Agonist-dependent difference in the relationship between cytosolic Ca2+ level and release of vascular relaxing factors in the endothelium of rabbit aortic valve

The correlation between changes in cytosolic Ca2+ levels ([Ca2+](i)) and the release of vascular relaxing factor(s) was investigated in the endothelium of rabbit aortic valve. ATP, carbachol and thapsigargin increased endothelial [Ca2+](i) in rabbit aortic valve loaded with a leakage resistant, fluorescent Ca2+ indicator, fura-PE3. Release of relaxing factors was bioassayed using the 'sandwich' preparation in which contraction was measured in the endothelium-denuded rabbit aorta attached to the endothelial surface of the valve. Addition of ATP, carbachol and thapsigargin induced sustained relaxation of the phenylephrine-induced contraction of the aorta in the 'sandwich' preparation, N-G-monomethyl-L-arginine (L-NMMA) greatly attenuated the relaxation induced by carbachol, and combined treatment with tetra-n-butylammonium completely inhibited the relaxation. These results suggest that the endothelial relaxing factors released from aortic valve are nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF). When the increase in endothelial [Ca2+](i) was plotted against the relaxation, the carbachol-induced increase in [Ca2+](i) elicited greater relaxation than did ATP or thapsigargin at a given [Ca2+](i). This suggests that various agonists differently modulate the relationship between [Ca2+](i) and release of NO. (C) 1999 Elsevier Science B.V. All rights reserved.