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

We assessed the role of endothelium in the arterial response to thapsigargin, the Ca2+-ATPase inhibitor of the endoplasmic reticulum, in rat isolated aortic rings. Thapsigargin induced an endothelium-dependent relaxation of phenylephrine-contracted aortic rings with an EC50 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 mu M) or methylene blue (1 mu M) 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 mu M N-G-nitro-L-arginine methyl ester. Pretreatment with indomethacin (3 mu M), glibenclamide (1 mu M) or iberiotoxin (100 nM) did not alter the thapsigargin-induced relaxation. In contrast, pretreatment with tetrapentylammonium ions (TPA(+), 1-3 mu M) Or With 300 mu M Ba2+ suppressed the relaxant response to thapsigargin. TPA(+) (3 mu M) also attenuated acetylcholine-induced relaxation. Thapsigargin-induced endothelium-dependent relaxation was primarily dependent on the presence of extracellular Ca2+ 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. Tnapsigargin 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 Ba2+-sensitive but not Ca2+-activated or ATP-sensitive K+ channels may be also involved in thapsigargin-induced relaxation of rat isolated aortic rings. (C) 2000 Elsevier Science B.V. All rights reserved.