Effect of ATP on Intracellular pH in Pancreatic Ducts Involves P2X7 Receptors

Abstract

Pancreatic acini release ATP, which can stimulate HCO₃^--secreting ducts that express purinergic receptors from both P2X and P2Y families. The aim of this study was to investigate whether extracellular ATP affects HCO₃^- or H⁺ transport across the plasma membrane of intralobular ducts, and determine which P2 receptors might be involved. Ducts were obtained from rat pancreas, and the pH sensitive fluorophore BCECF was used to measure pH_i and recovery rates from cellular acidosis induced by ammonium pre-pulses. In order to reveal Na⁺/H⁺ exchange, Cl^-/HCO₃^- exchange or a Na⁺-HCO₃^- cotransport, experiments were performed in solutions with or without bicarbonate buffers (+BIC or –BIC), with amiloride derivative EIPA, or with low extracellular Cl^- concentrations. Although these transporters contributed to pH_i recovery from acidosis, ATP had no effect. Nevertheless, ATP induced a small and reversible decrease in pH_i by 0.07 ± 0.02 pH-units and BzATP decreased pH_i by 0.29 ± 0.07 pH-units in –BIC (n=10, 11). These effects were abolished by Brilliant Blue and in Ca²⁺-free solutions. Our study shows that the pH_i effect of ATP is mediated by P2X₇ receptors. However, ATP does not affect H⁺/HCO₃^- transporters unmasked by cellular acidosis. Presumably, ATP alone does not stimulate HCO₃^- secretion in pancreatic ducts.