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M Field

Publications and source records attributed to M Field.

At least 163 records · Page 9Linked to original sources
PubMed1982

alpha 2-Adrenergic receptor regulation of ion transport in rabbit ileum.

Catecholamines are known to decrease short-circuit current (Isc), stimulate NaCl absorption, and inhibit HCO3 secretion in rabbit ileal mucosa in vitro. These effects appear to be mediated by alpha-adrenergic receptors because they are partially blocked by phentolamine and not by propranolol. To further characterize this receptor system, we determined the interactions of epinephrine (Epi) with alpha-subtype-selective antagonists. Prazosin (PZ), a specific alpha 1-antagonist, did not alter the Epi dose-response curve at concentrations up to 10(-5) M. Yohimbine (YO), a specific alpha 2-antagonist, completely inhibited the Epi effect on Isc. At 10(-5) M, YO increased by 70-fold the concentration of Epi required to produce a half-maximal effect (ED50; from 1.4 X 10(-7) M to 10(-5) M). YO and PZ by themselves had no significant effect on Isc in concentrations up to 10(-5) M. Clonidine, a specific alpha 2-agonist, decreased Isc with an ED50 similar to that of Epi; its effect was blocked by YO but not by PZ. Two alpha 1-selective agonists, methoxamine and phenylephrine, only caused a decrease in Isc in doses greater than 10(-5) M. This effect was reversed by YO but not by PZ. The effects of YO and PZ on Epi-modified Cl fluxes were also determined. YO completely aborted the effects of Epi on net Cl flux. No significant effects were seen with PZ. We conclude that the effects of Epi on ileal ion transport are mediated by a specific alpha 2-adrenergic receptor present in ileal mucosa and that no physiologic alpha 1-receptor function can be demonstrated.

Animals↗
PubMed1982

cGMP modulation of ileal ion transport: in vitro effects of Escherichia coli heat-stable enterotoxin.

Diarrheagenic strains of Escherichia coli have been shown to produce a heat-stable enterotoxin (ST) that simulates guanylate cyclase, increases short-circuit current (Isc), and inhibits active Cl absorption in the intestine. In rabbit ileum, the ion transport effects are smaller than those produced by cAMP-related agonists. Because ST may be a selective cGMP agonist, we further explored its mode of action in rabbit ileum. ST inhibits net Na and net Cl absorption. ST also inhibits the same fraction of Cl influx across the brush border that theophylline inhibits. At maximal doses, ST and 8-bromo-cGMP (8-Br-cGMP) had nearly equal, nonadditive effects of Isc that were about 66% of that produced by 8-Br-cAMP. ST increased mucosal cGMP concentration 16-fold, whereas epinephrine, an inhibitor of secretion, increased cGMP concentration by only 30%. This is insufficient to alter ion transport because doses of ST that increased cGMP concentration by 100% failed to alter Cl fluxes. Furthermore, epinephrine did not increase cGMP concentration in isolated enterocytes. We conclude that 1) cGMP mediates ST effects on ion transport, and 2) although ST and cAMP-related agonists have the same antiabsorptive effects, ST is less effective in stimulating electrogenic Cl secretion.

Animals↗
PubMed1981

Role of prostaglandins in the regulation of intestinal electrolyte transport.

The E prostaglandins (and to a lesser extent PGE2 alpha) stimulate active electrolyte secretion in mammalian small intestine and colon. They do so by stimulating intestinal mucosal adenylate cyclase and thereby increasing cAMP concentration. The diarrheagenic action of the prostaglandins is seen as a side effect of their therapeutic use, in certain hormone-secreting tumors, and in inflammatory lesions of the bowel in which leukocyte infiltrates are the probable sources of prostaglandin excess. Prostaglandins are also normally synthesized by intestinal epithelial cells and appear to play an important role in the physiologic regulation of intestinal fluid transport. In recent in vitro studies, we have shown that addition of arachidonic acid (K 1/2 congruent to 10(-6) M) also stimulates secretion, cAMP accumulation, and PGE2 production in rabbit ileal mucosa. In the continued presence of arachidonate, tachyphylaxis develops: both secretory and cAMP responses have a half-life of about twenty minutes and subsequent additions of arachidonate produce little or no further response. In contrast, PGE2 production continues undiminished. Similar tachyphylaxis develops when PGE2 itself is added. Resensitization following removal of PGE2 is rapid, 50% of the initial sensitivity being restored in 6-7 min. Prostaglandin desensitization has been noted in other cell systems and appears to be exerted on adenylate cyclase.

Adenylyl Cyclases↗
PubMed1981

Comparison of the biological actions of three purified heat-stable enterotoxins: effects on ion transport and guanylate cyclase activity in rabbit ileum in vitro.

