Observation that the
G protein-coupled
P2U receptor (
P2Y2 receptor) is activated by
UTP as well as
ATP provided the first indication that a class of
uridine nucleotide-responsive receptors might exist. This hypothesis was confirmed by our identification of a
uridine nucleotide-specific receptor on C6-2B rat
glioma cells and by the recent cloning of two
uridine nucleotide-responsive receptors, the
P2Y6 receptor [J. Biol. Chem. 270:26152-26158 (1995)] and the
P2Y4 receptor [J. Biol. Chem. 270:30849-30852 (1995) and J. Biol. Chem. 270:30845-30848 (1995)]. The relative
nucleotide selectivities of these
uridine nucleotide-activated receptors have not been established. Therefore, we cloned and expressed the P2Y6 and
P2Y4 receptors in 1321N1 human
astrocytoma cells and compared their relative selectivities for
UDP,
UTP, and other
uridine and
adenine nucleotides with that of the
P2Y2 receptor expressed in the same cells. These comparisons were made by measuring
inositol phosphate accumulation under conditions in which the initial purity and stability of agonists were rigidly ensured and quantitatively assessed. The data indicate that the
P2Y2 receptor is activated with similar potencies by
ATP and
UTP but not by
ADP or
UDP; the
P2Y6 receptor is activated most potently by
UDP but weakly by
UTP,
ATP, and
ADP; and the
P2Y4 receptor is activated most potently by
UTP, less potently by
ATP, and not at all by
nucleotide diphosphates. Furthermore, the
P2Y6 receptor, which displays a
uridine nucleotide selectivity essentially identical to that of the
uridine nucleotide-specific receptor in C6-2B cells, was shown to be natively expressed in C6-2B cells and to account for the
uridine nucleotide responses originally identified in these cells. These results define the
uridine nucleotide selectivity of three
phospholipase C-linked receptors: a receptor that is selectively activated by
UDP (
P2Y6 receptor), selectively activated by
UTP (
P2Y4 receptor), and activated by
UTP and
ATP but not by
diphosphate nucleotides (
P2Y2 receptor).