPGE2) and dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP) on protein phosphorylation were studied in relation to stimulation of chloride transport in cell suspensions of the human colon epithelial cell line Caco-2. In 36Cl-loaded cells, VIP and DMPGE2 within 1 min decreased cellular chloride content 35-40%, with half-maximal effects being elicited at 1.0 and 85 nM concentration, respectively. A similar effect on chloride content occurred after 10 min of treatment with 0.5 mM DBcAMP. For all three secretagogues, decreases in cellular chloride content were associated with increases in membrane permeability to chloride. DMPGE2 and VIP within 1 min, and DBcAMP within 10 min, increased the phosphorylation of an unidentified soluble protein of Mr = 42,000 and pI = 6.1, and of a protein of Mr = 20,200 and pI = 4.9 identified as myosin regulatory light chain. Between 10 and 30 min of stimulation, however, phosphorylation of the Mr = 42,000 protein and chloride transport activity remained elevated in DMPGE2- and DBcAMP-treated cells, whereas light chain phosphorylation returned to control level. No effect of secretagogues on phosphorylation was detected in the total particulate fraction or an integral membrane protein fraction. It is concluded that increased membrane permeability to chloride induced by cAMP-mediated secretagogues in Caco-2 is temporally associated with the increased phosphorylation of a Mr = 42,000 soluble protein.

Antidiarrheal activity of alpha-2 adrenoceptor agonist SK&F 35886.

Alpha-2 adrenoceptor agonists exhibit antidiarrheal activity in animal models and in humans. However, hypotensive and sedative side effects seriously limit the use of these agents to treat diarrhea. SK&F 35886 (2,6-dimethyl phenylamino imidazoline) is an alpha-2 adrenoceptor agonist with little central nervous system activity. In Ussing chamber preparations of rabbit ileum, SK&F 35886 produces a concentration-dependent decrease in basal short-circuit current (Isc) (EC50 0.2 microM) that is dependent on the presence of mucosal HCO3. This concentration-response curve is shifted to the right of rauwolscine, increasing the EC50 to 30 microM. Prazosin had no effect on this response. Flux studies indicate that SK&F 35886 increases net Cl absorption and enhances HCO3 absorption without altering net Na flux. After PGE2 stimulation of Isc, SK&F 35886, applied either serosally or mucosally, immediately returns the Isc to base line. This effect is due to a reversal of the PGE2-induced inhibition of Na and Cl absorption. In vivo SK&F 35886 dose-dependently inhibits PGE2-induced enteropooling when given orally (ED50 approximately 31 micrograms/kg). This effect is attenuated significantly by rauwolscine (1.0 micrograms/kg s.c.). In cecectomized rats, SK&F 35886 abolishes PGE2-induced diarrhea within 1 hr after oral administration of the drug. SK&F 35886 (500 micrograms/kg p.o.) did not alter hexobarbital sleep time or elicit piloerection or lethargy, whereas clonidine (37.3 micrograms/kg p.o.) significantly enhanced hexobarbital sleep time. These results illustrate the ability of a peripheral acting alpha-2 agonist to promote absorption and inhibit secretion and diarrhea in the mammalian intestine.

Selective alpha-2 adrenoceptor agonists alter fluid and electrolyte transport in mammalian small intestine.

