Vasoactive intestinal polypeptide, an erectile neurotransmitter, improves erectile function more significantly in castrated rats than in normal rats

What’s known on the subject? and What does the study add? Vasoactive intestinal polypeptide (VIP) is an important erectile neurotransmitter, and our previous study found that the mRNA expression of VIP was independent of androgens. The present study further investigated the vivo effect of VIP on erection. We found that not only the expression of VIP was independent of androgens, but also the effect of VIP on erection was independent of androgens. In fact, we found that VIP played a more significant role on erection in castrated rats than in normal rats.

OBJECTIVE

• ? To investigate the regulatory role of androgen in VIP‐mediated erectile effect. Androgen is essential for physiological erection. Vasoactive intestinal polypeptide (VIP) is an important erectile neurotransmitter. While previous studies demonstrated that VIP expression in the penis was androgen‐independent, it remains controversial whether androgen has any effect on VIP‐mediated erection.

MATERIALS AND METHODS

• ? Male SD rats were divided into a control group, a castration group, and a castration‐with‐testosterone‐replacement group. Four weeks later, each group was subdivided into low and high‐dose VIP subgroups and subjected to intracavernous injection of 0.5 and 2 µg VIP, respectively.
• ? Erectile function was tested by recording intracavernosal pressure (ICP) and mean arterial blood pressure (MAP) before and after VIP injection.
• ? The expressions of the VIP‐receptor (VPAC2), G‐protein stimulatory and inhibitory alpha subunits (Gs‐α, Gi‐α), and PDE3A in rat corpus cavernosum (CC) was qualified by real‐time PCR and Western blot analysis.

RESULTS

• ? Castration reduced erectile function while testosterone restored it. VIP improved erectile function in a dose‐dependent manner.
• ? High‐dose VIP significantly enhanced erectile function in castrated rats and there was no difference of ICP/MAP among three groups after injection of high‐dose VIP.
• ? Low‐dose VIP also resulted in a higher improvement of erectile function in castrated rats, although the ICP/MAP was lower in these rats than in the other two groups. VPAC2 and Gs‐α were up‐regulated while Gi‐α and PDE3A were down‐regulated in CC of castrated rats.

CONCLUSION

• ? VIP improves erectile function much more significantly in hypogonadal condition, mainly due to the higher expression of VPAC2, Gs‐α, and lower expression of Gi‐α and PDE3A in CC of castrated rats. Androgen may negatively regulate the erectile effect of VIP.