Segmental heterogeneity of the rat colon in the response to activators of secretion on the cAMP-, the cGMP- and the Ca(2+)-pathway

Decreased electrogenic anionic secretory response in the porcine colon following in vivo challenge with Brachyspira spp. supports an altered mucin environment

Brachyspira spp. cause diarrheal disease in multiple animal species by colonization of the colon, resulting in colitis, mucus induction, and disrupted ion transport. Unique to spirochete pathogenesis is the immense production of mucus, resulting in a niche mucin environment likely favoring spirochete colonization. Mucin rheological properties are heavily influenced by anionic secretion, and loss of secretory function has been implicated in diseases such as cystic fibrosis. Here, the effects on the agonist-induced electrogenic anionic secretory response by infectious colonic spirochete bacteria Brachyspira hyodysenteriae and Brachyspira hampsonii were assessed in the proximal, apex, and distal sections of colon in Ussing chambers. Activation of secretion via isoproterenol, carbachol, and forskolin/3-isobutyl-1-methylxanthine demonstrated a significantly decreased change in short-circuit current ( I_sc) in Brachyspira-infected pigs in all sections. Tissue resistances did not account for this difference, rather, it was attributed to a decrease in anionic secretion as indicated by a decrease in bumetanide inhibitable I_sc. Quantitative RT-PCR and Western blot analyses determined that the major anionic channels of the epithelium were downregulated in diarrheic pigs paired with altered mucin gene expression. The investigated cytokines were not responsible for the downregulation of anion channel gene transcripts. Although IL-1α was upregulated in all segments, it did not alter cystic fibrosis transmembrane conductance regulator (CFTR) mRNA expression in Caco-2 monolayers. However, a whole cell Brachyspira hampsonii lysate significantly reduced CFTR mRNA expression in Caco-2 monolayers. Together, these findings indicate that these two Brachyspira spp. may directly cause a decreased anionic secretory response in the porcine colon, supporting an altered mucin environment likely favoring spirochete colonization. NEW & NOTEWORTHY This research demonstrates for the first time that the niche mucin environment produced by two infectious spirochete spp. is supported by a decrease in the electrogenic anionic secretory response throughout the porcine colon. Our findings suggest that the host's cytokine response is not likely responsible for the decrease in anionic secretory function. Rather, it appears that Brachyspira spp. directly impede ion channel transcription and translation, potentially altering colonic mucin rheological properties, which may favor spirochete colonization.

The Effect of Deoxycholic Acid on Secretion and Motility in the Rat and Guinea Pig Large Intestine

Background/Aims

Bile acid is an important luminal factor that affects gastrointestinal motility and secretion. We investigated the effect of bile acid on secretion in the proximal and distal rat colon and coordination of bowel movements in the guinea pig colon.

Methods

The short-circuit current from the mucosal strip of the proximal and distal rat colon was compared under control conditions after induction of secretion with deoxycholic acid (DCA) as well as after inhibition of secretion with indomethacin, 1,2-bis (o-aminophenoxy) ethane-N,N,N′,N′-tetra-acetic acid (an intracellular calcium chelator; BAPTA), and tetrodotoxin (TTX) using an Ussing chamber. Colonic pressure patterns were also evaluated in the extracted guinea pig colon during resting, DCA stimulation, and inhibition by TTX using a newly developed pressure-sensing artificial stool.

Results

The secretory response in the distal colon was proportionate to the concentration of DCA. Also, indomethacin, BAPTA, and TTX inhibited chloride secretion in response to DCA significantly (P < 0.05). However, these changes were not detected in the proximal colon. When we evaluated motility, we found that DCA induced an increase in luminal pressure at the proximal, middle, and distal sensors of an artificial stool simultaneously during the non-peristaltic period (P < 0.05). In contrast, during peristalsis, DCA induced an increase in luminal pressure at the proximal sensor and a decrease in pressure at the middle and distal sensors of the artificial stool (P < 0.05).

Conclusions

DCA induced a clear segmental difference in electrogenic secretion. Also, DCA induced a more powerful peristaltic contraction only during the peristaltic period.

