Evidence against inhibition of sarcoplasmic reticulum Ca2+-pump as mechanism of H2O2-induced contraction of rat aorta

AIM: To test whether inhibition of sarcoplasmic reticulum (SR) Ca2+-pump is involved in H2O2-induced contraction of endothelium-denuded rat aorta. MET HODS: Isometric tension recording of H2O2 and cyclopiazonic acid (CPA)-indu ced contractions of rat aortic rings were compared in the absence or presen ce of various pharmacological tools to discriminate their signaling pathway s involved. RESULTS: Both H2O2 and CPA contracted rat aortic rings, but wit h different contractile patterns. H2O2 triggered a fast and phasic contract ion, whereas CPA elicited a slow and sustained contraction. In Ca2+-free me dium, pretreatment of aortic rings with CPA 30 mu mol/L but not with H2O2 3 0 mu mol/L nearly abolished phenylephrine (10 mu mol/L)-induced contraction . In addition, upon the maximal contraction induced by thapsigargin 30 mu m ol/L, H2O2 but not CPA further contracted aortic rings. On the other hand, H2O2 (30 mu mol/L)- but not CPA (10 mu mol/L)-induced contraction could be inhibited by suramin and RB-2 (each 100 mu mol/L), two Pz-purinoceptor anta gonists. Furthermore, although pretreatment with 2-APB, a membrane permeabl e IP3 receptor blocker, inhibited both H2O2- and CPA-induced contractions, only H2O2 (30 mu mol/L)- induced contraction could be depressed, to differe nt degree, by various inhibitors of receptor-coupled or downstream signalin g enzymes, including PLC, PKC, PLA(2), COX, and protein tyrosine kinases. C ONCLUSION: inhibition of smooth muscle SR Ca2+-pump is unlikely the mechani sm responsible for H2O2-induced contraction of endothelium-denuded rat aort a.