Carbonic anhydrase isozyme-II-deficient mice lack the duodenal bicarbonate secretory response to prostaglandin E(2)

Duodenal bicarbonate secretion (DBS) is accepted as the primary mucosal defense against acid discharged from the stomach and is impaired in patients with duodenal ulcer disease. The secretory response to luminal acid is the main physiological stimulus for DBS and involves mediation by PGE(2) produced by mucosal cells. The aim of this investigation is to elucidate the role of carbonic anhydrases (CAs) II and IX in PGE(2)-mediated bicarbonate secretion in the murine duodenum. CA II- and IX-deficient mice and different combinations of their heterozygous and WT counterparts were studied. A 10-mm segment of the proximal duodenum with intact blood supply was isolated, and DBS was titrated by pH-stat (TitroLine-easy, Schott, Mainz, Germany). Mean arterial blood pressure (MAP) was continuously recorded, and blood acid/base balance and gastrointestinal morphology were analyzed. The duodenal segment spontaneously secreted HCO(3)(-) at a steady basal rate of 5.3 ± 0.6 μmol·cm(-1)·h(-1). Perfusing the duodenal lumen for 20 min with 47 μM PGE(2) caused a significant increase in DBS to 13.0 ± 2.9 μmol·cm(-1)·h(-1), P < 0.0001. The DBS response to PGE(2) was completely absent in Car2(-/-) mice, whereas basal DBS was normal. The CA IX-deficient mice with normal Car2 alleles showed a slight increase in DBS. Histological abnormalities were observed in the gastroduodenal epithelium in both CA II- and IX-deficient mice. Our data demonstrate a gastrointestinal phenotypic abnormality associated with CA II deficiency. The results show that the stimulatory effect of the duodenal secretagogue PGE(2) completely depends on CA II

Duodenal acidity “sensing” but not epithelial HCO3− supply is critically dependent on carbonic anhydrase II expression

Carbonic anhydrase (CA) is strongly expressed in the duodenum and has been implicated in a variety of physiological functions including enterocyte HCO3− supply for secretion and the “sensing” of luminal acid and CO2. Here, we report the physiological role of the intracellular CAII isoform involvement in acid-, PGE2, and forskolin-induced murine duodenal bicarbonate secretion (DBS) in vivo. CAII-deficient and WT littermates were studied in vivo during isoflurane anesthesia. An approximate 10-mm segment of the proximal duodenum with intact blood supply was perfused under different experimental conditions and DBS was titrated by pH immediately. Two-photon confocal microscopy using the pH-sensitive dye SNARF-1F was used to assess duodenocyte pHi in vivo. After correction of systemic acidosis by infusion of isotonic Na2CO3, basal DBS was not significantly different in CAII-deficient mice and WT littermates. The duodenal bicarbonate secretory response to acid was almost abolished in CAII-deficient mice, but normal to forskolin- or 16,16-dimethyl PGE2 stimulation. The complete inhibition of tissue CAs by luminal methazolamide and i.v. acetazolamide completely blocked the response to acid, but did not significantly alter the response to forskolin. While duodenocytes acidified upon luminal perfusion with acid, no significant pHi change occurred in CAII-deficient duodenum in vivo. The results suggest that CA II is important for duodenocyte acidification by low luminal pH and for eliciting the acid-mediated HCO3− secretory response, but is not important in the generation of the secreted HCO3− ions