Two Distinct P2Y Receptors Are Involved in Purine- and Pyrimidine-Evoked Ca²⁺ Elevation in Mammalian Brain Astrocytic...

ATP and 2-methyl-thio-ATP (2-Me-SATP) increase cytosolic calcium concentrations ([Ca²⁺]_i) in rat striatal astrocytes (Centemeri et al. [1997] Br J Pharmacol 121:1700-1706). The aim of the present study was to: (1) characterize pyrimidine-induced [Ca²⁺]_i increases in the same experimental system, and (2) try to identify the multiple P2Y receptor subtypes mediating Ca²⁺ mobilization. UDP and UTP triggered a concentration-dependent [Ca²⁺]_i elevation (EC₅₀s = 0.58 microM ± 0.4 and 31 microM ± 6, respectively).Pyrimidine-evoked [Ca²⁺]_i elevation was solely due to mobilization from intracellular stores, because: (1) removing calcium from extracellular medium or (2) blocking its influx with Ni²⁺ did not modify UTP responses; (3) the store-depleting agent thapsigargin completely abolished UTP-evoked [Ca²⁺]_i increments. Guanosine-5'-O-(2-thiodiphosphate) partially inhibited the UTP response, whereas pertussis toxin (PTx) had no effect. The phospholipase C inhibitor U-73122 significantly reduced the UTP-evoked [Ca²⁺]_i rise. Computer-assisted analysis indicated that the UTP and UDP responses are mediated by a single receptor, while ATP and 2-Me-SATP interact with two distinct receptors. The selective P2Y₁ receptor antagonist MRS2179 abolished the ATP higher potency component. Sequential challenges with the same nucleotides resulted in almost complete homologous desensitization. Pre-exposure to UTP lowered the subsequent responses to either ATP or 2-Me-SATP. Maximally active concentrations of UTP and ATP were not additive. In conclusion, [Ca²⁺]_i elevation in astrocytes by purines and pyrimidines is mediated by two distinct P2Y receptors, likely the P2Y₁ and P2Y₆ subtypes.