Altered endothelial Ca2+ regulation after ischemia/reperfusion produces potentiated endothelium-derived hyperpolarizing factor-mediated dilations

Abstract

Background and Purpose - Endothelium-derived hyperpolarizing factor (EDHF)-mediated dilations are potentiated after several pathologies, including ischemia/reperfusion (I/R). However, no study to date has addressed the mechanism by which this potentiation occurs. This study tested the hypothesis that potentiated EDHF-mediated dilations are due to altered endothelial Ca2+ handling after I/R. Methods - Rat middle cerebral arteries (MCAs) were isolated after 2 hours of MCA occlusion and 24 hours of reperfusion (or sham surgery). This model has been previously demonstrated to produce potentiated EDHF-mediated dilations. MCAs were studied in a pressurized/perfused vessel chamber equipped for the simultaneous measurement of endothelial Ca2+ (with fura 2) and artery diameter. Measures were made after luminal administration of UTP (P2Y2 purinoceptor agonist), 2 MeS-ATP (P2Y1 purinoceptor agonist), and Br-A23187 (receptor-independent Ca2+ ionophore) for sham and I/R MCAs. Results - I/R resulted in significantly potentiated UTP-mediated dilations (through a P2Y2 purinoceptor) and endothelial Ca2+ responses in the presence of NG-nitro-L-arginine methyl ester (L-NAME) and indomethacin. Endothelial Ca2+ and diameter responses were also significantly potentiated with 2 MeS-ATP (through a P2Y1 purinoceptor) when L-NAME and indomethacin were absent. Br-A23187, a receptor-independent Ca2+ ionophore, produced significantly potentiated endothelial Ca2+ responses after I/R in the presence of L-NAME/indomethacin. Evaluation of artery diameter as a function of endothelial Ca2+ demonstrated no differences between sham and I/R groups. Conclusions - These findings demonstrate that I/R results in augmented endothelial Ca2+ responses that appear to be downstream of the receptor level. Moreover, these data suggest that this augmented Ca2+ response contributes to the potentiated EDHF-mediated dilations after I/R.