A guanine nucleotide-independent inwardly rectifying cation permeability is associated with P2Y(1) receptor expression in Xenopus oocytes

The functional properties of the G protein-coupled P2Y(1) receptor were investigated in Xenopus oocytes. Incubation of oocytes expressing either the human or turkey P2Y(1) receptor with adenine nucleotide agonists resulted in an increase in Cl- current and activation of a novel cation current with an inwardly rectifying current-voltage relationship. Activation of either the human P2Y(2) (P-2U-purinergic) or M1 muscarinic receptor expressed in oocytes resulted in an increase in Cl- current similar to that observed in P2Y(1) receptor-expressing oocytes but had no effect on cation current. P2 receptor agonists stimulated both the cation current and Cl- current in P2Y(1) receptor expressing oocytes with EC(50) values and an order of potency (2-methylthioadenosine diphosphate > 2-methylthioadenosine triphosphate (2MeSATP) > ATP > UTP) that were similar to those previously observed for activation of phospholipase C in 1321N1 human astrocytoma cells stably expressing the human or turkey P2Y(1) receptor. The P2Y receptor antagonists suramin and pyridoxal phosphate 6-azophenyl-2'-4'-disulfonic acid both shifted to the right the concentration-response relationship for 2MeSATP for stimulation of oocyte currents. Although injection of oocytes with either GDP beta S (granuyl-5'-yl thiophosphate) or GTP gamma S (guanosine 5'3-O-(thio)triphosphate) resulted in loss of adenine nucleotide-promoted Cl- channel activation, neither guanine nucleotide altered the 2MeSATP-stimulated cation current. These data are consistent with the view that activation of the novel cation current by the P2Y(1) receptor does not involve a G protein. Tail current analysis of the novel P2Y(1) receptor-associated cation conductance revealed that the open channel current-voltage relationship was outwardly rectifying with a reversal potential of -38 mV for the turkey P2Y(1) receptor and -36 mV for the human P2Y(1) receptor. Replacement of Na+ with K+ ions in the bathing solution produced a shift in reversal potential to near zero mV, but significant outward rectification remained. The cation current was not permeable to either Ca2+ or Ba2+ and exhibited steady-state inactivation at holding potentials below -60 mV. These results indicate that the P2Y(1) receptor exhibits both metabotropic properties and a novel G protein-independent ionotropic response when expressed in Xenopus oocytes.