The effects of the selective alpha-2 adrenoceptor agonists B-HT 920 and B-HT 933 on fluid and electrolyte transport in mammalian small intestine were assessed in vitro and in vivo. In Ussing flux chamber preparations of rabbit ileum, B-HT 920 reduces basal short-circuit current (Isc) in a concentration-dependent manner. This in vitro effect is inhibited by rauwolscine (KB = 9.7 nM) but not by prazosin. Isotope flux and ion replacement studies suggest that this decrease in Isc is due primarily to stimulation of a HCO3-dependent transport process. B-HT 920 promptly attenuates the 16,16-dimethyl prostaglandin E2 (dmPGE2)-stimulated increase in Isc and completely reverses dmPGE2-stimulated Cl secretion to absorption. Oral administration of B-HT 933 dose-dependently inhibits dmPGE2-induced enteropooling in conscious rats. This effect of B-HT 933 is likewise blocked significantly by rauwolscine but not by prazosin. Similar effects of B-HT 933 are observed on enteropooling in the pithed rat as are the effects of B-HT 920 in the conscious rat. These results indicate that selective alpha-2 adrenoceptor agonists from the azepine class of compounds have significant proabsorptive and antisecretory activities in the rabbit small intestine in vitro and in the rat intestine in vivo. This in vivo effect does not appear to be central nervous system mediated. These studies suggest that these alpha-2 adrenoceptor agonists may be useful in converting the hypersecreting mammalian small bowel to its normal absorptive state.

Effects of putative thromboxane receptor agonists and antagonists on rat small intestinal ion transport.

Effects of a thromboxane mimic, U46619, on electrolyte transport were examined in vitro using stripped segments of rat ileal mucosa mounted in Ussing chambers. Addition of U46619 to the serosal bathing solution elicited a transient increase in short-circuit current (Isc) and decrease in transepithelial conductance (Gt). The increase in Isc was accompanied by a transient increase in Cl- secretion and decrease in Na+ absorption. In the steady-state, Isc was not increased whereas Gt remained decreased and Na+ and Cl- absorption were inhibited. Removal of Cl- or pretreatment with serosal and mucosal indomethacin (1 microM) or the thromboxane receptor antagonist, SK&F 88046, added to the serosal bathing solution, inhibited the increase in Isc stimulated by U46619 (apparent KB approximately 8 nM). The effects of U46619 on both Isc and Gt are qualitatively similar to those resulting from stimulation with leukotriene D4. However, the changes in Isc with leukotriene D4 (10 microM) are antagonized by SK&F 88046 only at high concentrations (1-10 microM). In addition, the secretagogues prostaglandin F2 alpha, lys-bradykinin, serotonin and histamine, produce qualitatively similar changes in Isc to those seen with U46619 without altering Gt. With the exception of prostaglandin F2 alpha, the effects of these secretagogues are not inhibited by SK&F 88046 (10 microM). These results indicate that U46619 acts at a thromboxane receptor to stimulate intestinal Cl- secretion and inhibit Na+ and Cl- absorption. These changes are inhibited selectively by the thromboxane receptor antagonist, SK&F 88046.

Intestinal 5-fluorouracil absorption: use of Ussing chambers to assess transport and metabolism.

We have employed an in vitro system to study transport and metabolism of organic molecules by gastrointestinal tissues. Such a system would aid in the evaluation of the potential for oral delivery of organic molecules. Transport and metabolism of 5-fluorouracil (5-FU) were studied using rabbit intestinal preparations. Unidirectional fluxes and metabolism were measured in vitro in Ussing chambers under short-circuit conditions. Results from these studies reveal that in ileum, proximal, and distal colon, steady-state fluxes of 5-FU (10 microM added to both bathing solutions) are established after 30 min and remain constant for at least 110 min. Transport of 5-FU under "sink" conditions with 10 microM 5-FU present in the mucosal or serosal bathing solution alone demonstrated similar rates of transport as under "nonsink" conditions. The concentration dependence of 5-FU fluxes indicates that the mucosal (m)-to-serosal (s) flux is composed of both a saturable and a linear component over the range of 1-100 microM in the ileum, whereas the s-to-m flux in the ileum and both fluxes in the colon are linear functions of concentration. Over the concentration range employed and the time course of these studies, 5-FU had no effect on the electrical properties of the ileum or colon. In the ileum, the m-to-s but not the s-to-m flux of 5-FU was reduced by (1) serosal ouabain (0.1 mM); (2) reduction of the bathing solution Na concentration; and (3) addition of uracil, thymine, thymidine, uridine, 2-deoxyuridine, or uridine-5'-monophosphate.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of sulfidopeptide leukotrienes D4 and E4 on ileal ion transport in vitro in the rat and rabbit.

