Deactivation of norepinephrine by peroxynitrite as a new pathogenesis in the hypotension of septic shock.

BACKGROUND: Vascular hyporeactivity to catecholamines limits successful treatment of hypotension in septic shock. Large amounts of nitric oxide (NO) and superoxide anion (O(2)(-1).) are produced in response to bacterial endotoxins and/or inflammatory cytokines. NO reacts with O(2)(-1). to form the potentially toxic NO metabolite, peroxynitrite (ONOO(-1)). The purpose of this study was to investigate whether ONOO(-1) decreases the vasocontractile activity of norepinephrine.
METHODS: Norepinephrine was treated with ONOO(-1) or 3-morpholinosydonimine-N-ethyl-carbamine (SIN-1; an ONOO(-1) producer) in a 5 x 10(-2) m sodium phosphate buffer solution at pH 7.4, and absorbance of the product was measured spectrophotometrically at 295 and 370 nm. Norepinephrine pretreated with ONOO(-1) was administered to isolated rat thoracic aortas to observe contractions in functional experiments. The rate constant between norepinephrine and ONOO(-1) was determined via a competition assay with cysteine in functional experiments. Norepinephrine pretreated with ONOO(-1) was injected intravenously into anesthetized rats to measure blood pressure.
RESULTS: Norepinephrine pretreated with ONOO(-1) was confirmed spectrally as oxidized norepinephrine. Norepinephrine pretreated with ONOO(-1) decreased its vasocontractile force in an ONOO(-1) (10(-6), up to 3 x 10(-4) m) concentration-dependent manner (EC(50) = 5.1 x 10(-5) m). The decrease in its force was lower at pretreatment with ONOO(-1) in a lower pH buffer. A rate constant for the ONOO(-1)-norepinephrine reaction was 6 x 10(2) m/s. Norepinephrine (10(-7) m) incubated with SIN-1 (10(-3) m) decreased its vasocontractile force in an incubation time-dependent manner. Administration of norepinephrine pretreated with ONOO(-1) to anesthetized rats caused no significant change in arterial blood pressure.
CONCLUSIONS: These results indicate that norepinephrine was oxidized and deactivated by ONOO(-1). This deactivation may, at least in part, account for the hyporeactivities of vasocontraction to norepinephrine in septic shock.