Endothelial purinoceptors play an important role in vascular responses to extracellular adenine nucleotides and hemodynamic forces. Here we report that P2X4 purinoceptor expression in human umbilical vein endothelial cells is transcriptionally downregulated by fluid shear stress. When human umbilical vein endothelial cells were subjected to a laminar shear stress of 15 dyn/cm(2), P2X4 mRNA levels began to decrease within 1 h and further decreased with time, reaching 60% at 24 h. Functional analysis of the 1.9-kb P2X4 5'-promoter indicated that a 131-bp segment (-112 to +19 bp relative to the transcription start site) containing a consensus binding site for the Sp1 transcription factor was critical for the shear stress responsiveness. Mutations of the Sp1 site decreased the basal level of transcription and abolished the response of the P2X4 promoter to shear stress. Electrophoretic mobility shift assays showed a marked decrease in binding of Sp1 to the Sp1 consensus element in shear-stressed cells, suggesting that Sp1 mediates the shear stress-induced downregulation of P2X4 gene transcription.