Agonist-dependent difference in the relationship between cytosolic Ca 2+ level and release of vascular relaxing factors in the endothelium of rabbit aortic valve

Abstract

The correlation between changes in cytosolic Ca 2+ levels ([Ca 2+] i) and the release of vascular relaxing factor(s) was investigated in the endothelium of rabbit aortic valve. ATP, carbachol and thapsigargin increased endothelial [Ca 2+] i in rabbit aortic valve loaded with a leakage resistant, fluorescent Ca 2+ 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 [Ca 2+] i was plotted against the relaxation, the carbachol-induced increase in [Ca 2+] i elicited greater relaxation than did ATP or thapsigargin at a given [Ca 2+] i. This suggests that various agonists differently modulate the relationship between [Ca 2+] i and release of NO.