Proximal Colonic Mucosa Research Articles

Alpha 2-adrenergic receptor-mediated regulation of intestinal calcium transport.

The role of alpha 2-adrenergic receptors in the regulation of intestinal Ca2+ transport was studied in vitro in rabbit proximal colon and distal ileum. Unidirectional mucosal-to-serosal and serosal-to-mucosal fluxes (Jm----s and Js----m) of 45Ca2+ were measured across isolated mucosal sheets bathed on both sides with identical Ringer solutions (1.25 mM Ca2+) and short-circuited. In the proximal colon, mucosal-to-epithelial influx (Jm----e) of Ca2+ was also determined. Proximal colonic mucosa secreted Ca2+ (in nmol X h-1 X cm-2, Jm----s = 12.1 and Js----m = 42.2). Epinephrine (10 microM) abolished Ca2+ secretion (Jm----s = 24.1 and Js----m = 24.9) but did not affect Jm----e (21.4 in controls and 23.6 after epinephrine). These effects were mediated by alpha 2-adrenergic receptors: yohimbine abolished the effect of epinephrine, but prazosin and propranolol did not; furthermore, clonidine mimicked the effects of epinephrine, whereas methoxamine and isoproterenol had no effect. Ileal mucosa did not transport Ca2+ spontaneously; epinephrine (10 microM) increased Jm----s in the ileum without affecting Js----m, thereby stimulating net Ca2+ absorption. The mechanism for these alpha 2-receptor-mediated effects was examined in proximal colon. The effect of epinephrine on Jm----s was dependent on serosal Na+ but was unaffected by ouabain or the Na+-Ca2+ exchange inhibitor, benzamil. The effect of epinephrine on Js----m was dependent on serosal Na+ and was blocked by both ouabain and benzamil. In the absence of epinephrine benzamil did not alter Jm----s or Js----m.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of sialoadenectomy and salivary gland extracts on gastrointestinal mucosal growth and gastrin levels in the rat.

We have examined gastrointestinal mucosal growth 30 days after surgical removal of the submandibular-sublingual salivary glands and ligation of the parotid ducts of rats. The rate of [3H]thymidine uptake in vitro as an estimation of DNA synthesis and the content of DNA and RNA were examined in the oxyntic, duodenal and proximal colonic mucosa. DNA synthesis, DNA and RNA content of oxyntic mucosa were reduced in sialoadenectomized rats when compared to sham-sialoadenectomized control animals. There was no change in the degree of [3H]thymidine incorporation or DNA content of the duodenal or colonic mucosa. Intraperitoneal injection of an aqueous extract of the submandibular-sublingual salivary glands of rats (4.0 mg tissue protein in 0.1 M-sodium phosphate buffer administered twice a day for 15 days) increased the rate of DNA synthesis and the total mucosal DNA and RNA content in the oxyntic mucosa. Injections of extracts of spleen or muscle did not produce consistent results. Administration of epidermal growth factor (10 micrograms/kg) or pentagastrin (250 micrograms/kg) resulted in an increase of the level of DNA synthesis observed in the oxyntic mucosa of sialoadenectomized rats. Plasma and antral tissue levels of the trophic hormone, gastrin, were not significantly decreased in sialoadenectomized rats treated with 0.1 M-sodium phosphate-buffered saline. However, treatment with the salivary tissue extract did result in significant reductions in both plasma and tissue levels of gastrin. We conclude that elimination of the major salivary glands in the rat results in a decrease in [3H]thymidine uptake and DNA content of the gastric oxyntic mucosa. These effects are not mediated via a reduction in endogenous levels of the trophic hormone, gastrin. Administration of an aqueous extract of salivary tissue exerted a small but significant trophic influence on the oxyntic mucosa of the rat.

Mechanism of active potassium absorption and secretion in the rat colon.

