Calcium signalling through nucleotide receptor P2Y2 in cultured human vascular endothelium

Microfluorometric measurements in Fura-2-loaded single cultured human vascular endothelial cells were used to characterize the intracellular calcium [Ca2+](i) responses triggered by extracellular application of adenosine 5'-triphosphate (ATP) and other nucleotides. Application of ATP or uridine 5'-triphosphate (UTP) gave rise to dose-dependent elevations of [Ca2+](i) in all the cells tested. At saturating concentrations of agonist, the [Ca2+](i) response was biphasic, with an early peak and a sustained plateau. Unlike peak responses, the sustained Ca2+ plateau was sensitive to removal of Ca2+ from the external medium. Mn2+ quenching revealed the presence of Ca2+ influx during the agonist-induced calcium plateau. The agonist-evoked calcium plateau was inhibited in a dose-dependent manner by the Cl- channel blocker NPPB, by the divalent cation Ni2+ and by the imidazole antimycotic econazole, Previously, these compounds have been shown to block store-operated Ca2+ entry. The two phases of the agonist-evoked [Ca2+](i) response were blocked by the specific phospholipase C inhibitor U-73122 and by intracellular injection of low molecular weight heparin, suggesting the involvement of IP3-sensitive intracellular Ca2+ stores. The pharmacological profile of the response, using different nucleotides and analogues, ATP = UTP > ADP = UDP, and no responses to P-2X1 and P-2Y1 agonists, suggested the involvement of P-2Y2 receptors. The expression of mRNA for the P-2Y2 receptor was detected by RT-PCR analysis. These results indicate that P-2Y2 receptors linked to intracellular Ca2+ mobilization are present in human vascular endothelial cells. The initial [Ca2+](i) mobilization is followed by a phase of elevated [Ca2+](i) influx.