Carbonic anhydrase isozyme-II-deficient mice lack the duodenal bicarbonate secretory response to prostaglandin E2

Abstract

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 PGE2 produced by mucosal cells. The aim of this investigation is to elucidate the role of carbonic anhydrases (CAs) II and IX in PGE2-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 HCO3(-) at a steady basal rate of 5.3 +/- 0.6 micromol x cm(-1) x h(-1). Perfusing the duodenal lumen for 20 min with 47 microM PGE2 caused a significant increase in DBS to 13.0 +/- 2.9 micromol x cm(-1) x h(-1), P < 0.0001. The DBS response to PGE2 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 PGE2 completely depends on CA II.

Fig. 1.

CA IX-deficient mice. PGE₂ in the duodenal lumen induced a marked and significant increase in HCO₃^- secretion. * indicates significant changes between the WT group (n = 5) and the Car2^+/+/Car9^-/- group (n = 4). The secretory rates in the Car2^+/-/Car9^-/- group (n = 5) were not significantly different from the WT group. No differences in MAP were observed between these groups.

Fig. 2.

Secretory response in Car2^+/-/Car9^+/- mice. PGE₂ induced a marked and significant increase in HCO₃^- secretion in both the WT group (n = 5) and the Car2^+/-/Car9^+/- group (n = 3). In the Car2^-/-/Car9^-/- mouse (n = 1), the basal secretion was low, and no secretory increase in response to PGE₂ was observed. No differences in MAP were observed between these groups.

Fig. 3.

CA II-deficient mice. Perfusing the duodenal lumen with PGE₂ (47 μM) caused a marked increase in duodenal mucosal HCO₃^- secretion in the WT group (n = 5). No secretory increase in response to PGE₂ was observed in animals lacking the CA II isoform [Car2^-/-/Car9^+/- (n = 5) or Car2^-/-/Car9^+/+ (n = 3)]. #, Car2^-/-/Car9^+/- and *, Car^-/-/Car9^+/+ indicate significant difference compared with the WT group. No changes in MAP were observed in these experiments.

Fig. 4.

Morphological changes in the stomach and intestine. Hematoxylin/eosin staining of gastric (A, C, and E) and duodenal (B, D, and F) mucosa in WT (A and B), CA II-deficient (C and D), and CA IX-deficient (E and F) mice. The WT and CA II-deficient mice have normal histological structure of the mucosa. The CA IX-deficient mouse shows gastric pit-cell hyperplasia, glandular atrophy, and enlargement of intestinal crypts. Original magnifications, ×200.