The Role of Nitric Oxide (Formerly Endothelium-Derived Relaxing Factor-EDRF) in Vasodilatation and Vasodilator Therapy

Nitric oxide is widely distributed in the body. It has an important role in the regulation of the circulation and as yet, ill-defined roles in nervous and immune systems. It is derived from L-arginine from a reaction catalysed by a constitutive intracellular enzyme, nitric oxide synthase. It is recognised as the endogenous nitrovasodilator whose action is mimicked by all exogenous nitrovasodilators. After production in the vascular endothelial cell, it diffuses to the smooth muscle cell where it activates the enzyme guanylate cyclase which leads to an increase in cyclic GMP and thence to muscle relaxation. The duration of its action is brief, a few seconds. Disorders of NO metabolism underlie many disease states including endotoxic shock in which prolonged production of nitric oxide may be induced by cytokines. Deficiencies in endogenous production may account for hypertension in various disease states including atherosclerosis and chronic renal failure. NO therapy been used experimentally to successfully treat idiopathic pulmonary hypertension and pulmonary hypertension associated with cardiac and respiratory diseases. However, the long-term benefits have yet to be studied. Administration of NO requires the use of a device to monitor the concentrations of both NO and of NO2. The latter is a noxious agent and a time-related product of the reaction between NO and O2 and is a possible contaminant of preparations of NO. Precautions must be taken to prevent contamination of the work-place atmosphere with NO and NO2. These include gas scavenging and the use of a leak-free system for spontaneous and mechanical ventilation. Using NO in its gaseous form, clinicians have at long last been provided with the means to treat pulmonary hypertension without adversely causing systemic hypotension. The therapy is most suited to short-term use in mechanically ventilated patients. Safe practical long-term NO therapy must await the development of agents which release NO from aerosol preparations.