Phorbol ester-stimulated secretion in chicken ileum: role of arachidonic acid metabolism

Eicosanoids and the large intestine

During the past three decades, many studies have been conducted to determine the precise role of eicosanoids in colorectal physiology and pathophysiology. This research has increased our understanding of bioactive lipid signaling, and may contribute to the development of more effective therapeutic modalities for digestive diseases in the future. The purpose of this report is to provide a brief overview of the role of eicosanoids in the colon and rectum. This information has been organized according to both functional and disease-related categories. The role of eicosanoids in colonic secretion, motility, inflammatory bowel disease, and colorectal neoplasia will be discussed.

The regulation of epithelial cell cAMP- and calcium-dependent chloride channels

This chapter has focused on two types of chloride conductance found in epithelial cells. The leap from the Ussing chamber to patch-clamp studies has identified yet other conductances present which have also been electrophysiologically characterized. In the case of the swelling activated wholecell chloride current, a physiological function is apparent and a single-channel basis found, but its genetic identity remains unknown (see reviews by Frizzell and Morris, 1994; and Strange et al., 1996). The outwardly rectified chloride channel has been the subject of considerable electrophysiological interest over the past 10 years and is well characterized at the single-channel level, but its physiological function remains controversial (reviewed by Frizzell and Morris, 1994; Devidas and Guggino, 1997). Yet other conductances related to the CLC gene family also appear to be present in epithelial cells of the kidney (reviewed by Jentsch, 1996; Jentsch and Gunter, 1997) where physiological functions for some isoforms are emerging. Clearly, there remain many unknowns. Chief among these is the molecular basis of GCa2+Cl and many of other the conductances. As sequences become available it is expected that the wealth of information gained by investigation into CFTR function will provide a conceptual blueprint for similar studies in these later channel clones.

Calcium regulated chloride permeabilities in primary cultures of rabbit colonocytes

To determine if calcium-dependent secretagogues directly act on epithelial cells to elicit Cl- secretion, their effects on Cl- transport and intracellular Ca(2+) concentrations ([Ca2+]i) were determined in primary cultures of rabbit distal colonic crypt cells. The Cl- sensitive fluorescent probe, 6-methoxyquinolyl acetoethyl ester, MQAE and the Ca(2+)-sensitive fluorescent probe, fura-2AM were used to assess Cl- transport and [Ca2+]i, respectively. Basal Cl- transport (0.274 +/- 0.09 mM/sec) was inhibited significantly by the Cl- channel blocker diphenylamine-2-carboxylate (DPC, 50 microM, 0.068 +/- 0.02 mM/sec; P < 0.001) and the Na+/K+/ 2Cl- cotransport inhibitor furosemide (1 microM, 0.137 +/- 0.04 mM/sec; P < 0.01). Ion substitution studies using different halides revealed the basal influx to be l- > F- > or = Cl- > Br-. DPC inhibited l- influx by approximately 50%, F- influx by 80%, Cl-influx by 85%, and Br- influx by 90%. Furosemide significantly inhibited influx of Br- (84%) and Cl- (81%) but not of F- and l-. The effects of agents known to alter biological response by increasing [Ca2+]i in other epithelial systems were used to stimulate Cl- transport. Cl- influx in mM/second was stimulated by 1 microM histamine (0.58 +/- 0.05), 10 microM neurotensin (2.07 +/- 0.32), 1 microM serotonin (1.63 +/- 0.28), and 0.1 microM of the Ca2+ ionophore A23187 (2.05 +/- 0.40). The Cl- permeability stimulated by neurotensin, serotonin, and A23187 was partially blocked by DPC or furosemide added alone or in combination. Histamine-induced Cl- influx was significantly inhibited by only furosemide. Indomethacin blocked histamine-stimulated Cl- permeability but had no effect on the actions of the other agents. These studies, focusing on isolated colonocytes without the contribution of submucosal elements, reveal that (1) histamine stimulates Cl- transport by activating the Na+/K+/2Cl- cotransporter via a cyclooxygenase-dependent pathway; (2) neurotensin, serotonin, and A23187 activate both Cl- channels and the cotransporter, and their actions are cyclooxygenase-independent.

Eicosanoids and the gastrointestinal tract

Determining the role of eicosanoids in gastrointestinal physiology and pathophysiology has been an active area of investigation over the past 20 years. The landmark discovery of prostaglandin endoperoxide synthase and other enzymes involved in the production of arachidonic acid products (lipoxygenases and epoxygenases) ushered in a new era of research. The goal of this review is to distill a large body of work pertaining to studies of eicosanoids in the gastrointestinal tract. This review has been organized according both to functional (secretion and motility) and disease-related (inflammation, mucosal injury, and neoplasia) effects. The aim of this article is to present a clear summary of this area of gastroenterology so that future research can be directed in a logical and productive manner.

