We assessed the role of endothelium in the arterial response to thapsigargi
n, the Ca2+-ATPase inhibitor of the endoplasmic reticulum, in rat isolated
aortic rings. Thapsigargin induced an endothelium-dependent relaxation of p
henylephrine-contracted aortic rings with an EC50 of 2.6 +/- 0.4 nM and a 7
5% 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 re
laxation 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 wi
th tetrapentylammonium ions (TPA(+), 1-3 mu M) Or With 300 mu M Ba2+ suppre
ssed the relaxant response to thapsigargin. TPA(+) (3 mu M) also attenuated
acetylcholine-induced relaxation. Thapsigargin-induced endothelium-depende
nt 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 ace
tylcholine or A23187 was markedly reduced. While thapsigargin (3 nM) did no
t influence the relaxation induced by endothelium-independent dilators, sod
ium nitroprusside and verapamil. These results indicate that thapsigargin p
roduced complex vascular effects primarily by acting on the endothelial cel
ls. Tnapsigargin causes an endothelial nitric oxide-dependent relaxation; o
n the other hand, it inhibits nitric oxide-mediated relaxation at the simil
ar concentrations. Activation of TPA(+)- and Ba2+-sensitive but not Ca2+-ac
tivated or ATP-sensitive K+ channels may be also involved in thapsigargin-i
nduced relaxation of rat isolated aortic rings. (C) 2000 Elsevier Science B
.V. All rights reserved.