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-Oestradiol acutely regulates Cl- secretion in rat distal colonic epithelium
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ABSTRACT |
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-oestradiol (E2) on electrogenic transepithelial ion transport in rat distal colonic epithelium.
M)-induced Cl- secretion. E2 also down-regulated Cl- secretion which was pre-stimulated by forskolin. Cl- secretory responses to the Ca2+-dependent secretagogue carbachol (10 M) were also significantly reduced in the presence of E2 (10- 100 nM). However, E2 had no effect on amiloride-sensitive Na+ absorption.
M) or the protein kinase C (PKC) inhibitor chelerythrine chloride (1 M). However, in the presence of the nuclear oestrogen receptor antagonist tamoxifen (10 M), E2 still produced an inhibition of Cl- secretion. Testosterone, progesterone and 17
-oestradiol had no significant effect on colonic Cl- secretion. Also, E2 (100 nM) did not alter Cl- secretion in colonic epithelia isolated from male rats.
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INTRODUCTION |
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The primary function of the colon is the absorption of salt and water and the secretion of an isotonic fluid rich in K+ and HCO3-. Both secretion and absorption are tightly regulated by a variety of peptides including serotonin, somatostatin, vasoactive intestinal peptide and acetylcholine (Chang & Rao, 1994). Also, it is well established that mineralocorticoid and glucocorticoid steroid hormones play important roles in controlling colonic function, especially in the upregulation of Na+ absorption (Binder et al. 1989). Although oestrogen is considered a salt-retaining steroid hormone and has been shown to alter ion transport in various other epithelia (Zeitlin et al. 1989; Sweezey et al. 1996), a regulatory role for this steroid in colonic ion transport processes has not been demonstrated.
The most biologically potent oestrogen, 17-oestradiol (E2), influences target cell function via interacting with a specific nuclear receptor protein belonging to the steroid hormone receptor superfamily (Evans, 1988). The binding of the hormone causes the oestrogen receptor to undergo a conformational change that increases its affinity for DNA. In this conformation, it binds to specific genes in the nucleus, regulating their transcription, resulting in de novo protein synthesis (Gronemeyer, 1992). The presence of oestrogen receptors in colonic epithelial cells suggests this tissue is a target for E2 (Thomas et al. 1993). However, the physiological role of colonic oestrogen receptors remains unclear. One possibility is that E2 regulates colonic cell proliferation and growth as it does in other target tissues. A role for E2 in stimulating the proliferation of colonic crypt epithelial cells was postulated by Cheng & Bjerknes (1988) who observed significant variation in the mitotic activity during the oestrous cycle of enterocytes isolated from the colon of female mice. Also, E2 significantly stimulated cell growth in a human colonic cell line, with this effect being sensitive to the oestrogen receptor antagonist tamoxifen (Di Domenico et al. 1996).
In contrast to this long term genomic response, E2 also elicits rapid responses which are observed within seconds to minutes and occur independently of genome interaction and protein synthesis. Specifically, non-genomic responses to E2 include rapid increases in intracellular Ca2+ concentration ([Ca2+]i) (Picotto et al. 1996) phosphatidylinositol hydrolysis (Lieberherr et al. 1993), cyclic nucleotide levels (Aronica et al. 1994), protein kinase (Ansonoff & Etgen, 1998) and ion channel activity (Valverde et al. 1999). Previous studies from our laboratory have shown that E2 modulates [Ca2+]i, cAMP-dependent protein kinase (PKA) (Doolan et al. 2000), protein kinase C (PKC) (Doolan & Harvey, 1996) and K+ channel activity (McNamara et al. 1995; Condliffe et al. 1998) in colonic epithelium. Since K+ channels and second messengers such as Cai2+ assume important roles in ion transport processes, the aim of this study was to determine whether E2 rapidly alters the transporting functions of the distal colon.
Using the short circuit current technique, we demonstrate that E2 rapidly decreases the Cl- secretory capacity of the female distal colon but has no acute effect on electrogenic Na+ absorption. The effect of E2 on Cl- secretion is dependent on [Ca2+]i and PKC activation but is not altered by the oestrogen receptor antagonist tamoxifen. In addition, we demonstrate that the anti-secretory response of the colon to E2 is gender specific, being absent in colonic epithelium isolated from male rats.
Part of this work was presented to The Physiological Society (Newcastle meeting) and has been published in abstract form (Condliffe et al. 1999).
