Loss of penile erectile response to intracavernous injection of acetylcholine in castrated dog.

Castrated dogs showed a slightly weak erectile response to cavernous nerve stimulation compared with normal dogs. Intracavernous injection of acetylcholine induced a dose-dependent increase of intracavernous pressure in normal dogs, however, there was almost no penile response in castrated dogs. Histological study showed a significant higher ratio of collagen tissue/smooth muscle in the corpus cavernosum of castrated dogs, but no difference in acetylcholine-esterase staining and the number of endothelial cells lining the sinusoidal spaces between castrated and normal dogs. These results indicate that the corpus cavernosum of castrated dogs nearly lost responsiveness to acetylcholine probably due to a high ratio of collagen/smooth muscle fibers and that there are different mechanisms for erectile response to nerve stimulation and intracavernous injection of acetylcholine.