Subcellular mechanism of desensitization of the ATP-induced Ca2+ mobilization in human umbilical vein endothelial cells: role of intracellular Ca2+ stores

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 P2X1 and P2Y1 agonists, suggested the involvement of P2Y2 receptors. The expression of mRNA for the P2Y2 receptor was detected by RT-PCR analysis. These results indicate that P2Y2 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.

Chloride-sensitive Ca2+ entry by histamine and ATP in human aortic endothelial cells

The regulation of intracellular free Ca2+ concentration ([Ca2+]i) was studied in cultured human aortic endothelial cells loaded with the fluorescent Ca2+ indicator fura-2. Histamine and ATP at concentrations of 10 microM and higher produced a biphasic change in [Ca2+]i, which consisted of an initial transient elevation followed by a sustained elevation. Reduction of the extracellular Cl- concentration to 40 mM, or exposure to the Cl- channel antagonist N-phenylanthranilic acid selectively prevented the sustained response to histamine or ATP, but they did not affect the sustained response to the Ca2+ ionophore ionomycin. Elevation of extracellular K+ concentration to 90 mM had no influence on the sustained response to histamine or ATP. These results suggest that the sustained elevation of [Ca2+]i in response to histamine and ATP is due to the Cl(-)-sensitive entry of extracellular Ca2+ in cultured human aortic endothelial cells.