The role of vasoactive intestinal polypeptide as a neurotransmitter in canine penile erection: a combined in vivo and immunohistochemical study.

Vasoactive intestinal polypeptide (VIP), a 28-amino-acid polypeptide found in the human gut and genitourinary tract, primarily affects vasodilation and smooth-muscle relaxation. These effects have led to speculation that this neuropeptide may be a neurotransmitter in certain bodily functions, such as penile erection. We therefore designed an in vivo animal model to elucidate the influence of VIP and VIP antibody on the different stages of penile erection. We also performed immunohistochemical studies of the penile tissue to obtain further information about the distribution of VIP in the corpora cavernosa. Intracavernous injection of VIP induced penile erection. Its effect on arterial inflow was minor, but it caused active venous outflow restriction and was important in maintaining erection. VIP antibody blocked venous outflow restriction during neurostimulation-induced erection. VIP was found in the cavernous tissue (in the area between the smooth-muscle cells and the sinusoidal spaces) in close proximity to the arteries. We conclude that VIP is a neurotransmitter in the erectile tissue of the penis, and that its effects are similar to those from electrostimulation of the cavernous nerve. VIP increases arterial flow, decreases venous flow, and induces sinusoidal relaxation.