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METHODS |
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Tissue preparation
Sprague-Dawley rats were obtained from The Biological Services Unit, University College Cork. All experimental procedures were approved by the university ethics committee. Female rats were used for all experiments unless stated otherwise. Animals were maintained on a 12-12 h light-dark cycle with free access to normal food and water. For some experimental procedures, rats were maintained on a specialised low Na+ diet (0.13% NaCl) (Special Diet Services, Kent, UK) with normal access to water for 2 weeks prior to experimentation. Animals were killed by cervical dislocation, the distal colon was removed and placed in Krebs-Heinsleit solution containing (mM): 140 NaCl, 5 KCl, 1 MgCl, 2 CaCl, 10 Hepes, 10 Tris and 10 D-glucose (pH = 7.4). The colon was rinsed free of its faecal contents with Krebs-Heinsleit solution and a small plastic rod (diameter 4 mm) was inserted into the lumen allowing the serosal muscle layers to be removed by blunt dissection. Freshly dissected rat distal colonic epithelial sheets were mounted in modified Ussing chambers with an exposed surface area of 1 cm2. Tissues were perfused with 10 ml Krebs-Heinsleit solution on both sides from an apical and basolateral reservoir. Apical and basolateral solutions were mixed and oxygenated with room air by a gas lift system and maintained at 37°C.
Short circuit current measurements
The spontaneous transepithelial potential difference (VTE) was measured via calomel electrodes in 3 M KCl connected to an EVC 4000 amplifier (World Precision Instruments, UK). The voltage clamp feature of the amplifier was used to pass a short circuit current (ISC) via Ag-AgCl electrodes to clamp VTE at 0 mV. Both pairs of electrodes were connected to the solution reservoirs with 4% agar bridges. Current signals were sampled at a frequency of 1 Hz and digitised by a MP100 analog-digital converter (Biopac Systems Inc., USA) before being displayed and analysed by Acknowledge software (version 3.0; Biopac Systems Inc., USA) on an Apple Macintosh Quadra 650 personal computer. The ISC was defined as positive for cation flow from the apical to basolateral chamber.
Materials
1,2-Bis(2-aminophenoxy)ethane N,N,N',N'-tetracetic acid acetomethoxy ester (BAPTA AM) and chelerythrine chloride were purchased from Calbiochem. All other chemicals were purchased from Sigma.
Data analysis
ISC was calculated as the difference between the basal ISC and maximum ISC observed in response to drug treatments or at a specified time after drug treatment. Data are expressed as the mean ± S.E.M. of n experiments. Measures of statistical significance were obtained using either Student's t tests for paired data, or one way analysis of variance (ANOVA) for multiple testing. A P value of less than 0.05 was considered significant.
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RESULTS |
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Baseline transport parameters
The proportion of the basal short circuit current (ISC) determined by Cl- secretion and Na+ absorption varies in the rat distal colon depending on the diet (Binder & Sandle, 1994). In order to characterise the relative contribution of the respective ion movements in the basal ISC of the distal colon of rats on a normal NaCl diet (0.75% NaCl), the sensitivity of the basal ISC to inhibitors of transepithelial Cl- secretion and Na+ absorption was assessed. Amiloride (10 M), a blocker of apical epithelial Na+ channels, reduced the basal ISC by an average of 0.6 ± 0.4 A cm-2. Basal ISC values were not significantly different either pre- or post-amiloride, indicating that there is minimal electrogenic Na+ current under these conditions (n = 6, P = 0.25). In contrast, when tissues were subsequently exposed to bumetanide (100 M), an inhibitor of transepithelial Cl- secretion, there was a significant reduction of the basal ISC by 3.9 ± 1 A cm-2 (n = 6, P <= 0.01). Therefore, basal ISC of the rat distal colon under these dietary conditions is dominated by an electrogenic Cl- secretory component with minimal Na+ conductive absorption.
Effect of 17-oestradiol (E2) on the basal ISC
Previous studies have shown that E2 rapidly alters the activity of colonic basolateral K+ channels (McNamara et al. 1995; Condliffe et al. 1998). Since basolateral K+ channels play an important role in Cl- secretion, experiments were performed to determine if E2 had any effect on basal Cl- secretion in rat distal colon. Basolateral addition of E2 (100 nM) significantly reduced basal ISC within 15 min by an average of 2.6 ± 0.3 A cm-2 (Fig. 1) (n = 6, P <= 0.05). A smaller but significant decrease in basal ISC was also observed in response to E2 at a concentration of 10 nM (Fig. 1) (n = 6, P <= 0.05). However, there was no significant effect of 1 nM E2 on baseline Cl- secretion (Fig. 1) (n = 6, P = 0.37). As basal ISC is primarily generated by Cl- secretion, these results suggest E2 rapidly downregulates Cl- secretory processes in distal colonic epithelium.