Site-specific regulation of ion transport by prolactin in rat colon epithelium

The effect of prolactin (PRL) on ion transport across the rat colon epithelium was investigated using Ussing chamber technique. PRL (1 μg/ml) induced a sustained decrease in short-circuit current (I(sc)) in the distal colon with an EC(50) value of 100 ng/ml and increased I(sc) in the proximal colon with an EC(50) value of 49 ng/ml. In the distal colon, the PRL-induced decrease in I(sc) was not affected by Na(+) channel blocker amiloride or Cl(-) channel blockers, NPPB, DPC, or DIDS, added mucosally. However, the response was inhibited by mucosal application of K(+) channel blockers glibenclamide, quinidine, and chromanol 293B, whereas other K(+) channel blockers, Ba(2+), tetraethylammonium, clotrimazole, and apamin, failed to have effects. The PRL-induced decrease in I(sc) was also inhibited by Na(+)-K(+)-2Cl(-) transporter inhibitor bumetanide, Ba(2+), and chromanol 293B applied serosally. In the transverse and proximal colon, the PRL-induced increase in I(sc) was suppressed by DPC, glibenclamide, and bumetanide, but not by NPPB, DIDS, or amiloride. The PRL-induced changes in I(sc) in both distal and proximal colon were abolished by JAK2 inhibitor AG490, but not BAPTA-AM, the Ca(2+) chelating agent, or phosphatidylinositol 3-kinase inhibitor wortmannin. These results suggest a segment-specific effect of PRL in rat colon, by activation of K(+) secretion in the distal colon and activation of Cl(-) secretion in the transverse and proximal colon. Both PRL actions are mediated by JAK-STAT-dependent pathway, but not phosphatidylinositol 3-kinase pathway or Ca(2+) mobilization. These findings suggest a role of PRL in the regulation of electrolyte transport in mammalian colon.

Comparison of the Secretory Actions of 5-Hydroxytryptamine in the Proximal and Distal Colon of the Rat

The ability of 5-hydroxytryptamine (5-HT) to induce a secretory response in rat proximal and distal colon was examined both in-vivo and in-vitro by measuring transintestinal electrical activity.

In-vivo 5-HT caused a dose-dependent increase in the potential difference (PD) in both regions of the colon (maximum PD change = 7·2 ± 0·5 (n = 17)mV in proximal colon and 9·2 ± 0·7 (n = 17)mV in distal colon), an effect that was also observed in stripped (outer muscle layers removed) colonic sheets where the PD change was found to result from a rise in short-circuit current (SCC, maximum change = 150 ± 24 (n = 15)μA cm−2 in proximal colon and 126 ± 10 (n = 19)μA cm−2 in distal colon).

The effects of 2-methyl-5-hydroxytryptamine (2-Me-5-HT), a relatively selective agonist at 5-HT3 receptors, and 5-methoxytryptamine (5-MT), an agonist at all 5-HT receptors except 5-HT3, were also tested, their specificity of action being confirmed by their actions on cardiovascular function in-vivo. 2-Me-5-HT produced a similar response to 5-HT in proximal colon, but was less effective in the distal region, particularly in-vitro where it failed to induce any significant change in electrical activity. In contrast, 5-MT was more effective in the distal colon. Frusemide (10−3 M) inhibited the rise in SCC induced by both 2-Me-5-HT and 5-MT, indicating that, like 5-HT, these agonists stimulated electrogenic Cl· secretion. The 5-HT3 antagonist granisetron abolished the effects of 2-Me-5-HT, both in-vivo (8·6 times 10−8 molkg−1) and in-vitro (1·4 times 10−6M, 1·4 times 10−4M), but only caused a slight inhibition of the response to 5-HT in-vivo and no inhibition at all in stripped colonic sheets.

It is concluded that although 5-HT induces a secretory response in both proximal and distal colon, the mechanisms responsible differ, with 5-HT3 receptors making a greater contribution in the proximal region.

Segmental heterogeneity of epithelial ion transport induced by stimulants in rat distal colon

Mammalian colon plays an important role in electrolyte and water balance, and exhibits significant segmental heterogeneity. The different responses to stimulants even exist in different segments of rat distal colon. The present study focus on the segmental heterogeneity of epithelial responses to different stimulants and investigates the possible mechanism by using short circuit current recording technique. Baseline I(SC) in the segment 4 of distal colon (DC(4), 20.8+/-2.8 microA.cm(-2)) was significantly lower than that in the segment 1 of distal colon (DC(1), 40.5+/-1.9 microA.cm(-2)). Basolateral application of indomethacin induced a larger reduction of the baseline I(SC) in DC(4) (-28.2+/-3.9 microA.cm(-2)) than that in DC(1) (-10.1+/-3.9 microA.cm(-2)). Moreover DC(4) are more sensitive to foskolin (a cAMP activator, apical side), Ach (basolateral side) and 5-HT (basolateral side) than DC(1), which was not affected by pretreatment with amiloride, a blocker of epithelial Na(+) channel. Basolateral pretreatment with atropine (muscarinic cholinergic receptor antagonist) for 10 min, Ach-induced DeltaI(SC) increases in both DC(1) and DC(4 )were totally blocked. Otherwise, 5-HT(4) receptor antagonist GR113808 (basolateral side) and SB-204070 (basolateral side) completely inhibited 5-HT-induced I(SC) increases in both DC(1) and DC(4). Taken together, the results suggested that the segmental heterogeneity of epithelial responses to different stimulants exists in rat distal colon. And it is more likely related to the dissimilar distribution of membrane proteins involved in the ion transports within different segments of rat distal colon.

