Phosphodiesterase isozymes involved in regulation of HCO3− secretion in isolated mouse duodenum in vitro
Introduction
Duodenal mucosal HCO3− secretion is a key process that helps prevent acid-peptic injury [1], [2], [3]. The mechanisms that govern mucosal HCO3− secretion involve neuro-humoral factors and luminal acid [1], yet both endogenous prostaglandins (PGs) and nitric oxide (NO) play a particularly important role in the local control of this secretion [4], [5], [6]. The stimulatory action of PGE2 is known to be mediated by the activation of both EP3 and EP4 receptors and coupled intracellularly with Ca2+ and 3′,5′-cyclic-adenosine monophosphate (cAMP) [7], [8], [9], [10], [11]. It is also known that NO stimulates soluble guanylate cyclase and elevates the intracellular level of 3′,5′-cyclic-guanosine monophosphate (cGMP) [12]. We showed using the isolated bullfrog duodenum in vitro that NOR-3, a NO donor, increased the secretion of HCO3− via endogenous PGs in a cGMP-dependent manner [13], results later confirmed in the rat duodenum in vivo[6]. It is thus assumed that both PG/cAMP and NO/cGMP are involved in the local regulatory mechanism of HCO3− secretion in the duodenum.
These nucleotides are degraded into inactive metabolites due to hydrolysis by phosphodiesterase (PDE). At present, the PDE in mammalian tissues has been subdivided into 11 isozymes, each derived from separate gene families and having pharmacologically distinct roles in the body [14]. PDE1–PDE5 have been well characterized, and selective inhibitors of these isozymes are used for the treatment of heart disease, depression, asthma, inflammatory disease, and erectile dysfunction [15], [16], [17]. Since the secretion of HCO3− in the duodenum is intracellularly mediated by both cAMP and cGMP, it is possible that PDE affects the response by altering the levels of cyclic nucleotides. Indeed, Simson et al. [18] showed that theophylline, a nonselective PDE inhibitor, enhanced HCO3− secretion in the isolated bullfrog duodenum in vitro. We have reported a potentiation by isobutylmethylxanthine (IBMX) of PGE2-induced HCO3− secretion in the rat duodenum in vivo[8], [10]. It remains, however, unexplored which PDE isozyme(s) is involved in the regulation of duodenal HCO3− secretion.
In the present study, we examined the effects of subtype-selective inhibitors of PDE1–PDE5 on HCO3− responses to PGE2 or NO in the isolated mouse duodenum in vitro and investigated which isozyme(s) of PDE is involved in the local regulation of duodenal HCO3− secretion.
Section snippets
Animals
Male DDY mice weighing 25–30 g (Charles River, Japan) were used in all experiments. Animals were kept in stainless steel cages with raised mesh bottoms, and deprived of food but allowed free access to tap water for 18 h before the experiments. All experimental procedures used were carried out in accordance with the Helsinki Declaration and have been approved by the Committee for Animal Experimentation established by Kyoto Pharmaceutical University.
Determination of HCO3− secretion
Under deep diethyl ether anesthesia, the mouse
Effects of PGE2 or NOR-3 on duodenal HCO3− secretion
The isolated mouse duodenum consistently secreted HCO3− at rates of 0.4–0.7 μEq/h as basal secretion, in the absence or presence of 0.1% DMSO, a solvent for the agents used in the present study. Serosal addition of PGE2 (10−7 to 10−6 M) caused a gradual increase of HCO3− secretion in a concentration-dependent manner, the ΔHCO3− output at 10−6 M being 0.26 ± 0.02 μEq/h (Fig. 1A and B). The HCO3− stimulatory effect of PGE2 (10−6 M) was significantly attenuated by pretreatment of the tissue with either
Discussion
It has been well established that the secretion of HCO3− from the duodenal surface epithelial cells is regulated by both humoral and neuronal factors, including endogenous PGs, NO, and sensory neurons [5], [6], [21], and intracellularly mediated by cAMP and cGMP as well as Ca2+[1], [10], [13], [22]. These nucleotides are degraded into inactive metabolites by the catalytic action of PDE. It remains, however, unexplored which PDE isozyme(s) is involved in the regulation of duodenal HCO3−
Acknowledgements
This research was supported in part by the Kyoto Pharmaceutical University's “21st Century COE” program and the “Open Research” Program from the Ministry of Education, Science and Culture of Japan. We thank Ono Pharmaceutical Co. Ltd. for generously supplying ONO-AE5-599 and ONO-AE3-208.
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