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Data are expressed as the mean + S.E.M. (n = 6) change in basal ISC after 15 min exposure to different E2 concentrations. *Significant difference at the level of P <= 0.05 between values linked by a horizontal bar.
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Effect of E2 on secretagogue-stimulated
Forskolin stimulates Cl- secretion in rat distal colon via cAMP-dependent activation of apical Cl- channels (Schulthei§ et al. 1998). As E2 had an inhibitory effect on basal Cl- secretion, it was of interest to determine if E2 was also capable of altering the normal secretory response to forskolin. In control tissues, addition of forskolin (20 M) to apical and basolateral baths stimulated a large, sustained increase in ISC that was inhibited by bumetanide (100 M) (Fig. 2A). When tissues were pre-treated with E2 (100 nM) for 10 min, the ability of forskolin (20 M) to induce a Cl- secretory event was abolished (Fig. 2A). The effects of lower, physiological concentrations of E2 were also tested to determine a physiological role for the inhibitory effects of E2 on the cAMP-dependent Cl- secretory response. In the presence of 0.01 nM E2, forskolin caused an average increase in ISC from 9.2 ± 2.1 to 14 ± 1.9 A cm-2 which was not significantly different from the control forskolin response (Fig. 2B) (n = 6, P = 0.3). However, pre-treating tissues with 0.1 nM E2 significantly reduced the magnitude of the subsequent forskolin-induced Cl- secretory event (Fig. 2B) (n = 6, P <= 0.05). A progressively greater effect on forskolin-induced Cl- secretion was also observed when tissues were pre-treated with E2 in the concentration range of 1-100 nM (Fig. 2B). These results indicate that a physiological range of E2 concentrations significantly reduces the Cl- secretory response to a cAMP agonist. The effect of E2 on Cl- secretion which was pre-stimulated by forskolin was also examined. Forskolin (20 M) stimulated a sustained increase in ISC and application of E2 (100 nM) at the plateau of this response resulted in a significant decrease in ISC after 10 min (n = 6, P <= 0.05) compared to a vehicle control (treated with a 0.01% methanol vehicle at the same time) (Fig. 3A). After 20 min E2 exposure, ISC had decreased by an average of 3.8 ± 0.3 A cm-2 (Fig. 3B) (n = 6, P <= 0.01). A smaller but significant effect on forskolin-stimulated Cl- secretion was observed with E2 at a concentration of 10 nM whereas no significant effect was detected when E2 was used at a concentration of 1 nM (Fig. 3B).
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A, representative effect of forskolin (Fsk, 20
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A, representative ISC of female rat colonic epithelium pre-stimulated with forskolin (Fsk, 20
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The cholinergic agonist carbachol stimulates a transient Cl- secretory event in rat distal colon by triggering the Ca2+ second messenger pathway to activate basolateral Ca2+-dependent K+ (KCa) channels (Bšhme et al. 1991). Application of carbachol (10 M) to the basolateral bath of a rat distal colonic epithelium preparation caused a large, but transient increase in ISC from 15.9 ± 5.7 A cm-2 to a peak value of 48.7 ± 11.8 A cm-2 (n = 7, P <= 0.01). Pre-treating tissues with E2 significantly reduced the carbachol-induced increase in ISC (Fig. 4A). In the presence of E2 (100 nM), the carbachol-stimulated increase in ISC was only 12.1 ± 2.6 A cm-2 (Fig. 4B) (n = 7, P <= 0.01). The ability of carbachol to increase ISC was also reduced in the presence of 10 nM E2 (Fig. 4B) (n = 7, P <= 0.01). In contrast, addition of carbachol to tissues pretreated with 1 nM E2 for 10 min resulted in a large increase in ISC that was not significantly different to control observations (n = 7, P = 0.47). Overall, these data demonstrate that E2 is capable of inhibiting the Cl- secretory response of rat distal colonic epithelium to both cAMP and Ca2+-dependent secretagogues.
