Introduction
Chronic constipation is a common clinical problem. Although most cases do not reach medical attention due to the plethora of over-the-counter remedies available, severe constipation can be intractable to most remedies and, in its severe form, can be debilitating.
Recently, lubiprostone, a bicyclic fatty acid was approved by the FDA for the treatment of chronic constipation in adults. This compound appears to be in a new therapeutic class and works by a mechanism that has been described in only a few publications. In a detailed investigation of the effect of lubiprostone, Cuppoletti et al. tested the hypothesis that lubiprostone increased the activity of the chloride channel ClC-2. To test the hypothesis, they demonstrated that ClC-2 was expressed in the stomach and in the epithelial cells of the small and large intestine by PCR and in situ hybridization, and demonstrated that ClC-2 was apically expressed in transfected intestinal T84 cells. Electrophysiological measurements in T84 and in ClC-2 transfected human epithelial kidney (HEK) cell monolayers and in patch-clamped individual cells revealed that lubiprostone increased transepithelial and transmembrane currents in T84 only in ClC-2 transfected cells [
1]. These data suggest that lubiprostone increases Cl
− secretion by activation of the intestinal epithelial cell ClC-2 channel.
Even given these compelling data, the role of ClC-2 channels in epithelial ion secretion is by no means established. First described in 1992 [
2], ClC-2 channels are expressed in the stomach, intestine, colon, brain, heart, and muscle. They are part of a family of channels and transporters [
3], four of which are plasma membrane expressed, with ClC-2 being the only member to which epithelial transport function is attributed. Its enterocyte expression is thought to be mostly basolateral in situ, based on studies which report localization below the junctional protein ZO-1 [
4‐
6].
The discovery that the mutations of the cystic fibrosis transmembrane regulator (CFTR), an epithelial Cl
− channel, are responsible for the disease phenotype has increased interest in the function of all epithelial Cl
− channels. With the discovery of ClC-2 channels, investigators hypothesized that ClC-2 function might compensate for CFTR loss-of-function, which might explain the lack of overt pulmonary or pancreatic phenotype in CFTR knockout (KO) transgenic mice. Studies of ClC-2 KO transgenic mice revealed that ClC-2 dysfunction actually increased Cl
− secretion, and that double transgenic mice had a survival advantage over CFTR mutant mice, data most compatible with ClC-2 serving to recycle Cl
− across the basolateral membrane [
7]. Others have implicated ClC-4 in intestinal Cl
− secretion [
8]. Although apical Cl
− channels are hypothesized to facilitate gastric HCl secretion in parietal cells, ClC-2 could not be immunolocalized in the stomach by one group [
9], although gastric expression was documented by another [
1].
In one of the few published clinical studies addressing the intestinal effects of lubiprostone, Camilleri et al. [
10] found that lubiprostone inhibited gastric emptying, but accelerated small and large intestinal transit, which is associated with postprandial fullness. Subsequently, investigators have reported a beneficial effect of lubiprostone in subjects with chronic constipation and functional bowel disease on the basis of clinical trials [
11,
12].
To date, one full-length published study has addressed the effects of lubiprostone on gut fluid secretion. Fei et al. reported that lubiprostone increased
I
sc in Ussing-chambered guinea pig distal ileum and colon [
13]. Although the response was unaffected by the neurotoxin tetrodotoxin, or by EP receptor antagonists, consistent with a non-neurally and non-prostaglandin mediated mechanism, serosal application also produced a robust response, calling into question the role of ClC-2 if it was indeed apically expressed. Furthermore, the response was also inhibited by mucosal application of anion channel blockers such as glibenclamide and DIDS.
Given the aforementioned Ussing chamber studies, functional confirmation in situ in vivo would help further understand the effect of the compound on intestinal fluid secretion. We thus propose to examine the effect of lubiprostone on fluid and electrolyte secretion in the duodenum, a locus of enterocyte anion secretion, with a focus on HCO
3
− secretion, which protects the mucosa from injury due to luminal acid [
14]. Furthermore, given the possible confounding effect of CFTR-mediated secretion, we intend to examine the effect of acute CFTR inhibition on duodenal fluid and electrolyte secretion with a systemic, pharmacologic highly selective CFTR inhibitor.
Discussion
We demonstrated that luminally-perfused lubiprostone increased ion secretion with a concentration-related manner; a high concentration (10 μM) increased HCO3
− secretion, whereas a low concentration (0.1 μM) increased Cl− and water secretion. Lubiprostone-induced HCO3
− secretion was partially CFTR-dependent, whereas Cl− secretion was not apparently CFTR-dependent, presumably involving non-CFTR mediated Cl− secretion. High concentration lubiprostone may directly stimulate EP4 receptors, increasing duodenal HCO3
− secretion. CFTR inhibition decreased net water output, reversed by a high or low concentration of lubiprostone, consistent with CFTR-independent Cl− secretion. This is the first study demonstrating that lubiprostone stimulates duodenal ion secretion in vivo. Furthermore, lubiprostone may act as a dual activator of CFTR-independent Cl− secretion and as a PG receptor agonist.