Segmental heterogeneity of electrogenic secretions in human ascending colon and rectum

AIMS

We have attempted to ascertain putative segmental differences in the secretory responses of the human ascending colon and rectum.

METHODS

From the mucosal biopsy samples of two segments, the short-circuit current (I(sc)) and tissue resistance (R(te)) were compared under control conditions, as well as after the induction of secretion, using a modified Ussing chamber. We also performed semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) to detect and quantify transport proteins.

RESULTS

The spontaneous I(sc) in the ascending colon was found to be greater than that in the rectum (P<0.01), whereas isobutylmethylxanthine/forskolin and carbachol (CCh) induced a greater rise in I(sc) in the rectum than in the ascending colon (P<0.05). When coupled with indomethacin pretreatment, the increase in Delta I(sc) after the addition of CCh and forskolin was significant as compared to that observed without pretreatment (P<0.05). However, in the rectum, the secretory response to CCh and forskolin was abolished to a significant degree by indomethacin (P<0.05). Moreover, these indomethacin-induced changes were reversed by the addition of PGE2. Upon semiquantitative RT-PCR analysis, the amounts of cystic fibrosis transmembrane regulator, KCNQ1, and CLCA1 mRNAs were not found to be different between the two segments.

CONCLUSION

There was a clear segmental heterogeneity with regard to electrogenic secretion in the human colon, and this difference can be explained by differences in the ascending colon and rectum.

Cyclic AMP-dependent Cl- secretion induced by thromboxane A2 in isolated human colon

Increased release of thromboxane A(2) (TXA(2)) has been shown to be involved in inflammatory bowel diseases. In the present study, we have investigated the effect of a stable TXA(2) analogue (STA(2)) on the electrical parameters in isolated human colonic mucosa. In the human mucosa set between Ussing chambers, STA(2) stimulated Cl- secretion in a concentration-dependent manner with an EC(50) of 0.06 microm. The STA(2)-induced Cl- secretion was significantly inhibited by ONO-3708 (10 microm), a specific TXA(2) receptor antagonist. The effect of STA(2) (0.3 microm) was independent of the colonic segment from which the tissue was obtained, from caecum to rectum. Chromanol 293B, an inhibitor of the cAMP-dependent KvLQT1 channel, attenuated the STA(2)-induced Cl- secretion in the human colonic mucosa (IC(50) value 1.18 microm). We found that KvLQT1 mRNA and protein were expressed in all the tested segments of the human colon. The STA(2)-induced Cl- secretion was significantly inhibited by 8-bromo-2'-monobutyryladenosine-3',5'-cyclic monophosphorothioate (50 microm), a membrane-permeant cAMP antagonist. STA(2) (0.3 microm) significantly increased the intracellular cAMP levels and the short-circuit current via TXA(2) receptor in a human colonic cell line. These results suggest that the TXA(2)-induced Cl- secretion in the colon is mediated via the cAMP pathway in addition to the Ca(2+)-calmodulin pathway which was previously reported.

Effect of radiation on cAMP, cGMP and Ca(2+)(i) pathways and their interactions in rat distal colon

The secretory response implicated in the intestinal response to luminal attack is altered by radiation. The cAMP, cGMP and Ca(2+)(i) pathways leading to secretion as well as the interactions between the cAMP pathway and the cGMP or Ca(2+)(i) pathway were studied in the rat distal colon 4 days after a 9-Gy abdominal X irradiation, when modifications mainly occurred. The secretory response in Ussing chambers and cAMP and cGMP accumulation in single isolated crypts were measured. The muscarinic receptor characteristics were determined in mucosal membrane preparations. The secretory response by the cAMP pathway (stimulated by vasoactive intestinal peptide or forskolin) and the cAMP accumulation in crypts were decreased (P < 0.05) after irradiation. The weak secretory response induced by the cGMP pathway (stimulated by nitric oxide or guanylin) was unaltered by radiation, and the small amount of cGMP determined in isolated crypts from the control group became undetectable in the irradiated group. Inducible NOS was not involved in the hyporesponsiveness to VIP after irradiation (there was no effect of an iNOS inhibitor). The secretory response by the Ca(2+)(i) pathway (stimulated by carbachol) was unaffected despite a decreased number and increased affinity of muscarinic receptors. The non-additivity of VIP and carbachol co-stimulated responses was unmodified. In contrast, VIP and SNP co-stimulation showed that NO enhanced the radiation-induced hyporesponsiveness to VIP through a reduced accumulation of cAMP in crypts. This study provides further understanding of the effect of ionizing radiation on the intracellular signaling pathways.