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A, the effect of carbachol (CCh, 10
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Specificity of the effect of E2 on Cl- secretion
We tested the effect of other sex steroids on forskolin-induced Cl- secretion to determine whether the anti-secretory effect of E2 was specific for that hormone and not due solely to physicochemical properties of steroids. Rat distal colonic epithelial tissues were pre-incubated with the male sex steroid testosterone (100 nM) for 10 min before being treated with forskolin (20 M). Forskolin stimulated a significant increase in ISC from 8.8 ± 0.8 to 14.5 ± 1.7 A cm-2 in testosterone-treated tissues (Fig. 5) (n = 6, P <= 0.01). In addition, in the presence of the biologically inactive isomer of E2, 17-oestradiol (100 nM), forskolin stimulated an increase in Cl- secretion that was not significantly different from the control forskolin response (Fig. 5) (n = 6, P = 0.19). In contrast, pre-incubating colonic tissues with another female sex steroid, progesterone (100 nM), significantly reduced the forskolin-induced increase in ISC (Fig. 5) (n = 6, P <= 0.01). However, at a concentration of 10 nM, progesterone stimulated an increase in ISC of 4.8 ± 1.3 A cm-2, which was not significantly different from the control forskolin response (n = 6, P = 0.27). Overall, these results indicate that the ability of E2 to inhibit forskolin-induced Cl- secretion is E2 specific and is not a feature of sex steroids in general.
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Mean + S.E.M. (n = 6) change in ISC caused by forskolin (20
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As E2 has fewer physiological functions in the male, we assessed if the inhibitory action of E2 on forskolin-induced Cl- secretion was also effective in the distal colon isolated from male rats. In the presence of E2 (100 nM), the male distal colon still retained a normal response to forskolin. Forskolin (20 M) stimulated an increase in ISC of 5.4 ± 0.7 A cm-2 after pretreatment with E2 which was not significantly different from the control forskolin response (Fig. 5) (n = 6, P = 0.62). Therefore, the anti-secretory response of the colon to E2 is not only steroid specific but also gender specific, as it is restricted to female rats.
E2 requires [Ca2+]i and PKC to inhibit Cl- secretion
Since previous experiments demonstrated that E2 stimulates an increase in [Ca2+]i in rat distal colonic crypts (Doolan et al. 2000), the contribution of a Ca2+ second messenger component in the E2 effect on Cl- secretion was investigated. To prevent increases in [Ca2+]i, tissues were pre-incubated with the intracellular Ca2+ chelator BAPTA (50 M). Pre-incubating tissues with BAPTA for 20 min had no significant effect on basal ISC (n = 7, P = 0.36). In addition, pre-incubation with BAPTA had no significant effect on forskolin-induced Cl- secretion (n = 7, P = 0.74). Addition of E2 (100 nM) 10 min after BAPTA also had no effect on forskolin-induced Cl- secretion (Fig. 6). Under these conditions, forskolin induced an increase in ISC of 7 ± 2.6 A cm-2 which was not significantly different to the control forskolin response (n = 7, P = 0.15) (Fig. 6). Therefore the inhibitory effect of E2 on forskolin-induced Cl- secretion is Ca2+ dependent.
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Mean + S.E.M. (n = 6) change in ISC caused by forskolin (20
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PKC activity in rat distal colonic crypts is rapidly stimulated by E2 (Doolan & Harvey, 1996). To investigate whether PKC was involved in the anti-secretory effect of E2, rat colonic epithelial tissues were pre-treated with the specific PKC inhibitor chelerythrine chloride (ChCl). Pre-incubating control tissues with ChCl (1 M) for 20 min had no effect on basal ISC or the subsequent response to forskolin, as forskolin stimulated an increase from 11.3 ± 1.6 to 19 ± 1.4 A cm-2, which was not significantly different from control tissues (n = 7, P = 0.4). When tissues were treated with E2 (100 nM) after 10 min ChCl pre-incubation, forskolin increased ISC by 6.0 ± 1.2 A cm-2, which was not significantly different from control responses to forskolin (Fig. 6) (n = 7, P = 0.28). Therefore the anti-secretory effect of E2 on forskolin-induced Cl- secretion is dependent on PKC activation.
Effects of E2 on Cl- secretion occur independently of the genomic E2 receptor
Tissues were exposed to the oestrogen receptor antagonist tamoxifen to determine whether the anti-secretory effect of E2 was mediated via a genomic pathway. As tamoxifen has oestrogenic properties in some tissues (MacGregor & Jordan, 1998), we investigated whether tamoxifen alone could influence forskolin-induced Cl- secretion. Exposing colonic tissues to tamoxifen for 20 min had a weak although insignificant inhibitory effect on forskolin-induced Cl- secretion suggesting tamoxifen has some oestrogenic activity in this tissue (n = 6, P = 0.1) (Fig. 7). However, when E2 (100 nM) was added to tissues in the presence of tamoxifen (10 M), the increase in ISC caused by forskolin was significantly reduced (n = 6, P <= 0.01) (Fig. 7). Therefore, inhibition of Cl- secretion by E2 is not prevented by tamoxifen.
