Intestinal alkaline phosphatase regulates protective surface microclimate pH in rat duodenum

Area of Science:

• Gastroenterology
• Physiology
• Biochemistry

Background:

• Localized extracellular pH (pH(o)) regulation is vital for organ function.
• An alkaline microclimate at the duodenal enterocyte brush border protects the mucosa from luminal acid.
• Ecto-purinergic signaling involves cell-surface receptors and ATP hydrolysis.

Purpose of the Study:

• To investigate the role of intestinal alkaline phosphatase (IAP) in regulating duodenal pH(o).
• To determine if IAP functions as part of an ecto-purinergic system involving P2Y receptors and duodenal bicarbonate secretion (DBS).
• To elucidate the protective mechanisms of this system against acid-induced duodenal injury.

Main Methods:

• Perfused rat duodenal loop model to measure DBS.
• Utilized IAP inhibitors (glycerol phosphate, l-cysteine), ecto-nucleoside triphosphate diphosphohydrolase inhibitor (ARL67156), P2Y receptor agonists/antagonists (2-methylthio-ADP, MRS2179, MRS2500), and exogenous nucleotides.
• Measured perfusate ATP concentration using a luciferin-luciferase bioassay.
• Immunofluorescence to localize P2Y(1) receptors.

Main Results:

• IAP inhibition increased DBS and luminal ATP output, partially CFTR dependent and not due to injury.
• P2Y(1) receptors were localized to the duodenal brush border.
• P2Y(1) receptor activation increased DBS, while antagonism reduced ATP- or IAP inhibitor-induced DBS.
• Acid perfusion augmented DBS and ATP release, enhanced by IAP inhibition and reduced by apyrase.
• Antagonism of P2Y(1) receptors during acid perfusion induced epithelial injury, indicating a protective role.

Conclusions:

• The duodenal brush border IAP-P2Y-HCO(3-) system regulates microclimate pH, protecting the mucosa from acid injury.
• This ecto-purinergic system involves ATP hydrolysis by IAP and ATP-mediated stimulation of DBS via P2Y(1) receptors.
• A negative feedback loop exists where increased DBS raises pH(o), augmenting IAP activity, degrading ATP, and reducing DBS.