Neurogenic nitric oxide mediates relaxation of canine corpus cavernosum

The aim of the present study is to analyze mechanisms underlying neurogenic relaxation of the corpus cavernosum which are believed to participate in penile erection.

Mechanical responses to nerve stimulation by electrical pulses and nicotine were measured in strips of canine corpus cavernosum precontracted with phenylephrine. Cyclic guanosine monophosphate (GMP) contents in the strips were also measured by radioimmunoassay. Immunohistochemistry for nitric oxide synthase (NOS) and vasoactive intestinal polypeptide (VIP) was performed.

Transmural electrical stimulation and nicotine produced relaxations in the isolated canine corpus. The neurogenic relaxation was abolished by N omega-nitro-L-arginine, a NOS inhibitor, and the inhibition was reversed by L-arginine. Relaxations induced by nerve stimulation and exogenous nitric oxide (NO) were depressed by oxyhemoglobin and methylene blue. Vasoactive intestinal polypeptide (VIP)-induced relaxations were not influenced by these inhibitors. In the controls strips and those made unresponsive to VIP by its repeated application, the responses to nerve stimulation did not differ. The content of cyclic GMP in the tissue increased in response to nicotine, the effect being abolished by the NO synthase inhibitor. Immunohistochemical study demonstrated neurons containing NOS and VIP.

It appears that the relaxation induced by nerve stimulation is mediated solely by NO liberated from the nerve that activates soluble guanylate cyclase and increases the production of cyclic GMP in smooth muscle, whereas VIP does not play a role in the regulation of muscle tone under the experimental conditions used.