Effects of leukotrienes D4 and E4 (LTD4 and LTE4) on electrolyte transport were examined, employing stripped segments of rat and rabbit ileum mounted in Ussing chambers. Addition of LTD4 or LTE4 to the serosal but not the mucosal bathing solution elicited a transient increase in short-circuit current (Isc) with maximal responses seen at 10(-5) M and 10(-8) M in rat and rabbit respectively and a sustained decrease in transepithelial conductance (Gt) in the rat only. In the rat, Cl replacement, reduction of bathing solution [Ca2+] to 1 microM or pretreatment with 1 microM indomethacin or meclofenamic acid inhibited the LTD4- or LTE4-induced Isc changes with no effect on the decrease in Gt. LTD4 (10 microM) transiently increased net Cl secretion and produced a sustained decrease in both unidirectional and net Na transport and mucosal-to-serosal Cl flux in rat ileum. The decrease in unidirectional Na fluxes is accounted for predominantly by a change in the potential independent flux of Na. These results suggest that the increase in Isc in both rat and rabbit is mediated by arachidonic acid metabolites, whereas the decrease in Gt and net Na absorption in rat ileum is mediated by a cyclooxygenase-independent pathway.

The effects of phorbol esters on fluid transport and blood flow in the small intestine.

Studies were designed to examine the effects of phorbol esters on intestinal fluid transport and blood flow in the anesthetized cat and enteropooling in the conscious rat. Intraluminal administration of phorbol ester into a segment of isolated small bowel produced a copious intestinal secretion and a concomitant mesenteric hyperemia in the cat. Net fluid movement in the intestine was converted from absorption in the control state to secretion following phorbol ester administration. Intravenous atropine reduced the phorbol ester-induced secretion by 56%; clonidine abolished the remaining secretory response. In the rat, intragastric administration of phorbol ester produced enteropooling comparable to that of other potent intestinal secretagogues. Since phorbol esters are known to activate protein kinase C, these studies suggest that activation of protein kinase C in the small intestine may lead to a full secretory response. The evidence suggests that this secretion is accompanied by a metabolic hyperemia. These results suggest that protein kinase C plays an important role in the regulation of intestinal fluid transport.

Effect of auranofin on absorptive processes in the rat small bowel.

Mucosal auranofin (AF) caused a concentration dependent inhibition of fluid, Na+, glycine and galactose transport by everted sacs of rat small intestine (IC50 = 2 X 10(-4) M). Inhibition of nutrient absorption was not due to reduced fluid uptake since a similar reduction in fluid uptake induced by a mucosal osmotic load failed to alter glycine transport. Inhibitory effects of AF were not observed when metabolism was reduced, suggesting that carrier mediated entry processes were unaltered. AF inhibited Na+, K+-ATPase activity in isolated enterocytes (IC50 = 2 X 10(-4) M), without affecting Mg2+-dependent ATPase activity. Our studies suggest that the actions of AF on small intestinal absorption may result from inhibition of the Na+ pump.

The effect of auranofin on the colonic transport of Na+ and fluid in the rat.

Auranofin in the mucosal fluid caused a dose-dependent inhibition of fluid and Na+ absorption by everted sacs of rat colon. Serosal auranofin was without effect. (Na+ + K+)ATPase activity of homogenates of mucosal scrapes of rat colon was inhibited by auranofin in a dose-related manner, while Mg2+-ATPase activity was little affected. These actions of the drug on colonic transport mechanisms could contribute to the diarrhoea associated with auranofin therapy.

Effect of auranofin (SK&F 39162) on water and electrolyte flux in canine small bowel: a possible diarrheogenic mechanism.