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Mean + S.E.M. (n = 6) change in ISC caused by forskolin in control conditions, in the presence of tamoxifen (Tamox, 10
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Effect of E2 on electrogenic Na+ absorption
The effects of E2 on fluid retention may be mediated both by the anti-secretory effect described above and also via an upregulation of Na+ absorption. To test this hypothesis, the Cl- secretory component of the ISC was inhibited by bumetanide (100 M) and the effect of E2 on the remaining amiloride-sensitive ISC was examined. In tissues from female rats on a normal diet, E2 (100 nM) had no significant effect on amiloride-sensitive ISC at any stage of a 1 h time course (n = 6, P = 0.18; data not shown). After 30 min exposure to E2, amiloride-sensitive ISC had changed from 7 ± 1.3 to 4.7 ± 1.3 A cm-2, which was not significantly different from control values (n = 6, P = 0.52; data not shown).
To determine if E2 had any rapid effect on electrogenic Na+ absorption in the distal colon with elevated epithelial Na+ channel (ENaC) expression, E2 was tested on tissues from female rats maintained on a low Na+ diet. The low Na+ diet stimulated a significant increase in amiloride-sensitive ISC from 0.6 ± 0.4 to 11.3 ± 1.4 A cm-2, consistent with an upregulation of ENaC expression. After pre-treatment with bumetanide (100 M) to remove the Cl- secretory component, tissues from female rats fed on a low Na+ diet were then exposed to E2. After 30 min exposure to E2 (100 nM), amiloride-sensitive ISC had decreased from 25 ± 4 to 22.5 ± 3.5 A cm-2, which was not significantly different to control values (n = 6, P = 0.9). These results demonstrate that E2 has no rapid effect on electrogenic Na+ absorption in conditions where ENaC expression is either very low or upregulated.
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DISCUSSION |
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The present study demonstrates that E2 rapidly decreases the Cl- secretory capacity of the distal colon. Physiological concentrations of E2 reduced basal Cl- secretion, prevented secretagogue stimulation of Cl- secretion and decreased Cl- secretion pre-stimulated by secretagogues. These results provide the first evidence of an anti-secretory effect of E2 in the distal colon which could play a role in the fluid retention that occurs in high oestrogen states.
The rapid responses to E2 are likely to be mediated by a novel receptor protein in colonic epithelial cells. This is supported by the acute onset of the effect and by experiments which demonstrated that pre-incubation with the oestrogen receptor antagonist tamoxifen had no effect on the anti-secretory ability of E2. Although a non-genomic E2 receptor has not been successfully sequenced to date, various other studies conclude that rapid effects of E2 are mediated by a receptor protein distinct from the oestrogen receptor (Morley et al. 1992; Nadal et al. 1998; Watson & Gametchu, 1999). Furthermore, immunohistochemical evidence indicates that this receptor is localised to the cell membrane (Nadal et al. 1998; Watson & Gametchu, 1999).
E2 stimulates a rapid increase in intracellular Ca2+, PKA and PKC activity in female rat distal colon while having no effect on these second messengers in male colon (Doolan & Harvey, 1996; Doolan et al. 2000). This study has provided evidence that both an increase in intracellular Ca2+ and PKC activity are required for E2 to alter Cl- secretion. Therefore, the absence of an effect of E2 on Cl- secretion in male colon may be because E2 does not activate the necessary second messenger components involved in the anti-secretory effect in male tissue. This could be due to a gender-specific distribution of colonic protein kinase isoforms, as has been observed in rat liver (Zangar et al. 1995). Alternatively, colonocytes from male animals may not express the most proximal component of the E2 non-genomic signalling pathway, a putative E2 receptor. This would explain why E2 fails to activate second messenger components and alter Cl- secretion in male rats.