Stimulation of enterocyte protein kinase C by laxatives in-vitro

To elucidate the role of protein kinase C in the mechanism of action of stimulatory laxatives, experiments were performed with preparations of rat lysed enterocytes. The phorbol ester 4-beta-phorbol 12-myristate 13-acetate (PMA) concentration-dependently (2-200 micrograms mL-1) stimulated the activity of protein kinase C in this preparation. Ricinoleic acid, the active principle of castor oil, deacetylbisacodyl, the active principle of bisacodyl, and deoxycholic acid exerted the same effect, although less efficiently. This reflects their potency for inducing intestinal fluid secretion and prostaglandin release, effects that are also induced more potently by PMA. Accordingly, the potency of the three C18 fatty acids, ricinoleic acid, stearic acid and oleic acid on protein kinase C activity in-vitro, on prostaglandin E2 release and on net fluid secretion in-vivo runs in parallel. It is therefore concluded that stimulatory laxatives activate protein kinase C, leading to prostaglandin E2 release, thus resulting in net fluid secretion.

Characterization of chicken intestinal brush border membrane Ns/H exchange

1. Na/H exchange is the major pathway for Na uptake in brush border membrane vesicles from chicken small intestine. Hanes-Woolf analysis demonstrated that Na and H competed at the same extravesicular site. The KNa for Na+ at extravesicular pH 6.6 is 35 mM and at pH 7.4, 12 mM. 2. Similar to mammalian intestinal cells, the Na/H exchanger does not appear to have an internal proton modifier site. Varying intravesicular pH from 6.1 to 7.8 stimulates uptake, but a sigmoidal relationship is not observed. 3. The ability of several amiloride analogs to inhibit the exchanger was tested and the inhibitory profile was similar, but not identical to Na/H exchangers in mammalian tissues. The potency series (from most to least potent) is hexamethylamiloride approximately ethylisopropylamiloride > methylisobutylamiloride > dimethylamiloride >> amiloride.

Regulation of apical Cl- conductance and basolateral K+ conductances by phorbol esters in HT-29cl.19A cells

The effect of phorbol esters [4 beta-phorbol 12,13-dibutyrate (PDB) and phorbol 12-myristate 13-acetate (PMA)] on potential differences and resistances was studied with the conventional microelectrode technique applied to confluent filter-grown monolayers of the human colon carcinoma cell line HT-29cl.19A. Phorbol esters (PDB or PMA from 10(-7) to 10(-6) M) evoked 1) a transient increase in the transepithelial potential difference (peak value 3.5 +/- 0.5 mV), 2) a depolarization of the cell potential by 23 +/- 2 mV at the peak of the transepithelial potential change and a continued decrease during the decline of the transepithelial potential, and 3) a decrease of the fractional resistance of the apical membrane consisting of two phases, a relative rapid one (time constant 1.2 +/- 0.2 min) and a much slower further decrease during the second phase (time constant 34 +/- 1 min). Ion replacements and electrical circuit analyses indicate that PDB activates an apical Cl- conductance and slowly inhibits the basal K+ conductance of the basolateral membrane. PDB reduced the transepithelial response to forskolin due to inhibition of the basal K+ conductance. The Ca2+ ionophore ionomycin accelerated that effect of PDB. Staurosporine inhibited the effects of PDB, suggesting that the PDB effects are mediated via activation of a protein kinase C.

Bethanechol inhibition of chicken intestinal brush border Na/H exchange: Role of protein kinase C and other calcium-dependent processes

Bethanechol, a muscarinic agonist, inhibits the initial rate of amiloride-sensitive Na uptake by intact mucosa of avian small intestine as well as by isolated chicken villus enterocytes, an effect that is maximal at 90 seconds and reverses by 6 minutes. Bethanechol similarly decreases intracellular pH in isolated cells suspended in bicarbonate-free buffer in a time course similar to inhibition of enterocyte Na uptake, suggesting inhibition of Na/H exchange. In brush border membrane vesicles rapidly prepared from cells stimulated with bethanechol, proton-dependent 22Na uptake is transiently inhibited in a time course similar to inhibition of cell Na uptake. Bethanechol also stimulates transient translocation of protein kinase C from the cytosol to the particulate fraction, a portion of this activity translocating to the brush border membrane. To determine the calcium dependence of bethanechol's action, enterocytes were loaded with varying concentrations of the calcium buffering agent quin-2. Inhibition of cell Na uptake by the calcium ionophore ionomycin could be completely reversed by quin-2 buffering in a concentration-dependent manner. In contrast, quin-2 buffering had little or no effect on the inhibition of Na uptake caused by the protein kinase C activators phorbol esters and oleoylacetylglycerol. Bethanechol's inhibitory effects were partially, but not completely reversed by quin-2 buffering. These data suggest that the effects of bethanechol on chicken villus enterocyte brush border Na/H exchange are mediated by calcium-dependent process(es) as well as by protein kinase C.