Effect of nitric oxide on electrolyte transport across the porcine proximal colon

The effect of nitric oxide (NO) on ion transport in the porcine proximal colon was investigated in slide-stripped epithelia mounted in Ussing chambers. The serosal addition of the NO-donors sodium nitroprusside (SNP, 0.5 mM) or S-nitroso-N-acetylpenicillamine (SNAP, 0.5 mM) induced a steep increase of short-circuit current ( I(sc)). The stimulatory effect of SNP on I(sc) could not be blocked by piroxicam or tetrodotoxin. Potassium channel inhibitors (quinidine, tetraethylammonium or barium) added serosally reduced the SNP- or SNAP-induced increases of I(sc). In chloride-free solutions, the SNP-induced increase of I(sc) was smaller than in chloride-containing solutions. Cl(- )and Na(+) flux measurements demonstrated that SNP diminished Cl(-) and Na(+) net absorption. Pre-treatment with barium was able to block the inhibitory effect of SNP on NaCl net absorption totally. NO effects on paracellular pathways were assessed by measuring flux rates of [(14)C]-D-mannitol. SNP did not change unidirectional D-mannitol flux rates. In conclusion, NO inhibits NaCl net absorption in the proximal colon of pigs by acting directly on the enterocyte. The antiabsorptive (and/or prosecretory) effect of NO depends on a functional basolateral potassium conductance.

Nitric oxide-induced Cl- secretion in isolated rat colon is mediated by the release of thromboxane A2

We have shown previously that thromboxane A2 (TXA2), which may be released by the anti-tumour drug irinotecan and by platelet-activating factor (PAF), causes Cl- secretion in the isolated rat colon. In the present study, the involvement of TXA2 in nitric oxide-induced Cl- secretion in isolated rat colon was investigated. In colonic mucosa set between Ussing chambers, the NO donor sodium nitroprusside (SNP; 100 microM) caused Cl- secretion, an effect that was almost completely inhibited by the NO scavenger carboxy-PTIO at 200 microM. The SNP-induced Cl- secretion was inhibited in a concentration-dependent manner by the TXA2 receptor antagonist ONO-3708 (IC50 = 2 microM) and the TX synthase inhibitor Y-20811 (IC50 = 0.4 microM). SNP significantly increased the release of TXA2 (measured as TXB2 release) from the mucosa. The SNP-induced increases in Cl- secretion and TXA2 release were blocked by a NO-sensitive guanylate cyclase inhibitor (ODQ). Dibutyryl cGMP (500 microM) also induced Cl- secretion, which was sensitive to ONO-3708 (10 microM) and Y-20811 (1 microM), and increased the release of TXA2 from the mucosa. PAF-induced (10 microM) Cl- secretion was inhibited by carboxy-PTIO (200 microM) and ODQ (10 microM), whereas irinotecan-induced (500 microM) Cl- secretion was not significantly inhibited by these drugs. A stable TXA2 analogue (STA2) but not SNP (100 microM) changed the membrane potential of epithelial cells in isolated colonic crypts under the whole-cell current-clamp condition. These results indicate that PAF elicits the NO-cGMP pathway and then stimulates the release of TXA2, which is a stimulant of colonic Cl- secretion. In contrast, the NO-cGMP pathway is not involved in the TXA2-mediated Cl- secretion induced by irinotecan.

Electrolyte transport in the mammalian colon: mechanisms and implications for disease

The colonic epithelium has both absorptive and secretory functions. The transport is characterized by a net absorption of NaCl, short-chain fatty acids (SCFA), and water, allowing extrusion of a feces with very little water and salt content. In addition, the epithelium does secret mucus, bicarbonate, and KCl. Polarized distribution of transport proteins in both luminal and basolateral membranes enables efficient salt transport in both directions, probably even within an individual cell. Meanwhile, most of the participating transport proteins have been identified, and their function has been studied in detail. Absorption of NaCl is a rather steady process that is controlled by steroid hormones regulating the expression of epithelial Na(+) channels (ENaC), the Na(+)-K(+)-ATPase, and additional modulating factors such as the serum- and glucocorticoid-regulated kinase SGK. Acute regulation of absorption may occur by a Na(+) feedback mechanism and the cystic fibrosis transmembrane conductance regulator (CFTR). Cl(-) secretion in the adult colon relies on luminal CFTR, which is a cAMP-regulated Cl(-) channel and a regulator of other transport proteins. As a consequence, mutations in CFTR result in both impaired Cl(-) secretion and enhanced Na(+) absorption in the colon of cystic fibrosis (CF) patients. Ca(2+)- and cAMP-activated basolateral K(+) channels support both secretion and absorption of electrolytes and work in concert with additional regulatory proteins, which determine their functional and pharmacological profile. Knowledge of the mechanisms of electrolyte transport in the colon enables the development of new strategies for the treatment of CF and secretory diarrhea. It will also lead to a better understanding of the pathophysiological events during inflammatory bowel disease and development of colonic carcinoma.

