Threshold concentrations of ET-1 inhibited NO-mediated relaxant responses in penile arteries from LZR and OZR. Effect of ET-1 (0.3 nM) on (A, B) the average contractions response curves to ACh, (C, D) the concentration-response curves to the NO donor SNAP and (E, F) the concentration-response curves to the AC activator forskolin in penile arteries precontrated with phenylephrine (Phe; 1 μM) from LZR (A, C, E) and OZR (B, D, F). Results are means ± SEM of n = 3–5 animals (one to two arteries from each animal). *P < 0.05, **P < 0.01 versus control Emax; Student's t-test for paired observations.