Dual role for protein kinase C alpha as a regulator of ion secretion in the HT29cl.19A human colonic cell line

The involvement of protein kinase C (PKC) in the regulation of intestinal ion secretion was studied in polarized monolayers of the HT29cl.19A human colon carcinoma cell line. Carbachol, phorbol esters [PMA (phorbol 12-myristate 13-acetate) and PDB (phorbol 12,13-dibutyrate)] and 8-bromo cyclic AMP (8-Br-cAMP) induced Cl secretion, as measured by a rise in the short-circuit current (ISC). The electrical response to carbachol coincided with a transient translocation of PKC alpha from the soluble to the particulate fraction. The carbachol-, PDB- and 8-Br-cAMP-induced ISC responses were inhibited by pretreatment of the cells with PMA (0.5 microM) for 2 h, a time period in which PKC alpha, beta 1 and gamma levels were not changed. As shown by 86Rb+ and 125I- efflux studies, the main targets for this inhibition were basolateral K+ transporters rather than apical Cl- channels. Prolonged exposure to PMA (24 h) led to a 60% recovery of the 8-Br-cAMP response, but not of the carbachol- or PDB-provoked secretion. As shown by immunoblotting with PKC-isoenzyme-specific antisera, the recovery of the 8-Br-cAMP response coincided with the down-regulation of PKC alpha, whereas the levels of PKC beta 1 and gamma were unmodified. These results suggest that PKC alpha, but not PKC beta 1 or gamma, is involved in both acute stimulation and chronic inhibition of ion secretion in the HT29cl.19A colonic cell line.

Ca-mediated stimulation of Cl secretion by reactive oxygen metabolites in human colonic T84 cells

Monochloramine (NH2Cl), a granulocyte-derived reactive oxygen metabolite (ROM), increases short-circuit current (Isc) in cultured T84 monolayers in a concentration-dependent manner up to nonlethal concentrations of 75 microM. Isc increases slowly after NH2Cl, reaching a peak value of 18 +/- 2 microA/cm2 20 min after addition. The Isc changes are persistent (lasting over 20-30 min), depend on medium Cl, and are inhibitable with bumetanide. 36Cl flux studies demonstrated that NH2Cl increases serosa-to-mucosa flux of Cl without changing mucosa-to-serosa flux, consistent with stimulation of electrogenic Cl secretion. Isc responses to NH2Cl, but not PGE2, are dependent on medium calcium. As demonstrated in fura-2-loaded T84 cells, NH2Cl increases free cytosolic calcium by influx of extracellular Ca2+ and by release of Ca2+ from endogenous stores. However, NH2Cl had no effect on phosphatidylinositol metabolism or cyclic nucleotide levels. We conclude that ROM directly stimulate electrolyte secretion, an effect in part mediated by increases in cytosolic Ca2+, possibly through increasing Ca2+ permeability of cellular membranes.

Phorbol ester inhibition of chicken intestinal brush-border sodium-proton exchange

Phorbol esters, specific activators of protein kinase C, inhibit amiloride-sensitive Na uptake from the mucosal medium in intact intestinal mucosa as well as in isolated chicken villus enterocytes. In isolated cells, maximal inhibition is observed at 60 s, and influx returns to control values within 15 min. This effect can be measured either as initial 22Na influx rates or by following changes in intracellular pH using the pH-sensitive fluorescent dye 5,6-carboxyfluorescein. The effects of amiloride and phorbol esters were not additive, suggesting inhibition of a common transport system, i.e., Na-H exchange. In brush-border membrane vesicles (BBMV) made from villus enterocytes, amiloride-sensitive Na-H exchange activity was significantly inhibited in phorbol ester-treated cells. The degree of inhibition of 22Na uptake by BBMV had the same time course and dose-effect relationship as phorbol ester-inhibited cellular Na uptake. Similarly, the time course of protein kinase C translocation from cytosol to particulate or brush-border membrane fractions correlated with Na uptake measurements made in whole cells and BBMV. These results suggest that protein kinase C activation in chicken villus enterocytes inhibits brush-border membrane Na-H exchange activity.