The biological activities of three purified preparations of heat-stable enterotoxin (ST), elaborated by different strains of Escherichia coli and known to differ in their amino acid composition and molecular size, were compared in the rabbit ileum. The mechanisms of action and potencies of all three purified STs were similar and resembled those previously demonstrated for partially purified ST. They all increased electrical potential difference and short-circuit current, inhibited active Cl- absorption, increased cyclic guanosine 3',5'-monophosphate production, and stimulated particulate guanylate cyclase activity in ileal mucosa. Their molar potencies were also similar, the concentrations of toxin required for half-maximal response differing less than fourfold in short-circuit current response and twofold in guanylate cyclase activity. However, there were 10-fold differences in potency when activity was expressed in mouse units per milliliter. Thus, heterogeneity in the size of these three ST molecules is not reflected in a difference in their mechanisms of action or potencies.

Animals↗
PubMed1981

Physiologic and pharmacologic effects of glucocorticoids on ion transport across rabbit ileal mucosa in vitro.

Physiologic and pharmacologic effects of glucocorticoids on ileal ion transport were examined in vitro. Tissues were obtained from the three following groups of rabbits: (a) normal; (b) glucocorticoid deficient, which were treated with aminoglutethimide (AG), 100 mg twice daily for 3 d, with a resulting marked reduction in urinary cortisol excretion but no decrease in urinary aldosterone; and (c) methylprednisolone-treated (MP), 40 mg daily for 2 d. Transileal NaCl fluxes were measured with radioisotopes under short-circuit conditions, and the net HCO(3) flux was assumed equal to that portion of the short-circuit current (I(sc)) not accounted for by Na and Cl. In NaCl Ringer's solution containing 25 mM HCO(3) (pH 7.4), normals absorbed both Na and Cl and secreted HCO(3); the I(sc) was greater in both AG and MP groups than in normals; in the AG group, no Na was absorbed, and Cl as well as HCO(3) was secreted; in the MP group, more Na was absorbed and more HCO(3) secreted than in normals. Addition of glucose to the luminal side caused similar increments in I(sc) in all three groups, suggesting similar rates of Na-coupled glucose absorption. Secretory response was assessed with a maximal secretory simulus (8-Br-cAMP) and also a submaximal, cGMP-related secretory stimulus (Escherichia coli heat-stable enterotoxin). After addition of 8-Br-cAMP, the rates of net Cl secretion were similar in all three groups, suggesting no effect of glucocorticoids on maximal secretory capacity. Because the AG group was already secreting Cl, however, the cAMP-induced change in net Cl flux was least in this group. After addition of heat-stable enterotoxin, there were similar changes in net Cl flux in all three groups. To examine specifically Cl-independent, electrogenic Na transport, we used a 10 mM HCO(3), Cl-free SO(4)-Ringer (ph 7.2) in which net Na absorption was previously shown to be equal to the I(sc). Under these conditions, I(sc) was greatest in the MP group and least in the AG group. In vitro addition of hydrocortisone, 50 mug/ml, to AG tissues had no effect on Cl fluxes or I(sc) over a 3.5-h period. No differences among groups were observed with respect to morphology, electrical resistance, or cGMP concentration. We conclude that (a) the effect of glucocorticoid deficiency is similar to that of a submaximal secretory stimulus in that Na absorption is inhibited and some Cl secretion develops; (b) electrogenic Na absorption is depressed in glucocorticoid deficiency and enhanced in glucocorticoid excess; (c) glucocorticoid excess increases HCO(3) secretion; and (d) glucocorticoid status does not affect maximal secretory capacity.

Aminoglutethimide↗
PubMed1981

Antisecretory effects of indomethacin on rabbit ileal mucosa in vitro.

Prior in vivo studies have shown that indomethacin, which inhibits prostaglandin (PG) synthesis, affects fluid transport in the small bowel, enhancing spontaneous fluid absorption and reducing the amount of fluid that accumulates in response to cholera toxin and other secretory stimuli. To further explore the mechanisms involved, we determined the effects of indomethacin on ion transport, cAMP concentration, and PGE2 production in rabbit ileal mucosa in vitro. Indomethacin (1 mM), when added alone, had no significant effect on short-circuit current (either basal or glucose-stimulated), Cl fluxes, or cAMP concentration. Indomethacin did, however, inhibit the ion transport changes caused by several secretagogues: Effects of theophylline, Ca-ionophore A23187, and arachidonate were reversibly inhibited by at least 65%, whereas effects of dibutyryl cAMP, 16,16-dimethyl PGE2, cholera toxin, and heat-stable Escherichia coli enterotoxin were inhibited by about 30%. Indomethacin also inhibited the theophylline-evoked increase in cAMP concentration. Indomethacin decreased PGE2 production under basal conditions and in the presence of theophylline and A23187, which may partly explain the antisecretory action of the drug. Since arachidonate increased PGE2 release from the mucosa more than 10-fold and indomethacin did not inhibit this effect, indomethacin at high concentration (0.5-1 mM) appears to also inhibit the action of intestinal secretagogues by a prostaglandin-independent mechanism. This study also demonstrates that the antisecretory effect of indomethacin is not simply due to stimulation of an unrelated absorptive process.

Animals↗
PubMed1981

Inhibition of intestinal secretion in rats by colchicine and vinblastine.