Cell volume-induced changes in K+ transport across the rat colon

The effect of cell swelling and cell shrinkage on K+ transport across the rat colonic epithelium was studied by measuring unidirectional fluxes, uptake and efflux of 86Rb+, a marker for K+. Exposure to a hypotonic medium stimulated the secretory, serosa-to-mucosa flux of K+, whereas exposure to a hypertonic medium inhibited the absorptive, mucosa-to-serosa flux of K+ in the distal, but not in the proximal colon. Neither manoeuvre had any effect on the uptake of K+ across the apical or the basolateral membrane. Cell swelling induced a sustained increase in the apical and basolateral K+ efflux from both colonic segments, whereas cell shrinkage reduced the efflux. Ba2+ (10(-2) mol l(-1)) inhibited the swelling-induced stimulation of the apical, quinine (10(-3) mol l(-1)) that of the basolateral K+ efflux in the distal colon. Incubation of the tissue in Ca2+-free buffer or La3+, which blocks Ca2+-influx into the epithelium, strongly reduced the basal K+ efflux across the basolateral membrane. The same was observed with brefeldin A, a blocker of the transport of newly synthesized proteins out of the endoplasmatic reticulum. Swelling-induced K+ efflux, however, was not reduced. In the presence of colchicine, an inhibitor of the polymerization of microtubules, swelling evoked only a transient increase in mucosal efflux, which, especially in the proximal colon, fell after 6 min to the level of the isotonic control period. These results demonstrate that the cell volume is involved in the regulation of transepithelial K+ transport across the rat colonic epithelium and suggest a role of the cytoskeleton in the control of a part of the volume-sensitive K+ channels.

Imidazolines inhibit secretory responses of rat colonic mucosa to calcium-dependent but not cyclic AMP-dependent secretagogues

The purpose of this study was to investigate whether imidazolines have an anti-secretory action on intestinal epithelial cells. Muscle-stripped preparations of rat colon and monolayers of T84 human colonic epithelial cells were set up in Ussing chambers for measurement of short-circuit current. In rat colon acetylcholine, histamine, vasoactive intestinal polypeptide and forskolin elicited secretory responses which were recorded as increases in short-circuit current. Secretory responses to acetylcholine were inhibited in a concentration-dependent manner by the imidazolines phentolamine, idazoxan and clonidine. The effect of clonidine was not reversed by pre-incubation of mucosal preparations with yohimbine. Secretory responses to vasoactive intestinal polypeptide were unaffected by the three imidazolines. Phentolamine reduced responses of colonic mucosa to histamine but had no effect on responses to forskolin. Responses to vasoactive intestinal polypeptide and forskolin were significantly reduced in the presence of barium. In T84 cell monolayers phentolamine significantly reduced responses to acetylcholine. Three imidazolines, two with alpha-adrenoceptor-antagonist properties and one with alpha-agonist properties, have anti-secretory effects in rat colonic mucosal preparations. The anti-secretory action appears to discriminate between calcium-dependent and cyclic AMP-dependent secretagogues, inhibiting the former but not the latter.

Effect of zaldaride maleate, an antidiarrheal compound, on intracellular cyclic nucleotide-mediated intestinal ion secretion in rats

The purpose of this study is to clarify the mechanisms of action of zaldaride, a calmodulin inhibitor. 16,16-Dimethyl prostaglandin E(2), forskolin, 8-bromo cAMP, nitroprusside, 8-bromo cGMP and Escherichia coli heat-stable enterotoxin STa increased the short-circuit current in rat colonic mucosa. Zaldaride at >/=10 microM significantly attenuated the 16,16-dimethyl prostaglandin E(2) and Escherichia coli heat-stable enterotoxin STa-induced increase in short-circuit current; whereas it did not affect other secretagogues-induced effects. These results suggest that zaldaride inhibits the activation of Ca(2+)/calmodulin-sensitive adenylate cyclase or guanylate cyclase linked to a receptor.

Effect of short-chain fatty acids on cyclic 3',5'-guanosine monophosphate-mediated colonic secretion

