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

Abstract

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 N^G-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.