Duodenal brush border intestinal alkaline phosphatase activity affects bicarbonate secretion in rats

Abstract

We hypothesized that duodenal HCO(3)(-) secretion alkalinizes the microclimate surrounding intestinal alkaline phosphatase (IAP), increasing its activity. We measured AP activity in rat duodenum in situ in frozen sections with the fluorogenic substrate ELF-97 phosphate and measured duodenal HCO(3)(-) secretion with a pH-stat in perfused duodenal loops. We examined the effects of the IAP inhibitors L-cysteine or L-phenylalanine (0.1-10 mM) or the tissue nonspecific AP inhibitor levamisole (0.1-10 mM) on AP activity in vitro and on acid-induced duodenal HCO(3)(-) secretion in vivo. AP activity was the highest in the duodenal brush border, decreasing longitudinally to the large intestine with no activity in stomach. Villous surface AP activity measured in vivo was enhanced by PGE(2) intravenously and inhibited by luminal L-cysteine. Furthermore, incubation with a pH 2.2 solution reduced AP activity in vivo, whereas pretreatment with the cystic fibrosis transmembrane regulator (CFTR) inhibitor CFTR(inh)-172 abolished AP activity at pH 2.2. L-Cysteine and L-phenylalanine enhanced acid-augmented duodenal HCO(3)(-) secretion. The nonselective P2 receptor antagonist suramin (1 mM) reduced acid-induced HCO(3)(-) secretion. Moreover, L-cysteine or the competitive AP inhibitor glycerol phosphate (10 mM) increased HCO(3)(-) secretion, inhibited by suramin. In conclusion, enhancement of the duodenal HCO(3)(-) secretory rate increased AP activity, whereas inhibition of AP activity increased the HCO(3)(-) secretory rate. These data support our hypothesis that HCO(3)(-) secretion increases AP activity by increasing local pH at its catalytic site and that AP hydrolyzes endogenous luminal phosphates, presumably ATP, which increases HCO(3)(-) secretion via activation of P2 receptors.

The role of intestinal alkaline phosphatase in pediatric inflammatory bowel and celiac diseases

“…Az iAP alkalikus pH-n működik a legjobban, és aktivitása a duodenum kezdeti szakaszán a legmagasabb. Az iAP szabályozza a zsírfelszívódást és az organikus foszfátok hidrolizálását, valamint a bél mucosalis integritásának fenntartását [7] (1. ábra).…”

Characterization of CFTR High Expresser cells in the intestine

“…Intestinal ALP is a glycosylphosphatidylinositol-anchored ectoenzyme that is highly expressed in the BBM of duodenal epithelial cells, with expression declining along the proximal-caudal axis. ALP activity affects HCO 3 Ϫ secretion and regulates protective surface microclimate pH in rat duodenum (1,35). The lack of ALP in CHE cells provides further evidence against a major role for HCO 3 Ϫ secretion by CHE cells.…”

“…Intestinal ALP is found on the apical BBM of mature enterocytes, where it plays an important role in intestinal barrier functions (46) and affects HCO 3 Ϫ secretion and surface microclimate pH in rat duodenum (1). To further investigate the BBM characteristics of CHE cells, we examined whether the enzyme ALP was present.…”

Imaging Enzymes at Work: Metabolic Mapping by Enzyme Histochemistry

“…The substrates are particu-larly used to analyze microbial hydrolytic ectoenzymes that are important for nutrient fluxes in aquatic environments because these enzymes cleave dissolved organic matter. The substrates can also be applied to cryostat sections to localize enzyme activity in tissues (Akiba et al 2007;Strojsová and Vrba 2007;Wasaki et al 2008) and mammalian cells (Ali et al 2005). It should be realized that most of these substrates are usually not cleaved by single enzymes.…”