Short chain fatty acids (SCFA) prevent and reverse cyclic 3',5'-adenosine monophosphate (cAMP) but not Ca(2+)-mediated Cl- secretion. Mucosal [HCO3-]i has an opposite effect on these secretagogues. We examined whether SCFA and [HCO3-]i affect cyclic 3',5'-guanosine monophosphate (cGMP)-induced secretion. Stripped segments of male Sprague-Dawley rat (Rattus norvegicus) proximal and distal colon, and cultured T84 cells were studied in Using chambers, and pHi and [HCO3-]i were determined. Mucosal [cGMP] was measured in proximal colon. In T84 cells, the increase in Cl- secretion (measured as Isc) induced by mucosal 0.25 microM Escherichia coli heat-stable enterotoxin (STa) was prevented/reversed by bilateral 50 mM Na+ butyrate (71%/73%), acetate (58%/76%), propionate (68%/73%) and (poorly metabolized) isobutyrate (80%/79%). In proximal colon in HCO3- Ringer, basal Cl- secretion was not affected by [HCO3-]i or 25 mM butyrate. Mucosal 0.25 microM STa decreased net Na+ and Cl- absorption. Bilateral but not mucosal 25 mM SCFA reversed STa-induced effects on Na+ absorption and Cl- secretion. Bilateral and mucosal 25 mM SCFA but not [HCO3-]i prevented STa-induced Cl- secretion and increases in mucosal [cGMP]. STa did not produce Cl- secretion in distal colon. It was concluded that SCFA but not [HCO3-]i can prevent and reverse cGMP-induced colonic Cl- secretion.

The action of 5-hydroxytryptamine on normal and cystic fibrosis mouse colon: effects on secretion and intracellular calcium

The ability of mouse colon to generate a secretory response to stimulation by 5-hydroxytryptamine (5-HT) was investigated in intact colonic sheets mounted in Ussing chambers. A preparation of intact isolated crypts was used to determine whether 5-HT action was associated with an elevation of cytosolic calcium levels, measured using the calcium-sensitive fluorescent dye, fura-2. 5-HT increased the short-circuit current, an effect that was inhibited by 55% in the absence of chloride and by 83% in the presence of serosal frusemide, consistent with the stimulation of electrogenic chloride secretion. This was confirmed by the observation that colonic tissue from transgenic cystic fibrosis mice (n = 4) failed to respond to 5-HT, although wild-type tissues generated an increased short-circuit current of 52.4+/-1.1 microAcm(-2) (n = 9). The electrical response to 5-HT was calcium-dependent. 5-HT action was unaffected by tetrodotoxin and was not mimicked by the 5-HT3 agonist 1-phenylbiguanide, indicating that neural mechanisms are not involved. The cyclooxygenase inhibitor indomethacin, however, reduced the 5-HT-induced rise in short-circuit current by 73%, suggesting that prostaglandin production contributes to the response. Stimulation of crypts with acetylcholine elicited an increase in cytosolic calcium levels, but no such response was detected on application of 5-HT (10(-6) to 10(-4) M), suggesting that 5-HT does not directly modulate intracellular calcium in colonic crypt cells. It is concluded that mouse colon responds to 5-HT challenge with a stimulation of electrogenic chloride secretion and that this effect is mediated by indirect mechanisms that might involve immune elements within the colonic wall.

The protein tyrosine kinase pathway is not involved in the regulation of K+ transport across the rat colon

The protein tyrosine kinase inhibitor, genistein, is known to activate the cystic fibrosis transmembrane regulator (CFTR) Cl- channel and to inhibit K+ currents across the rat colonic epithelium. The aim of the present study is to answer the question whether these effects are involved in the regulation of transepithelial K+ transport. Therefore, the action of genistein on K+ transport in rat proximal and distal colon was studied by measuring unidirectional fluxes, uptake and efflux of Rb+ in mucosa-submucosa preparations. All effects of genistein (5 x 10(-5) mol L(-1)) were tested in the presence of a low concentration of forskolin (2 x 10(-7) mol L(-1)), because prestimulation of the cAMP pathway has been shown to be a prerequisite for a secretory action of genistein. Forskolin caused an increase in the serosa-to-mucosa flux of Rb+ (J(Rb)sm) thereby stimulating net K+ secretion in the proximal and distal colon. None of these effects was further enhanced after administration of genistein. Neither mucosal uptake of Rb+, representing mainly the activity of the H+-K+-ATPase in the distal colon, nor serosal Rb+ uptake, representing, e.g. the activity of the Na+-K+-2Cl- cotransporter, were affected by genistein. Also the efflux of Rb+ across the apical or the basolateral membrane, an indicator for the apical and basolateral K+ conductance, was unchanged in the presence of genistein. These results demonstrate that the K+ channels inhibited by genistein are not involved in transepithelial K+ transport.