The mechanism by which lubiprostone stimulates intestinal Cl
− secretion selectively via ClC-2 is controversial. Lubiprostone activates ClC-2 via a CFTR-independent mechanism in T84 cells [
1]. In Ussing-chambered ileum and colon, lubiprostone increases Cl
− secretion by a CFTR-independent mechanism [
13]. Furthermore, the localization of ClC-2 in the intestinal epithelial cells is also controversial. Transfected T84 and MDCK cells express ClC-2 on the apical membrane [
1], whereas immunohistochemistry on intact intestinal sections reveals the expression of ClC-2 on the enterocyte basolateral membrane [
6,
20] or in the region of the intercellular tight junctions [
4]. Since the duodenum secretes Cl
− and HCO
3
− via a CFTR-dependent mechanism, whereas ClC-2 is localized on the basolateral membrane of duodenocytes [
6], we assessed the involvement of CFTR in duodenal lubiprostone-induced ion secretion. Our results demonstrated that at high concentration, lubiprostone stimulated HCO
3
− secretion via a CFTR-dependent mechanism, whereas a low concentration increased Cl
− secretion via a CFTR-independent mechanism.
Furthermore, the site of lubiprostone action is still controversial. Since lubiprostone is derived from PGE
1 [
21], it may activate EP receptors. Indeed, EP receptor antagonists diminished lubiprostone-induced contraction of rat gastric and colonic smooth muscle with an apparent pK
B of 6.7–7.6 [
22]. In contrast, the lubiprostone-induced
I
sc
increase in guinea pig colon was not affected by EP receptor antagonists [
13]. Our results show that high concentration lubiprostone-induced HCO
3
− secretion is mediated via the EP
4 receptor, but not via EP
1/EP
2 or endogenous PGs by COX activation. Since EP
4 activation increases intracellular cAMP [
23], and elevated cAMP activates CFTR [
24], CFTR-dependent HCO
3
− secretion by lubiprostone is consistent with EP
4 receptor activation by lubiprostone.
Taken together, our results support the hypothesis that high concentration lubiprostone directly stimulates EP
4 receptors, activating CFTR and increasing HCO
3
− secretion in rat duodenum. Existing controversies regarding the CFTR dependence of lubiprostone action might reflect species, intestinal segment, and concentration differences. Our results also support the hypothesis that low concentration lubiprostone activates Cl
− secretion via non-CFTR channels. Unfortunately, the lack of selective ClC-2 inhibitors impedes further characterization of the mechanism by which low concentration lubiprostone augments Cl
− secretion. Very recent data generated from mucosal biopsies obtained from normal mice and humans and those homozygous for CFTR loss-of-function mutations, and CFTR null mice strongly implicated the CFTR in lubiprostone-stimulated anion secretion. Furthermore, this secretion was significantly reduced by EP
4 receptor antagonists, as is consistent with our data [
25].
Increased duodenal HCO3
− secretion induced by a high concentration of lubiprostone (10 μM) may protect the mucosa from acid-induced injury. Although the exact luminal concentration of lubiprostone in humans is not available, the recommended dose (24 μg; ~60 nmol) may produce duodenal luminal concentrations to the dose we tested (10 nmol/min). Since duodenal luminal concentrations are no doubt higher than in the distal gut, the 24 μg dose may protect the gastroduodenal mucosa. In the distal gut, the lower lubiprostone concentrations may increase Cl− secretion.
CFTR inhibition decreased water output, presumably explained by the reciprocal activation of Na
+/H
+ exchanger-3 (NHE3) on the apical membrane [
26] mediating Na
+ absorption followed by water absorption. Recovery of water output by low concentration lubiprostone during CFTR inhibition is consistent with CFTR-independent Cl
− secretion. ClC-2 channels, due to their basolateral location in the duodenum, may increase duodenal Cl
− absorption during HCO
3
− secretion as an alternative to CFTR-mediated Cl
− recycling across the apical membrane. Furthermore, lubiprostone stimulates CFTR-dependent HCO
3
− secretion without changing net Cl
− secretion. This effect supports the hypothesis that Cl
− secreted by CFTR is recycled across the apical membrane by anion exchangers. Interestingly, the non-specific anion exchange inhibitor DIDS inhibits lubiprostone-induced
I
sc increase in guinea pig ileum [
13], also supporting this hypothesis.
CFTR-dependent and -independent actions of lubiprostone may be beneficial for other organs, which manifest the CF phenotype. Recently, lubiprostone was reported to increase
I
sc and mucus secretion via CFTR-dependent and -independent pathways in airway submucosal glands, suggesting it might be of benefit outside of the gastrointestinal tract [
27].
In conclusion, lubiprostone stimulates duodenal HCO3
− and Cl− secretion via different pathways, dependent on its concentration. Increased duodenal HCO3
− secretion suggests that lubiprostone may protect the duodenum from acid-induced injury. Restoration of impaired water output during CFTR inhibition suggests that lubiprostone may improve the intestinal phenotype in CF patients.