Role of endothelium in thapsigargin-induced arterial responses in rat aorta

Abstract

We assessed the role of endothelium in the arterial response to thapsigargin, the Ca 2+-ATPase inhibitor of the endoplasmic reticulum, in rat isolated aortic rings. Thapsigargin induced an endothelium-dependent relaxation of phenylephrine-contracted aortic rings with an EC 50 of 2.6±0.4 nM and a 75% maximum relaxation, while it was less effective against 30 mM K +-induced contraction. Pretreatment of aortic rings with N G-nitro- l-arginine methyl ester (30 μM) or methylene blue (1 μM) reduced thapsigargin-induced relaxation by approximately 85%. Thapsigargin failed to relax the endothelium-denuded rings. l-Arginine (3 mM) partially, but significantly, antagonized the effect of 30 μM N G-nitro- l-arginine methyl ester. Pretreatment with indomethacin (3 μM), glibenclamide (1 μM) or iberiotoxin (100 nM) did not alter the thapsigargin-induced relaxation. In contrast, pretreatment with tetrapentylammonium ions (TPA +, 1–3 μM) or with 300 μM Ba 2+ suppressed the relaxant response to thapsigargin. TPA + (3 μM) also attenuated acetylcholine-induced relaxation. Thapsigargin-induced endothelium-dependent relaxation was primarily dependent on the presence of extracellular Ca 2+. Interestingly, when the tissues were exposed to very low concentrations of thapsigargin (1–3 nM) the nitric oxide-dependent relaxation induced by acetylcholine or A23187 was markedly reduced. While thapsigargin (3 nM) did not influence the relaxation induced by endothelium-independent dilators, sodium nitroprusside and verapamil. These results indicate that thapsigargin produced complex vascular effects primarily by acting on the endothelial cells. Thapsigargin causes an endothelial nitric oxide-dependent relaxation; on the other hand, it inhibits nitric oxide-mediated relaxation at the similar concentrations. Activation of TPA +- and Ba 2+-sensitive but not Ca 2+-activated or ATP-sensitive K + channels may be also involved in thapsigargin-induced relaxation of rat isolated aortic rings.