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

The present study was designed to evaluate the role of endothelial intracellular Ca2+ concentration ([Ca2+](i)) in the difference between P2Y(1)- and P2Y(2)-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 Call indicator, for simultaneous measurement of endothelial [Ca2+](i) and diameter. Luminal administration of 2-methylthioadenosine 5'-triphosphate (2-MeS-ATP), an endothelial P2Y(1) agonist, resulted in purely nitric oxide (NO)-dependent dilation and [Ca2+](i) increases up to similar to 300 DM (resting [Ca2+](i) = 145 nM). UTP, an endothelial P2Y(2) agonist, resulted in dilations that were both endothelium-derived hyperpolarizing factor (EDHF)and NO-dependent with [Ca2+](i) increases to >400 nM. In the presence of N-G-nitro-L-arginine-indomethacin to inhibit NO synthase and cyclooxygenase, UTP resulted in an EDBF-dependent dilation alone. The [Ca2+](i) threshold for NO-dependent dilation was 220 vs. 340 nM for EDBY In summary, the differences in the mechanism of vasodilatation resulting from stimulation of endothelial P2Y(1)- and P2Y(2) purinoceptors result in part from differential [Ca2+](i) responses. Consistent with this finding, these studies also demonstrate a higher [Ca2+](i) threshold for EDBF-dependent responses compared with NO.