Effect of ATP on intracellular pH in pancreatic ducts involves P2X7 receptors.

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 pHi 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 pHi recovery from acidosis, ATP had no effect. Nevertheless, ATP induced a small and reversible decrease in pHi by 0.07+/-0.02 pH-units and BzATP decreased pHi 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 pHi 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.