Nitric oxide in shock

Refractory hypotension with end-organ hypoperfusion and failure is an ominous feature of shock. Distributive shock is caused by severe infections (septic shock) or severe systemic allergic reactions (anaphylactic shock). In 1986, it was concluded that nitric oxide (NO) is the endothelium-derived relaxing factor that had been discovered 6 years earlier. Since then, NO has been shown to be important for the physiological and pathological control of vascular tone. Nevertheless, although inhibition of NO synthesis restores blood pressure, NO synthase (NOS) inhibition cannot improve outcome, on the contrary. This implies that NO acts as a double-edged sword during septic shock. Consequently, the focus has shifted towards selective inducible NOS (iNOS) inhibitors. The contribution of NO to anaphylactic shock seems to be more straightforward, as NOS inhibition abrogates shock in conscious mice. Surprisingly, however, this shock-inducing NO is not produced by the inducible iNOS, but by the so-called constitutive enzyme endothelial NOS. This review summarizes the contribution of NO to septic and anaphylactic shock. Although NOS inhibition may be promising for the treatment of anaphylactic shock, the failure of a phase III trial indicates that other approaches are required for the successful treatment of septic shock. Amongst these, high hopes are set for selective iNOS inhibitors. But it might also be necessary to shift gears and focus on downstream cardiovascular targets of NO or on other vasodilating phenomena.