Functional Characterization of Endothelin Receptors in the Bovine Retractor Penis Muscle and Penile Artery

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Abstract
The effects of endothelin‐1 and sarafotoxin 6c on the bovine retractor penis muscle and the bovine penile artery were studied, and a functional characterization of endothelin receptors in these tissues was performed by using the ETA‐receptor antagonist BQ‐123 and the ETB‐receptor antagonist IRL 1038. The retractor penis muscle and the penile artery were about equipotently contracted by endothelin‐1 in a concentration‐dependent manner the EC50values being 3.5 X 10‐9M and 1.3 X 10‐9M, respectively. In both tissues BQ‐123 (10‐6M) inhibited maximal contraction induced by endothelin‐1 by about 50%. Sarafotoxin 6c substantially relaxed the retractor penis muscle, and to a lesser extent also the penile artery, whereas endothelin‐1 did not relax either tissue. The sarafotoxin 6c‐induced relaxation of the retractor penis muscle was totally inhibited by IRL 1038 (3 X 10‐6M) and the nitric oxide synthase inhibitor L‐NNA (10‐4M). In both tissues L‐NNA enhanced the contraction induced by endothelin‐1 and lowered the threshold concentration for it. The results show that in both tissues the contraction induced by endothelin‐1 was mediated primarily by ETA‐receptors. The retractor penis muscle is also equipped with ETB‐receptors, probably at least in part located on the inhibitory nerves, which mediate relaxation via activation of the L‐arginine nitric oxide synthase pathway.