Heterogeneity of [Ca2+]i signaling in intact rat aortic endothelium

Most existing knowledge about [Ca2+](i) signaling in vascular endothelium has been based on studies using endothelial cells cultured ii vitro. To examine how endothelial cells behave in situ, we have developed a method to monitor single-cell [Ca2+](i) from Fura-2-loaded rat aortic segments, Fluorescence ratio images from large numbers of endothelial cells were acquired by using a flow chamber mounted on a dual-wavelength fluorescence microscope. Our results showed that either acetylcholine or histamine reversibly activated the vascular endothelium by eliciting M-3 or H-1 receptor-mediated [Ca2+](i) increases, respectively. The acetylcholine-evoked endothelial [Ca2+](i) elevation at the branch site (intercostal orifice) was much more pronounced than that at the non-branch area. However, endothelium at the branch site was relatively insensitive to histamine, Both acetylcholine-sensitive and histamine-sensitive endothelial cells were arranged in belts aligned along flow lines and were intercalated with each other. Data analyzed from 400 endothelial cells located at the non-branch site showed drastically heterogeneous [Ca2+](i) responses to a fixed concentration of either acetylcholine or histamine, differing by two orders of magnitude in individual cells. As a conclusion, vascular endothelial cells appear to have their own characteristic [Ca2+](i) 'fingerprint' to various agonists and they may function coordinately in situ.