Auranofin (AF) a new antiarthritic gold compound effective when administered orally, frequently causes diarrhea with abnormal stool electrolyte content. Studies were designed to determine the mechanism of the diarrhea caused by AF. In perfused canine Thiry-Vella loops, AF caused significant elevations in effluent volume, osmolarity, and sodium concentration and a significant decrease in potassium concentration. In mucosal homogenates of rat small bowel, AF inhibited sodium, potassium ATPase in a concentration dependent manner. AF did not alter canine colonic smooth muscle activity in vitro. We suggest that AF induced diarrhea results from interruption of normal water and electrolyte absorption by inhibition of enterocyte sodium, potassium ATPase activity.

Intestinal ion transport and diarrheal disease.

The physiology of intestinal electrolyte transport is currently an area of intense research interest. Also, reports regularly appear that define possible roles of various endocrine, paracrine, and neurohumoral substances in regulating intestinal ion and water flux. A vast body of knowledge has appeared recently that focuses on the action of specific intracellular mediators or second messengers and certain biochemical events that are thought to be involved in this transport process. This area of research has drawn the attention of the clinical investigator as well as the basic scientist because of the implications of these findings to the understanding of secretory disorders of the gastrointestinal tract, in particular diarrheal disease. The purpose of this review is to focus on recent findings reported in three major areas: the physiology of intestinal electrolyte transport and its regulation; the pathophysiology of secretory diarrhea; and current thoughts and practices in the therapeutic approach to the disease.

Evidence for protein kinase C as a regulator of intestinal electrolyte transport.

We examined the possibility that the Ca2+-and phospholipid-dependent protein kinase, protein kinase C, may regulate intestinal electrolyte transport. By use of two active phorbol esters, known activators of protein kinase C, we studied the secretory response in rat small intestine. In the in vivo enteropooling assay, 4 beta-phorbol 12-myristate 13-acetate given by gavage produced intestinal fluid secretion and accumulation comparable with that of known secretagogues. The response was dose dependent and only partially blunted at higher doses by atropine. Known inactive phorbol esters failed to elicit a secretory response. In Ussing chamber preparations, 4 beta-phorbol 12,13-dibutyrate applied on the serosal side produced a dose-dependent increase in short-circuit current (Isc). This response was totally dependent on the presence of Cl ion, and as seen in vivo atropine only partially attenuated the Isc response to high concentrations of phorbol ester. Only a minimal increase in Isc was observed when the ester was applied to the mucosal side. Protein kinase C was found to phosphorylate the microvillus membrane of the enterocyte, and the phosphorylation was stimulated by phorbol ester in a concentration-dependent manner. Two membrane proteins of Mr 51,000 and 46,000 were the primary substrates of the enzyme. These studies demonstrate that phorbol esters, specific activators of protein kinase C, elicit the secretory response in rat small intestine both in vivo and in vitro and that this response is mediated by an increase in Cl ion secretion. Furthermore, protein kinase C-mediated phosphorylation of specific microvillus membrane proteins suggests that protein kinase C may modify the functional state of the microvillus membrane of the enterocyte.

Action of serotonin on the gastrointestinal tract.

Serotonin is localized in the enterochromaffin cells of the gastrointestinal mucosa and within neurons in the enteric nervous system. It can be released into the blood or into the lumen of the gut. Serotonin inhibits gastric acid secretion and may be an endogenous enterogastrone. It appears to stimulate the production and release of gastric and colonic mucus. When placed on the serosal surface of the rabbit ileum in vitro, serotonin increases short-circuit current and inhibits the mucosal-to-serosal flux of NaCl. Serotonin potentially is involved in the pathogenesis of diarrhea due to amoebae or cholera. As an enteric neurotransmitter, serotonin affects neural modulation of gut smooth muscle function and may act either directly on mesenteric vascular smooth muscle or through enteric nerves to influence gastrointestinal blood flow. Thus, since serotonin may be involved in multiple physiological processes of digestion, this report reviews and summarizes the role of this ubiquitous substance in the major functions of the gastrointestinal system.