Intestinal alkaline phosphatase regulates protective surface microclimate pH in rat duodenum

Abstract

Regulation of localized extracellular pH (pH o ) maintains normal organ function. An alkaline microclimate overlying the duodenal enterocyte brush border protects the mucosa from luminal acid. We hypothesized that intestinal alkaline phosphatase (IAP) regulates pH o due to pH-sensitive ATP hydrolysis as part of an ecto-purinergic pH regulatory system, comprised of cell-surface P2Y receptors and ATP-stimulated duodenal bicarbonate secretion (DBS). To test this hypothesis, we measured DBS in a perfused rat duodenal loop, examining the effect of the competitive alkaline phosphatase inhibitor glycerol phosphate (GP), the ecto-nucleoside triphosphate diphosphohydrolase inhibitor ARL67156, and exogenous nucleotides or P2 receptor agonists on DBS. Furthermore, we measured perfusate ATP concentration with a luciferin-luciferase bioassay. IAP inhibition increased DBS and luminal ATP output. Increased luminal ATP output was partially CFTR dependent, but was not due to cellular injury. Immunofluorescence localized the P2Y 1 receptor to the brush border membrane of duodenal villi. The P2Y 1 agonist 2-methylthio-ADP increased DBS, whereas the P2Y 1 antagonist MRS2179 reduced ATP-or GP-induced DBS. Acid perfusion augmented DBS and ATP release, further enhanced by the IAP inhibitor l-cysteine, and reduced by the exogenous ATPase apyrase. Furthermore, MRS2179 or the highly selective P2Y 1 antagonist MRS2500 co-perfused with acid induced epithelial injury, suggesting that IAP/ATP/P2Y signalling protects the mucosa from acid injury. Increased DBS augments IAP activity presumably by raising pH o , increasing the rate of ATP degradation, decreasing ATP-mediated DBS, forming a negative feedback loop. The duodenal epithelial brush border IAP-P2Y-HCO 3 − surface microclimate pH regulatory system effectively protects the mucosa from acid injury.

Supplementation of enzyme-treated soy protein saves dietary protein and promotes digestive and absorptive ability referring to TOR signaling in juvenile fish

“…In the duodenum of human, the high AKP activity depressed pH (Lalles 2010). Nevertheless, to protect the PI mucosa from acid injury, the falling pH restrained AKP expression, forming a negative feedback loop in rat intestine (Mizumori et al 2009). However, this hypothesis requires further investigation in future work.…”

Luminal calcium concentration controls intestinal calcium absorption by modification of intestinal alkaline phosphatase activity

“…The hydrolysis of phosphoric esters by the enzyme produces phosphoric acid and reduces bicarbonate secretion by the enterocytes, and regulates surface microclimate pH in the duodenum of rats (25,26) . Experiments with intestinal everted sacs and cell cultures are being carried out to analyse the relationship between TRPV6 and IAP.…”

Purinergic signalling in the gastrointestinal tract and related organs in health and disease

“…The NANC relaxation of the human ileal longitudinal and circular muscle is inhibited by MRS2179, a selective P2Y 1 receptor antagonist [669]. Duodenal brush border intestinal alkaline phosphatase degrades ATP released from the epithelium and stimulates HCO 3 − secretion via P2Y receptor activation [483]. The ecto-purinergic system may regulate cell surface pH, maintaining a protective alkaline microclimate during acid stress.…”