The role of microtubules in small intestinal electrolyte secretion was investigated in rats by testing the effects of colchicine and vinblastine, both of which inhibit microtubule assembly. In the intact rat, intraperitoneal injection of colchicine (5 mg/kg) inhibited cholera toxin and prostaglandin-stimulated secretion without inhibiting their effects on adenylate cyclase or cAMP concentration. Pretreatment with colchicine had no effect on fluid transport in the absence of secretory stimuli. When added to rat ileum in vitro, colchicine reduced by 60% the short-circuit current (Isc) response to dibutyryl cAMP added 4 h later, whereas its structural isomer, lumicolchicine, which does not inhibit microtubule assembly, was ineffective. Vinblastine reduced by 55% the Isc response to dibutyryl cAMP and theophylline added 2 h later. Two hour pretreatment with vinblastine also reduced by 40% the Isc response to the cholinergic agonist, carbamylcholine, a Ca-dependent secretory stimulus which does not increase cAMP concentration. In contrast to their antisecretory actions, neither colchicine nor vinblastine inhibited glucose-stimulated active Na absorption. These results suggest a role for microtubules in active electrolyte secretion in the small intestine.

Adenylyl Cyclases↗
PubMed1980

Mode of action of heat-stable Escherichia coli enterotoxin. Tissue and subcellular specificities and role of cyclic GMP.

Some enteric strains of Escherichia coli release a heat-stable enterotoxin which, in contrast to cholera and heat-labile E. coli enterotoxins, stimulates guanylate cyclase (GTP pyrophosphate-lyase (cyclizing), EC 4.6.1.2). We have examined the tissue spcificity of its action and the relation of its action to those of the 8-bromo analogues of cyclic GMP and cyclic AMP. Heat-stable enterotoxin stimulated guanylate cyclase activity and increased cyclic GMP concentration throughout the small and large intestine. It increased transepithelial electric potential difference and short-circuit current in the jejunum, ileum and caecum but not in the duodenum or distal colon. This pattern of electrical responses was mimicked by 8-bromo-cyclic GMP. However, 8-bromo-cyclic AMP produced an electrical response in all intestinal segments. The enterotoxin failed to stimulate guanylate cyclase inliver, lung, pancreas or gastric antral mucosa. In the intestines, it stimulated only the particulate and not the soluble form of the enzyme. Preincubation of the toxin with intestinal membranes did not render it capable of stimulating pancreatic guanylate cyclase. Cytosol factors did not enhance the toxin's stimulation of intestinal guanylate cyclase. This study supports the role of cyclic GMP as intracellular mediator for heat-stable enterotoxin and suggests that the toxin affects a membrane-mediated mechanism for guanylate cyclase activation that is unique to the intestines.

Animals↗
PubMed1980

Ion transport across the isolated intestinal mucosa of the winter flounder, Pseudopleuronectes americans: II. effects of cyclic AMP.

Addition of cyclic AMP and theophylline to the intestinal mucosa of the winter flounder, Pseudopleuronectes americanus decreased short-circuit current and net Na and Cl absorption and increased total conductance and the serosa-to-mucosa unidirectional Cl flux (JsmCl). The last two changes were independent of the original rate of NaCl absorption and persisted even when net absorption of Na and Cl had been abolished by ouabain. Voltage clamp experiments revealed that the increment in JsmCl produced by cyclic AMP is PD-insensitive and therefore not due to an increase in the Cl conductance of the paracellular shunt. Cyclic AMP appears, therefore, both to inhibit net NaCl absorption and to increase the Cl permeability and total conductance of the intestinal epithelial cells; its failure to stimulate secretion (in contrast to its action on mammalian intestine) may be related to the absence of crypts in flounder intestinal epithelium.

Animals↗
PubMed1980

Control of guinea pig intestinal electrolyte secretion by a delta-opiate receptor.

The effects of opioids on transepithelial potential difference and short-circuit current across guinea pig ileum stripped of one muscle layer were measured in vitro in Ussing chambers. Opioid peptides such as [DAla2, DLeu5]enkephalin and [DAla2, DMet5]enkephalin, which are primarily agonists at delta-opiate receptors, were able to reduce transepithelial potential difference and short-circuit current at concentrations as low as 1 nM. The narcotic drug etorphine was also very potent in reducing short-circuit current, but fentanyl and morphine, which are primarily agonists at mu-opiate receptors, were almost completely ineffective. Ketocyclazocine was relatively ineffective, and beta-endorphin had intermediate potency. All opioid effects could be reversed by the opiate antagonist naloxone. Somatostatin also reduced short-circuit current, but its effect was not reduced by naloxone. Chloride flux measurements indicated that the effect of etorphine on short-circuit current is associated with an enhancement of active Cl- absorption. The relative effects of opioids in this system suggest that their actions are being mediated by a specific delta-opiate receptor. In contrast, opioid effects on guinea pig intestinal smooth muscle seem to be primarily mediated by a mu-opiate receptor.

Animals↗