Study of the role of pituitary adenylate cyclase activating polypeptide (PACAP) as a neurotransmitter in the urogenital organs.

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To investigate a possible role for pituitary adenylate cyclase polypeptide (PACAP) on penile erection, effects of PACAP and vasoactive intestinal polypeptide (VIP) on rabbit corpus cavernosum and penile vessels were examined by in vitro isometric tension experiments. Electrical field stimulation (EFS) evoked a neurogenic and transient contaction of tissue strips prepared from the corpus cavernosum. The contraction was caused through excitation of alpha-1 adrenoceptors. Both PACAP27 and PACAP38 augumented the EFS-induced contactions, but VIP did not, suggesting the augmentation was mediated with type I PACAP receptors. EFS caused a nueurogenic relaxation in strips from the corpus cavernosum which had been contracted by noradrenalin (NA). The relaxation was mediated at least in part by nitric oxide (NO). Neither PACAP nor VIP affected the relaxation. Both PACAP and VIP relaxed the contraction induced by NA in ring preparations prepared from penile arteries and veins in a concentration-dependent manner. VIP was more potent to relax arterial preparations than PACAP.The threshold concentration of the peptides to cause relaxation response was lower in venous preparations then in arterial preparations., suggesting that the peptides act primarily on the blood. outflow from the corpus cavemosum to reduce its resistance. The results indicate that PACAP may play a role as a neurotransmitter for penile tumescence and detumescence by regulating the smooth muscle tone of the penile erectile tissue and penile vessels, at least in part, in a distinct manner from VIP, suggesting the existence of type I PACAP receptors in these tissues.