To characterize and contrast the active potassium absorptive and secretory processes present in the rat colon, unidirectional 42K fluxes were performed under short-circuit conditions across isolated distal (D) and proximal (P) colonic mucosa of control rats and animals with hyperaldosteronism due to sodium depletion (aldosterone group). In the control D colon there was net potassium absorption (+0.51 +/- 0.07 mueq X h-1 X cm-2). The absorptive process appears electroneutral because net potassium flux ( JK net ) was unchanged in sodium-free Ringer solution (+0.76 +/- 0.12 mueq X h-1 X cm-2), whereas short-circuit current (Isc) was reduced to zero, and in chloride-free Ringer solution. In P colon of controls, net potassium secretion was -0.19 +/- 0.02 mueq X h-1 X cm-2 and was abolished by removal of sodium but not by removal of chloride. In both P and D colon aldosterone produced active potassium secretion (-0.39 +/- 0.06 mueq X h-1 X cm-2, P less than 0.001, and -0.90 +/- 0.07 mueq X h-1 X cm-2, P less than 0.001, respectively) that was sodium and chloride dependent. Although mucosal amiloride in D colon of aldosterone animals reduced net sodium flux to zero and reversed Isc from 4.1 +/- 0.6 to -1.1 +/- 0.1 mueq X h-1 X cm-2, net potassium secretion was not affected. Thus, in the presence of amiloride, Isc is accounted for by JK net (-0.93 +/- 0.12 mueq X h-1 X cm-2). These data indicate that 1) the active potassium absorptive process is electroneutral and could be explained by a potassium-proton exchange, and 2) the potassium secretory process is stimulated by aldosterone, is not inhibited by amiloride, and probably is electrogenic.

Cyclic Adenosine Monophosphate Stimulates Active Potassium Secretion in the Rat Colon

To determine whether cyclic adenosine monophosphate influences active potassium transport in the rat colon, we studied the effect of dibutyryl cyclic adenosine monophosphate and theophylline on unidirectional transmural 42K fluxes across proximal colonic mucosa under short-circuited conditions. Active potassium secretion (-0.19 +/- 0.02 microEq/h X cm2) was present in animals maintained on a normal potassium diet. Both 0.5 mM dibutyryl cyclic adenosine monophosphate and 5 mM theophylline significantly increased net potassium secretion by 0.49 +/- 0.04 and 0.33 +/- 0.03 microEq/h X cm2, p less than 0.001, respectively; the stimulation of net potassium secretion was secondary to an increase in serosal-to-mucosal potassium transport without change in mucosal-to-serosal potassium movement. A similar increase in active potassium secretion (from -0.15 +/- 0.03 to -0.32 +/- 0.03 microEq/h X cm2, p less than 0.005) was produced by bethanechol, a cholinergic muscarinic agonist that alters sodium and chloride transport by a noncyclic adenosine monophosphate, calcium-dependent process. In animals maintained on a high potassium diet, active potassium secretion was significantly increased to -0.79 +/- 0.17 microEq/h X cm2 (p less than 0.001). In these potassium-loaded animals, theophylline produced a greater increase in active potassium secretion (0.91 +/- 0.10 vs. 0.33 +/- 0.03 microEq/h X cm2, p less than 0.001) than in animals fed a normal potassium diet. These studies demonstrate that cyclic adenosine monophosphate and noncyclic adenosine monophosphate mediated secretogogues stimulate active potassium secretion. We speculate that the mechanism by which cyclic adenosine monophosphate increases active potassium secretion is related to an increase in luminal potassium conductance.

The neuroendocrine system of normal human appendix, ileum and colon, and in neurogenic appendicopathy.

Epithelial endocrine cells containing 5-hydroxytryptamine, substance P, somatostatin, enteroglucagon and vasoactive intestinal polypeptide-immunoreactivity were localized by immunocytochemistry in the mucosa of normal appendices, ileum and proximal colon, and in neurogenic appendicopathy. In neurogenic appendicopathy a large number of proliferating nerves were visualized independently of neurotransmitters by immunostaining for neuron-specific enolase. A large number of nerve fibers were shown to contain substance P-immunoreactivity and to be of intrinsic origin. Stromal endocrine cells containing 5-hydroxytryptamine, somatostatin- and possibly substance P-immunoreactivity, were observed in substantial numbers in neurogenic appendicopathy. Substance P may be involved as a neurotransmitter and/or as a paracrine/endocrine peptide in the pathogenesis of spastic contractions and abnormal peristalsis of the appendix, which are characteristic of neurogenic appendicopathy. Stromal endocrine cells may be considered to be the origin of certain carcinoids in the appendix.

