Purinoceptors evoke different electrophysiological responses in pancreatic ducts -: P2Y inhibits K+ conductance, and P2X stimulates cation conductance

In epithelia, extracellular nucleotides are often associated with regulation of ion transporters, especially Cl- channels. In this study, we investigated which purinoceptors are present in native pancreatic ducts and how they regulate ion transport. We applied whole-cell patch-clamp recordings, intracellular Ca2+ and pH measurements, and reverse transcription-polymerase chain reaction (RT-PCR) analysis. The data show two types of purinoceptors and cellular responses. UTP and ATP produced large Ca2+ transients, a decrease in intracellular pH, 8-10-mV depolarization of the membrane voltage, and a decrease in the whole-cell conductance. The membrane effects were due to closure of K+ channels, as confirmed by dependence on extracellular K+. UTP/ATP effects could be associated with P2Y(2) purinoceptors, and RT-PCR revealed mRNAs for P2Y(2) and P2Y(4) receptors. On the other hand, 2',3'-O-4-benzoylbenzoyl-ATP induced Ca2+ influx and similar to 20-mV depolarization of the membrane voltage with a concomitant increase in the whole-cell conductance. These effects were depend ent on extracellular Na+, not Cl-, indicating opening of cation channels associated with P2X(7) purinoceptors. RT-PCR showed mRNAs for P2X(7) and P2X(4) receptors. In microperfused ducts, luminal (but not basolateral) ATP caused large depolarizations of membrane voltages recorded with microelectrodes, consistent with luminal localization of P2X(7) receptors. Thus, P2Y(2) (and possibly P2Y(4)) purinoceptors inhibit K+ channels and may not support secretion in native duets. P2X(7) (and possibly P2X(4)) receptors are associated with cation channels and may contribute to regulation of secretion.