Role of endothelium-formed nitric oxide on vascular responses

Effects of the nitric oxide synthase inhibitor NG-nitro-L-arginine on the erectile response to cavernous nerve stimulation in the rabbit

Using a rabbit model, the involvement of the L-arginine/nitric oxide pathway in penile erection was investigated. The mean basal intracavernous pressure was 21 cm H2O. Cavernous nerve stimulation (4-8 V, 20-30 Hz) increased the pressure to approximately 130 cm H2O. This response was highly reproducible and usually associated with full penile erection. The pressure increase could be quantified in terms of: (1) the slope of the initial, ascending part of the pressure increase; (2) delta P, which was defined as the maximal pressure obtained by the stimulation minus the basal pressure before the stimulation; (3) T90, which was defined as the time to reach 90 per cent of delta P. Intrapenile administration of the L-arginine/nitric oxide synthesis inhibitor NG-nitro-L-arginine had no effect on systemic arterial blood pressure. However, NG-nitro-L-arginine (0.22 and 2.19 mg), administered via the same route, abolished the erectile response induced by cavernous nerve stimulation; T90 increased and slope and delta P decreased significantly. NG-nitro-D-arginine (2.19), on the other hand, had no inhibitory effect. L-arginine (21.07 mg), given either directly or after NG-nitro-L-arginine had no consistent effect on the functional response to cavernous nerve stimulation. The results suggest that pharmacologically induced effects on intracavernous pressure in the rabbit can be described quantitatively, and that this model may be useful to study the mechanisms controlling penile erection in vivo. The pronounced inhibitory action of NG-nitro-L-arginine demonstrates the important role of the arginine/nitric oxide pathway in mediating relaxation of penile smooth muscles necessary for erection.