Extracellular UTP stimulates electrogenic bicarbonate secretion across CFTR knockout gallbladder epithelium

The loss of cystic fibrosis transmembrane conductance regulator (CFTR)-mediated transepithelial HCO3- secretion contributes to the pathogenesis of pancreatic and biliary disease in cystic fibrosis (CF) patients. Recent studies have investigated P2Y(2) nucleotide receptor agonists, e.g., UTP, as a means to bypass the CFTR defect by stimulating Ca2+-activated Cl- secretion. However, the value of this treatment in facilitating transepithelial HCO3- secretion is unknown. Gallbladder mucosae from CFTR knockout mice were used to isolate the Ca2+-dependent anion conductance during activation of luminal P2Y(2) receptors. In Ussing chamber studies, UTP stimulated a transient peak in short-circuit current (I-sc) that declined to a stable plateau phase lasting 30-60 min. The plateau I-sc after UTP was Cl- independent, HCO3- dependent, insensitive to bumetanide, and blocked by luminal DIDS. In pH stat studies, luminal UTP increased both I-sc and serosal-to-mucosal HCO3- flux (J(s-->m)) during a 30-min period. Substitution of Cl- with gluconate in the luminal bath to inhibit Cl-/HCO3- exchange did not prevent the increase in J(s-->m) and Isc during UTP. In contrast, luminal DIDS completely inhibited UTP-stimulated increases in J(s-->m) and I-sc. We conclude that P2Y(2) receptor activation results in a sustained (30-60 min) increase in electrogenic HCO3- secretion that is mediated via an intracellular Ca2+-dependent anion conductance in CF gallbladder.