Segment-specific effects of epinephrine on ion transport in the colon of the rat

The effect of epinephrine on transport of K+, Na+, Cl-, and HCO-3 across the rat colon was studied using the Ussing chamber technique. Epinephrine (5 x 10(-6) mol/l) induced a biphasic change in short-circuit current (Isc) in distal and proximal colon: a transient increase followed by a long-lasting decay. The first phase of the Isc response was abolished in Cl--poor solution or after bumetanide administration, indicating a transient induction of Cl- secretion. The second phase of the response to epinephrine was suppressed by apical administration of the K+ channel blocker, quinine, and was concomitant with an increase in serosal-to-mucosal Rb+ flux, indicating that epinephrine induced K+ secretion, although this response was much smaller than the change in Isc. In addition, the distal colon displayed a decrease in mucosal-to-serosal and serosal-to-mucosal Cl- fluxes when treated with epinephrine. In the distal colon, indomethacin abolished the first phase of the epinephrine effect, whereas the second phase was suppressed by TTX. In the proximal colon, indomethacin and TTX were ineffective. The neuronally mediated response to epinephrine in the distal colon was suppressed by the nonselective beta-receptor blocker, propranolol, and by the beta2-selective blocker, ICI-118551, whereas the epithelial response in the proximal colon was suppressed by the nonselective alpha-blocker, phentolamine, and by the selective alpha2-blocker, yohimbine. These results indicate a segment-specific action of epinephrine on ion transport: a direct stimulatory action on epithelial alpha2-receptors in the proximal colon and an indirect action on secretomotoneurons via beta2-receptors in the distal colon.

Neurokinin-1 (NK-1) receptor is required in Clostridium difficile- induced enteritis

Toxin A, a 308,000-Mr enterotoxin from Clostridium difficile, mediates antibiotic-associated diarrhea and colitis in humans. Injection of toxin A into animal intestine triggers an acute inflammatory response characterized by activation of sensory neurons and immune cells of the intestinal lamina propria, including mast cells and macrophages, and migration of circulating neutrophils in the involved intestinal segment. In this study we show that mice genetically deficient in the neurokinin-1 receptor are protected from the secretory and inflammatory changes as well as from epithelial cell damage induced by toxin A. The protective effect of neurokinin-1R deletion correlates with diminished intestinal levels of the cytokine TNF-alpha and its mRNA and the leukocyte enzyme myeloperoxidase. These results demonstrate a major requirement for substance P receptors in the pathogenesis of acute inflammatory diarrhea.

Inhibition of potassium (KATP) channels reduces the short-circuit current response of rat colonic mucosa to acetylcholine

Intestinal secretion depends upon electrogenic chloride transport into the gut lumen, which requires maintenance of an electrically negative cell-membrane voltage. We have investigated whether secretory responses of rat colonic mucosa to acetylcholine were sensitive to inhibition of potassium channels and whether selective inhibition could indicate the nature of the channel involved. Rat colonic mucosa was set up in Ussing chambers, short-circuit current responses obtained to acetylcholine, and the sensitivity of such responses to inhibition of potassium channels was investigated. Non-selective potassium-channel blockade by barium induced concentration-dependent inhibition of responses to acetylcholine. Similar inhibitory effects were obtained using 4-aminopyridine and glibenclamide. 5-Hydroxydecanoate and phentolamine also inhibited the increase in short-circuit current. However, a combination of charybdotoxin plus apamin was without effect. We conclude that a basolateral outward movement of potassium ions is required for the secretory action of acetylcholine on rat colonic mucosa. The potassium channel involved seems to be ATP-dependent and calcium-insensitive.

The effect of neuropeptide Y on sodium, chloride and potassium transport across the rat distal colon

1. Neuropeptide Y (NPY; 10(-10)-10(-7) mol l-1) reduced basal short-circuit current (Isc) in a concentration-dependent manner in the rat distal colon but was ineffective in the proximal colon. 2. The action of NPY was dependent upon the presence of Cl- and HCO3- anions and was blocked by prior treatment of the tissue with a Cl- channel blocker. The decrease in Isc was associated with an increase in mucosa-to-serosa fluxes of Na+, Rb+ (K+) and Cl-, whereas the serosa-to-mucosa flux of Cl- was decreased. 3. The size of the inhibitory NPY effect was linearly correlated with the height of the basal Isc, i.e. it inhibited 55% of basal secretory Isc. 4. The action of NPY was unaffected by indomethacin and tetrodotoxin, when given alone, but was abolished, when the basal Isc was decreased to values near zero by a combination of both inhibitors. This inhibition could be overcome by restoring basal Isc with prostaglandin E2, indicating that the effect of NPY is not mediated by nerves or prostaglandins, but that NPY is only effective, when anion secretion is stimulated by the spontaneous release of neurotransmitters and prostaglandins. 5. NPY inhibited the increase in Isc induced by veratridine and prostaglandin E2, but it had no effect on the Isc induced by direct stimulation of the adenylate cyclase with forskolin, or on Isc induced by stimulation of the Ca(2+)-pathway with carbachol. Inhibition of the response to veratridine or prostaglandin E2 by NPY showed the same dependence on the height of the ISC just prior to addition of NPY as seen in control conditions, i.e. NPY inhibited 55% of cyclic AMP-mediated secretion.6. These results suggest that the effect of NPY is mediated by an inhibition of cyclic AMP-stimulated secretion, which is stimulated in the rat distal colon by a continuous release of prostaglandins and neurotransmitters.