The most potent effect of E2 was to prevent Cl- secretion being stimulated by the cAMP-dependent secretagogue forskolin, while inhibition of basal or pre-stimulated Cl- secretion occurred at higher E2 concentrations. This suggests that two different types of mechanisms may be employed by E2 to reduce Cl- secretion. McNamara et al. (1999) reported that blockade of basolateral K+ channels reduced both basal Cl- secretion and Cl- secretion pre-stimulated by forskolin in human colonic epithelia. Also, basolateral K+ channel inhibition decreases forskolin-stimulated Cl- secretion in rat colonic epithelia (Schulthei§ & Diener, 1998). Furthermore, E2 has been shown to rapidly decrease the activity of K+ channels in colonic epithelia (McNamara et al. 1995; Condliffe et al. 1998). Therefore, we propose that E2 decreases basal and pre-stimulated Cl- secretion by activating PKC which then phosphorylates basolateral K+ channels to reduce K+ recycling and thereby decrease the drive for Cl- secretion. In agreement with these findings, Reenstra (1993) has shown that the phorbol ester phorbol 12-myristate 13-acetate (PMA) inhibits the basolateral K+ conductance of T84 cells and that this effect reduces transepithelial Cl- secretion.
Although modulation of basolateral K+ channel activity by E2 may underlie the inhibition of basal and pre-stimulated Cl- secretion, it does not explain how E2 can completely prevent the normal secretory response to forskolin, as this response is largely mediated via cAMP-dependent activation of the cystic fibrosis transmembrane conductance regulator (CFTR; Schulthei§ et al. 1998). It is possible that, in addition to downregulating KCa activity, E2 could also prevent subsequent forskolin-induced Cl- secretion in female rat distal colon by regulating CFTR activity prior to activation by forskolin. Regulation of CFTR by E2 has been reported in other types of epithelia. Sweezey et al. (1997) found that pre-incubating lung epithelia with E2 for 16 h decreased the CFTR-mediated Cl- secretory response to cAMP. Interestingly, this response was not affected by tamoxifen and did not involve a decrease in CFTR mRNA. Therefore, like the anti-secretory effect of E2 in the colon, the E2-induced decrease in CFTR conductance in lung epithelia might also be manifested after a short time course. Also, Sweezey et al. (1996) postulated that E2-induced increases in [Ca2+]i in pancreatic epithelial cells may be responsible for the E2-stimulated inhibition of Cl- conductance via a Ca2+-dependent phosphorylation of CFTR. Therefore, we propose that E2 prevents forskolin-induced Cl- secretion in female distal colon by down-regulating CFTR activity. This mechanism seems to involve Cai2+ and PKC, as the anti-secretory effect of E2 could be abolished by Cai2+ chelation or PKC inhibition.
The present study also investigated potential rapid effects of E2 on electrogenic, transepithelial Na+ absorption in rat distal colon. In rat distal colon, electrogenic Na+ absorption occurs primarily in the surface cells of the crypt while Cl- secretory processes predominate in base cells of the crypt (Greger et al. 1997). The acute augmentation of ATP-dependent K+ (KATP) channel current by E2 in human colon led McNamara et al. (1995) to postulate that E2 increases the Na+ absorptive capacity of the colon. However, E2 had no effect on the electrogenic Na+ current of the distal colon from rats on a normal NaCl diet. In addition, the upregulated electrogenic Na+ current of the distal colon from rats on a low Na+ diet was unaffected by E2. Therefore, although E2 may increase basolateral KATP channel current in surface cells of the crypt, there does not appear to be a cross-talk effect to enhance the absorption of Na+ via the apical epithelial Na+ channel.
In the present study, we have demonstrated that E2 can rapidly inhibit Cl- secretion in female distal colonic epithelium. This suggests E2 may alter colonic ion transport during high oestrogen states such as pregnancy and certain phases of the menstrual cycle. Indeed, a marked salt and water retention is observed during the high oestrogen states of pregnancy (Atherton et al. 1982), use of the oral contraceptive (Blahd et al. 1974) and E2 treatment (Stachenfeld et al. 1998). We therefore conclude that the ability of E2 to limit secretory losses in the female colon may be involved in the retentive properties of this hormone.
In summary, this study has described a novel, gender-specific effect of E2 which causes an acute decrease in the Cl- secretory capacity of the rat distal colon. This mechanism involves intracellular Ca2+ and protein kinase C and is probably mediated by an inhibition of basolateral KCa and/or CFTR channel activity. The ability of E2 to decrease Cl- secretion in the female colon may contribute to E2-induced salt and water retention.
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Acknowledgements
This study was supported by The Health Research Board of Ireland (Grant 01/96) and a Wellcome Trust Programme Grant (WT 0400/7/93/ZMP/HA).
Corresponding author
S. Condliffe: Wellcome Trust Cellular Physiology Research Unit, Physiology Department, University College Cork, Ireland.
Email: mdpy6009{at}bureau.ucc.ie
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