inosine-triphosphate and diadenosine-tetraphosphate

The nucleotide selectivities of the human P2Y(4) (hP2Y(4)) and rat P2Y(4) (rP2Y(4)) receptor stably expressed in 1321N1 human astrocytoma cells were determined by measuring increases in intracellular [Ca(2+)] under conditions that minimized metabolism, bioconversion, and endogenous nucleotide release. In cells expressing the hP2Y(4) receptor, UTP, GTP, and ITP all increased intracellular [Ca(2+)] with a rank order of potency of UTP (0.55) > GTP (6.59) = ITP (7.38), (EC(50), microM). ATP, CTP, xanthine 5'-triphosphate (XTP), and diadenosine 5',5"'-P(1), P(4)-tetraphosphate (Ap(4)A), all at 100 microM, were inactive at the hP2Y(4) receptor. In cells expressing the rP2Y(4) receptor, all seven nucleotides increased intracellular [Ca(2+)] with similar maximal effects and a rank order of potency of UTP (0.20) > ATP (0. 51) > Ap(4)A (1.24) approximately ITP (1.82) approximately GTP (2. 28) > CTP (7.24) > XTP (22.9). Because ATP is inactive at the hP2Y(4) receptor, we assessed whether ATP displayed antagonist activity. When coapplied, ATP shifted the concentration-response curve to UTP rightward in a concentration-dependent manner, with no change in the maximal response. A Schild plot derived from these data gave a pA(2) value of 6.15 (K(B) = 708 nM) and a slope near unity. Additionally, CTP and Ap(4)A (each at 100 microM) inhibited the response to an EC(50) concentration of UTP by approximately 40 and approximately 50%, respectively, whereas XTP had no effect. The inhibitory effects of ATP, CTP, and Ap(4)A were reversible on washout. Thus, ATP is a potent agonist at the rP2Y(4) receptor but is a competitive antagonist with moderate potency at the hP2Y(4) receptor.

Topics: Adenosine Diphosphate; Adenosine Triphosphate; Animals; Dinucleoside Phosphates; Humans; Purinergic P2 Receptor Agonists; Purinergic P2 Receptor Antagonists; Rats; Receptors, Purinergic P2; Species Specificity; Tumor Cells, Cultured; Uridine Diphosphate