Evidence against direct activation of chloride secretion by carbachol in the rat distal colon

Carbachol (5 x 10(-5) mol.1-1) induced a biphasic increase in short-circuit current (Isc) consisting of an initial peak phase followed by a long-lasting plateau. Complete dependence on the presence of Cl- ions and sensitivity to bumetanide confirmed that carbachol induces Cl- secretion. The plateau phase was blocked by indomethacin, and both the plateau and the peak phase were suppressed in the combined presence of indomethacin and tetrodotoxin. Inhibition of the carbachol response could be overcome by agonists of the cAMP pathway like prostaglandin E2, forskolin or 8-(4-chlorophenylthio)-adenosine-3',5'-cyclic monophosphate. The increase in Isc was inhibited by a blocker of cAMP-activated Cl- channels, glibenclamide, but was resistant to an inhibitor of Ca(2+)-activated Cl- channels, 4,4'-diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS). The K+ channel blockers Ba2+ and charybdotoxin inhibited the first and suppressed the second phase of the carbachol response, whereas a less specific K+ channel blocker, quinine, suppressed both phases. These results suggest that the dominant effect of carbachol in the intact colonic mucosa is an opening of Ca(2+)-dependent, charybdotoxin- and Ba(2+)-sensitive K+ channels, which leads to hyperpolarization of the epithelial cells. This stimulates Cl- secretion only if there are spontaneously open apical Cl- channels which are basically stimulated by a continuous release of neurotransmitters and prostaglandins. Only during the first phase of the carbachol effect is there indirect evidence for activation of a Cl- conductance synergistically with the cAMP pathway as shown by the increase in tissue conductance resistant to K+ channel blockers.

The role of volume-sensitive Cl- channels in the stimulation of chloride absorption by short-chain fatty acids in the rat colon

The short-chain fatty acids acetate, propionate and butyrate induced a concentration-dependent decrease in short-circuit current (Isc) of the rat colon in vitro. The decrease in Isc, being more pronounced in the distal than in the proximal colon, was dependent on the presence of Cl- ions and partly on the presence of HCO3-. In the distal colon, the fall in Isc could be inhibited by amiloride, indicating that the activity of the Na+/H+ exchanger is necessary for the induction of this response. The decrease in Isc was diminished by the Cl- channel blocker, 5-nitro-2-(3-phenylpropylamino)-benzoate, and the lipoxygenase inhibitor, nordihydroguaiaretic acid. In contrast, inhibitors of the leukotriene pathway or a Cl- channel blocker did not affect the Isc response in the proximal colon. Measurements of unidirectional fluxes revealed that butyrate caused a stimulation of the mucosa to serosa fluxes (Fms) of Na+ and Cl- in the distal, but only of FNams in the proximal colon. Unidirectional Rb+ fluxes were not altered. The stimulation of Fclms correlated with the degree of metabolism of the short-chain fatty acid. The increase in FClms was most pronounced for butyrate, smaller for acetate and not observed with the poorly metabolizable short-chain fatty acid, isobutyrate. Consequently, two factors seem to be responsible for the stimulation of Cl- absorption by short-chain fatty acids in the distal colon: (1) the intracellular production of HCO3- during the oxidation of short-chain fatty acids as substrate for the apical Cl-/HCO3- exchanger, and (2) the activation of volume-sensitive basolateral Cl- channels.

Vitamin A deficiency and colonic electrogenic absorption and secretion in the rat

The effects of vitamin A deficiency on electrogenic transport in the colon were examined in rats made vitamin A deficient at weaning by feeding a vitamin A deficient diet for 40 days. A pair fed control group was given the same diet but supplemented with soluble vitamin A in their drinking water. The basal and stimulated electrogenic secretory and absorptive functions of the muscle stripped proximal, mid, and distal colon were examined in vitro using the short circuit current (Isc) as the index of net ion transport. A significant increase in the basal and secretory Isc (mainly Cl-ions) induced by the cholinergic agonist bethanechol was observed in the mid-colon of the vitamin A deficient rats. In the distal colon, however, vitamin A deficiency caused a significant reduction in both the basal and secretory Isc response to bethanechol compared with the vitamin A supplemented pair fed control. Secretory Isc induced by dibutyryl cyclic adenosine monophosphate was not significantly altered by vitamin A deficiency. The condition abolished the response of the distal colon to luminal amiloride (0.1 mmol/l). Thyroid hormone induced reduction in the distal colonic response to aldosterone is implicated in this lack of response. This is the first experimental linkage between vitamin A action, the thyroid hormone and aldosterone on colonic function. The colonic changes induced by vitamin A deficiency, namely hypersecretion and a reduced electrogenic distal absorptive function, together with the previously described small intestine hypersecretion may be the underlying basis for the diarrhoea observed in human and animal vitamin A deficiency.