We studied H2O2-induced contractions of isolated rabbit intrapulmonary arteries mounted in standard tissue baths. All vessels were pretreated with a thromboxane A2/prostaglandin H2 receptor antagonist, SQ 29,548, to block immediate transient contractions to H2O2 and to isolate slowly developing sustained contractions. When exposed to H2O2 (0.1, 0.2, 0.3, 0.6, and 1.0 mM) for 30 min, vessels contracted in (0.1, 0.2, 0.3, 0.6, and 1.0 mM) for 30 min, vessels contracted in a concentration-dependent fashion between 0.1 and 0.3 mM H2O2; contractions at 0.6 and 1.0 mM H2O2 were not significantly different from those at 0.3 mM H2O2. During recovery (90 min) from H2O2 exposures, baseline tension was significantly greater, but active tension (10 microM phenylephrine) was significantly less for vessels previously exposed to 0.6 and 1.0 mM H2O2. Contractions to 0.3 mM H2O2 were not blunted by the following interventions: 1) endothelium rubbing, 2) incubation in Ca(2+)-free 100 microM ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) Krebs-Ringer solution, 3) incubation in the Ca(2+)-free solution and depletion of ryanodine (20 microM)-sensitive Ca(2+) stores, or 4) pretreatment with the protein kinase C inhibitor 1-(5-isoquinolinylsulfonyl)-3-methyl-piperazine (20 microM). However, contractions were depressed by approximately 50% when vessels were pretreated with the phospholipase C/serine esterase inhibitor 2-nitro-4-carboxy-phenyl-N,N-diphenylcarbamate (50 microM). These results suggest that slow-developing contractions to H2O2 are concentration dependent and may result, in part, from activation of a serine esterase(s) and/or phospholipase C.
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