Anatomical Localization and Some Pharmacological Effects of Vasoactive Intestinal Polypeptide in Human and Monkey Corpus Cavernosum

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Abstract
Vasoactive intestinal polypeptide is hypothesized to be a nonadrenergic, noncholinergic neurotransmitter important in the physiology of penile erection. To further explore this concept, anatomical localization of vasoactive intestinal polypeptide, in vitro muscle bath studies and in vivo injection experiments were undertaken in the monkey and man. Using immunohistochemical techniques vasoactive intestinal polypeptide was localized at the light microscopic level to nerves within the monkey and human penis. Ultrastructurally, a modified peroxidase-antiperoxidase technique was used to identify large vasoactive intestinal polypeptide-positive vesicles within peptidergic and cholinergic varicosities. In the in vitro muscle bath, the addition of 10-9 M vasoactive intestinal polypeptide did not alter the baseline tension of strips of monkey and human corpus cavernosum. During contraction produced by norepinephrine stimulation, vasoactive intestinal polypeptide (10-7 M) caused relaxation of the monkey (41 .+-. 18%, n = 8) and human (23 .+-. 8%, n = 5) corpus cavernosum. Intracorporal injection of vasoactive intestinal polypeptide (0.75 .times. 10-9 to 3.75 .times. 10-9 mole/kg) had no effect on the monkey penis. Administration of vasoactive intestinal polypeptide (1.25 .times. 10-9 mol/kg) into the internal iliac artery of the monkey, while having no effect on the flaccid penis, caused detumescence of the erect penis obtained by cavernous nerve stimulation (2-5 V, 40 Hz, 2 ms). Although vasoactive intestinal polypeptide can be found within the nerves of the penis, its apparent in vitro and in vivo effects raise further questions concerning the role of this peptide in penile erection.