Activation of EP1 receptors by lubiprostone in rat isolated forestomach longitudinal muscle
Lubiprostone, a ClC-2 channel opener derived from prostaglandin E1 (PGE1) and indicated for the treatment of constipation, increases chloride ion transport into the intestinal lumen and thereby enhances fluid secretion. However, in healthy volunteers, lubiprostone may increase fasting gastric volume and retard gastric emptying, as well as accelerate intestinal transit (Camilleri et al., 2006). Nausea (31%) is the most frequently reported adverse event (Hussar, 2006). A weak ability of lubiprostone to activate prostanoid receptors has been reported in various tissues (Perentesis et al., 2005). We re-examined this activity of lubiprostone in rat isolated stomach, an area of the gut not previously studied. METHODS: Longitudinal muscle preparations of rat forestomach were used (male, Sprague-Dawley, 250-300 g). Muscle strips (~2 x 8 mm) were s uspended in Krebs solution (5% CO2/ 95% O2; pH 7.4; 37oC) under 10 mN tension for isometric recording; experiments were conducted in the presence of indomethacin (3 μM, at least 20 min preincubation). After obtaining contraction to PGE2 (1 μM; previously determined as maximally-effective) and subsequent wash-off, a cumulative concentration response curve to lubiprostone was generated (0.1 nM – 10 μM, ~3 min intervals) in the presence of antagonists at the prostaglandin EP1 ( 3-Pyridinecarboxylic acid, 6-[[[5-bromo-2-(phenylmethoxy)phenyl]methyl]ethylamino] , EP3 (Thiophene-2-sulfonic acid (3-[2-2-(4-methylsulfonylbenzyl)-phenyl]-acryloyl)-amide; L-798,106) or EP4 ( (N-(2-[4-(4,9-diethoxy-1-oxo-1,3-dihydro-2H-benzo[f]isoindol-2-yl)phenyl]acetyl) benzene sulphonamide)); GW627368X) receptors (Breault et al., 1996, Juteau et al., 2001 and Wilson RJ et al., 2006, respectively; 20 min pre-incubation) or vehicle (0.1 % DMSO). The resultant contractile responses were quantified as a % of the 1 μM PGE2-induced contraction (mean ± sem) and statistical analysis was performed using a Student’s unpaired t-test. RESULTS: Lubiprostone 3 nM – 10 μM caused a concentration-dependent contraction of the forestomach (pEC50 = 7.01 ± 0.04; n=4) which was TTX (1 μM)- and scopolamine (10 μM)-insensitive (n=6; P>0.05). The maximum effect was 95 ± 3 % of that evoked by 1μM PGE2. Lubiprostone-induced contractions were unchanged in the presence of the EP3 or EP4 receptor antagonists (both 1 μM; pEC50 = 7.10 ± 0.10 and 7.01 ± 0.09, respectively; n=4; P>0.05) but were concentration dependently antagonised by the EP1 receptor antagonist (e.g. 300 nM pEC50 = 6.19 ± 0.19, apparent pKB of 7.59, n=6), without significant suppression of the maximum response. CONCLUSIONS: Lubiprostone activates EP1 receptors to contract rat isolated forestomach longitudinal muscle with similar efficacy to PGE2. Since emesis may be induced by EP1 receptor activation (Kan et al., 2002), this action may at least partly explain the nausea experienced by patients taking this drug.
Breault GAet al. , (1996). Patent No. WO9603380A1.