W1715 Effect of Carbon Monoxide/Heme Oxygenase On Duodenal HCO3- Secretion in Rats

Abstract

Background/Aims: Carbon monoxide (CO) with free iron and biliverdin are generated from degradation of heme catalyzed by constitutive heme oxygenase (HO-2) and inducible HO1. Particularly, HO-1 is recognized as the stress protein, which is up-regulated by various stresses and exerts a protective action against tissue injury. By the way, HCO3secretion is one of protective mechanisms in the duodenum against acid stress. However, no information is available concerning the relation of HO-1/CO with HCO3secretion. In the present study, we examined the effect of a CO donor on HCO3secretion in the rat duodenum and investigated whether HO-1 is involved in the physiological regulation of this secretion. Methods: Male SD rats were used after 18 h fasting. Under urethane anesthesia, a proximal duodenal loop was perfused with saline, and HCO3secretion was measured at pH 7.0 using a pH-stat method. CORM-2 (a CO donor; 0.5-2 mM), biliverdin (10 mg/ml) or ruthenium (III) chloride (Ru; a negative control; 5 mM) was applied to the loop for 5 min. Mucosal acidification was performed by exposing the loop to 10 mM HCl for 10 min. Acetazolamide (an inhibitor of CA), indomethacin or L-NAME (a NOS inhibitor) was given SC 1 h or 3 h before the administration of agonists, while SnPP (an inhibitor of HO), CuPP (a negative inhibitor) or hemin was given IP 1 h before. Expression of HO protein was examined by western blot. Results: Topical application of CORM-2 significantly increased duodenal HCO3secretion in a concentration dependent manner, while neither Ru nor biliverdin have any effect. The stimulatory effect of CORM-2 was significantly attenuated by indoemthacin and acetazolamide but not L-NAME. The secretion of HCO3was increased by acidification of the mucosa, in an indomethacin-inhibitable manner. The acid-induced HCO3response was also markedly inhibited by SnPP but not CuPP. Furthermore, the inhibitory effect of SnPP was significantly reversed by the pretreatment with hemin. On the other hand, perfusion of the duodenal loop with 100 mM HCl for 2 h caused hemorrhagic lesions, and this response was significantly worsened by the prior administration of SnPP as well as indomethacin. In addition, the expression of HO-1 but not HO-2 protein was markedly up-regulated in the duodenum following acid perfusion for 4 h. Conclusions: These results suggest that CO stimulates HCO3secretion in the duodenum, and this effect is mediated by endogenous PGs and partly dependent on CO2 generated from the reaction of CO with O2. It is assumed that HO-1 plays an important role in maintaining the integrity of the duodenal mucosa when the tissue is exposed to acid stress.