Evidence That a S-Nitrosothiol, But Not Nitric Oxide, May Be Identical with Endothelium-Derived Relaxing Factor

Abstract

Summary: We recently reported that endothelium-derived relaxing factor (EDRF) cannot be identified as free nitric oxide (NO) by electron paramagnetic resonance (EPR) spectroscopy of nitrosylhemoglobin (NO-Hgb). The purpose of our present studies was to determine whether, in contrast to NO, S-nitroso-l-cysteine (cysNO) behaves similarly to EDRF in the EPR test (i.e., no detectable NO-Hgb at concentrations of cysNO that evoke vasorelaxation). Authentic NO and cysNO relaxed canine coronary artery rings with similar potency (ED50 = 23 and 12 nM, respectively). The lower detection limit by EPR of cysNO was more than two orders of magnitude higher (65 nmol) than that of NO (0.3 nmol). At concentrations that cause vasorelaxation similar to EDRF, NO but not cysNO could be detected as NO-Hgb by EPR. These findings suggest that cysNO (or other S-nitrosothiols with similar characteristics), but not free NO, is likely to be EDRF. This possibility is supported by the demonstration that bradykinin increased the efflux of 35S from cultured bovine pulmonary endothelial cells preincubated with [35S]cysteine.