The endothelin ETB receptor agonist [I-125]BQ-3020 binds predominantly to nerves in the bovine retractor penis muscle and penile artery

Preliminary pharmacological experiments have suggested that in the bovine r etractor penis muscle there are relaxation-mediating endothelin ETB recepto rs, at least part of which are located on the inhibitory nitrergic nerves. The present work was undertaken to test this hypothesis by means of recepto r autoradiography and additional pharmacological experiments. In the retrac tor penis muscle and the penile artery, specific binding of the ETB recepto r-selective agonist [I-125]BQ-3020 took place predominantly to nerve trunks and minor nerve branches. The situation was the same in the dorsal metatar sal artery, that was included as a reference because of its different inner vation. Throughout the nerves the silver grains were evenly distributed ove r the nuclei of Schwann cells and the spaces between them. In the retractor penis there was also a small amount of specific binding to smooth muscle. No specific endothelial binding was observed in any of the tissues examined . The pharmacological studies confirmed that the relaxation of the retracto r penis muscle induced by the ETB receptor-selective agonist, sarafotoxin S 6c, is susceptible to tetrodotoxin as well as to inhibition of nitric oxide synthase. The relaxation was also characterized by inconsistency, weakness and tachyphylaxis. The electrical field stimulation-induced submaximal rel axation of the retractor penis was unaffected by stimulation or blockade of ETB receptors. The autoradiography suggests that in all the three bovine t issues studied there are ETB receptors located on nerves independently of t he type of efferent nerve. The pharmacological experiments do not support t he concept that in the bovine retractor penis muscle neuronal ETB receptors exert important immediate effects on the functioning of the penile erectio n-mediating nitrergic nerves.