Purines such as ATP, long known as bioenergetically active, are also important for cellular signalling. Originally described for excitable tissues, epithelial purinergic signalling has been more recently studied We have formulated a novel hypothesis wherein purinergic signalling regulates the microclimate pH (pH_e) overlying HCO₃^- secreting epithelia: ATP is non-lytically released across the apical plasma membrane where it interacts with surface P2Y₁ receptors, increasing the rate of HCO₃^- secretion. HCO₃^- secretion in turn raises pH_e at the catalytic site of alkaline phosphatase (AP), bringing it closer to its pH optimum, increasing the rate of ATP hydrolysis, decreasing [ATP]_e, decreasing the rate of HCO₃^- secretion. Thus, AP acts as a pH_e sensor through its pH-dependent ATP hydrolytic activity and through ATP-induced HCO₃^- secretion. To test this hypothesis, we measured AP activity in vivo in rat duodenum using the fluorogenic dye ELF-97, finding that AP activity was dependent on the rate of HCO₃^- secretion and bulk luminal pH, with AP activity still measurable despite a luminal pH of 2, consistent with a large gradient between bulk pH and pH_e. ATP was released in response to luminal acid perfusion, augmented by AP inhibition. Exogenous ATP increased the rate of HCO₃^- secretion, blocked by P2Y receptor antagonists. Interesting, functional CFTR inhibition with CFTR_inh-172 impaired ATP release.

Since precise pH_e regulation is important for physiological processes such as a sperm capacitation, biliary secretion, pancreatic enzyme secretion, bone remodeling, airway ciliary beat frequency, duodenal mucosal protection, and jejunal fatty acid uptake, we hypothesized that ecto-enzymes with pH optima far removed from neutrality act as pH_e sensors as part of an ecto-purinergic pH_e system. Since the aforementioned organs also express the cystic fibrosis transmembrane receptor (CFTR) and AP, secrete HCO₃^-, and are affected by the disease CF, we further hypothesize that dysregulation of the ecto-purinergic signalling system may help explain the CF phenotype.