Effects of Dietary Wheat Bran on Rat Colonic Structure and Mucosal Cell Growth

The effects of dietary wheat bran on colonic morphology and mucosal cell growth were examined in male Wistar rats. Two groups of rats were fed defined diets for 2 weeks. The control group was fed a fiber-free diet while the experimental group received the same diet plus a 20% wheat bran supplement. Nutrient intake and body weight gain were similar in the two groups of rats. The bran-fed group developed an increase in proximal (cecum) and distal colonic weights (P < 0.02) but mucosal weight was only increased in the distal colon when compared to the control group (P < 0.005). Histological measurements revealed deeper colonic crypts (P < 0.001) and mucosal cell hyperplasia in the bran-fed group in addition to an increase in muscle thickness (P < 0.001). Mucosal cell hyperplasia, as measured by the increases in mucosal protein (P < 0.01), DNA (P < 0.01) and RNA (P < 0.005) was most marked in the distal colon. Proximal colon showed no significant change in mucosal cell mass. Cell RNA content was decreased only in the proximal colonic mucosa (P < 0.01). These results demonstrate that dietary wheat bran regulates colonic mucosal cell growth according to the anatomical region of the colon, producing distal mucosal hyperplasia and decreasing proximal colonic cell RNA.fiber colonic mucosal cell growth

Different mechanisms for neutral amino acid uptake by new‐born pig colon

1. Mucosal amino acid uptake by pig proximal colon, measured independently for fourteen different amino acids each used at a concentration of 1 mM, ranged from 0.6 to 8.6 n-mole. cm(-2). min(-1) in the new-born to 0 to 0.3 n-mole. cm(-2). min(-1) in the 2-day-old animal. Long chain amino acids entered the mucosa of new-born pig proximal colon much more readily than did short chain amino acids.2. Glycine was used extensively to inhibit the uptake of other neutral amino acids. The degree of maximal inhibition produced depended on the amino acid used. The relative inability of glycine to inhibit the uptake of long chain amino acids suggested that these compounds could cross the brush border on a carrier inaccessible to glycine. The glycine-sensitive uptake remained more or less constant for all amino acids tested (1-2 n-mole.cm(-2).min(-1)); the glycine-insensitive uptake varied from 0 to 7 n-mole.cm(-2).min(-1) (glycine and methionine respectively).3. It is suggested that at least two mechanisms exist for the entry of neutral amino acids into pig proximal colon, one showing specificity for hydrophobic amino acids and the other having broad specificity. The mechanism responsible for the uptake of long chain essential amino acids predominates over the less specific mechanism.4. These results are discussed in relation to previous work carried out on the rabbit ileum where two similar systems for neutral amino acid entry have been shown to be present. Both tissues transport hydrophobic amino acids on their own specific carrier at approximately the same rate; the ability of the pig colon to transport amino acids on the broad specificity carrier is eight times less than in the rabbit ileum. The possibility is raised that this system is subject to regulation.

The purification and properties of carbonic anhydrases from guinea-pig erythrocytes and mucosae of the gastrointestinal tract.

Procedures for isolating carbonic anhydrase (EC 4.2.1.1) enzymes from the erythrocytes and the mucosae of the gastrointestinal tract of guinea pigs are described. From a haemolysate, haemoglobin was removed by the addition of ammonium sulphate, and also by two other methods, namely by gel filtration or by adsorption on DEAE-Sephadex. The crude enzyme thus obtained was resolved into the different isoenzymes by chromatography with DEAE-cellulose. From particle-free supernatants of homogenates of some gastrointestinal tissues, carbonic anhydrases were purified by ammonium sulphate fractionation, gel filtration, and ion-exchange chromatography with DEAE-cellulose. The major isoenzymes from blood, stomach, proximal colonic mucosa and caecal mucosa were homogeneous during ion-exchange chromatography, acrylamide-gel electrophoresis, and centrifugal examination. From these tissues, carbonic anhydrase was isolated as two major isoenzymes. They resemble the pairs of isoenzymes discovered in the bloods of other species. The carbon dioxide hydratase activity of one isoenzyme (;high activity' carbonic anhydrase) was 40 times that of the other isoenzyme (;low activity' carbonic anhydrase), as measured at a single substrate concentration. Two other minor components of the enzyme are also found in guinea-pig erythrocytes. All of the enzymes isolated had molecular weights of nearly 30000 (sedimentation equilibrium). ;High activity' carbonic anhydrases from blood and gastrointestinal tissues were indistinguishable according to some chemical, physical and kinetic measurements; similarly ;low activity' carbonic anhydrases from those tissues were indistinguishable. ;High activity' carbonic anhydrase was markedly different from the ;low activity' carbonic anhydrase with respect to its amino acid composition, chromatographic behaviour and isoelectric pH value. Marked differences were also found in the tissue concentrations of the major isoenzymes. It is suggested that the characteristic and selective distribution of the different forms of carbonic anhydrase in the guinea-pig tissues is related to the specific and different physiological functions of the enzymes.