Mechanisms of endothelial P2Y1- and P2Y2-mediated vasodilatation involve differential [Ca2+]iresponses

The present study was designed to evaluate the role of endothelial intracellular Ca2+concentration ([Ca2+]i) in the difference between P2Y1- and P2Y2-mediated vasodilatations in cerebral arteries. Rat middle cerebral arteries were cannulated, pressurized, and luminally perfused. The endothelium was selectively loaded with fura 2, a fluorescent Ca2+indicator, for simultaneous measurement of endothelial [Ca2+]iand diameter. Luminal administration of 2-methylthioadenosine 5′-triphosphate (2-MeS-ATP), an endothelial P2Y1agonist, resulted in purely nitric oxide (NO)-dependent dilation and [Ca2+]iincreases up to ∼300 nM (resting [Ca2+]i= 145 nM). UTP, an endothelial P2Y2agonist, resulted in dilations that were both endothelium-derived hyperpolarizing factor (EDHF)- and NO-dependent with [Ca2+]iincreases to >400 nM. In the presence of NG-nitro-l-arginine-indomethacin to inhibit NO synthase and cyclooxygenase, UTP resulted in an EDHF-dependent dilation alone. The [Ca2+]ithreshold for NO-dependent dilation was 220 vs. 340 nM for EDHF. In summary, the differences in the mechanism of vasodilatation resulting from stimulation of endothelial P2Y1and P2Y2purinoceptors result in part from differential [Ca2+]iresponses. Consistent with this finding, these studies also demonstrate a higher [Ca2+]ithreshold for EDHF-dependent responses compared with NO.