Pharmacological characterization of endothelin receptor subtypes in the guinea‐pig prostate gland

Experiments have been conducted to investigate the actions of endothelins on the guinea‐pig prostate gland. Saturation experiments with [ 125 I]‐endothelin‐1 (2–800 p M ) in guinea‐pig prostatic homogenates indicated the presence of high affinity binding sites with an equilibrium dissociation constant ( K D ) of 230±50 p M , a maximum number of binding sites (B max ) of 52±16 fmol mg −1 protein or 269±61 fmol g −1 tissue and a Hill coefficient (n H ) of 1.01±0.03 ( n =3). Competition experiments revealed that binding of [ 125 I]‐endothelin‐1 (20 p M ) was inhibited with the following order of potency: endothelin‐1 >>BQ‐788 ( N‐cis ‐2,6‐dimethylpiperidinocarbonyl‐ L ‐γ‐methyl‐Leu‐ D ‐Trp[1‐CO 2 CH 3 ‐ D ‐Nle‐ONa])>BQ‐123 (cyclo‐ D ‐Asp‐ L ‐Pro‐ D ‐Val‐Leu‐ D ‐Trp) sarafotoxin S6c. At concentrations with negligible influence on smooth muscle tone, endothelin‐1, endothelin‐2 and sarafotoxin S6b (1 n M –0.1 μ M ) produced concentration‐dependent potentiation of the contractions evoked by electrical field stimulation with trains of 20 pulses at 10 Hz every 50 s, 0.5 ms pulse width and a dial setting of 60 V. In contrast, the endothelin ET B receptor‐preferring agonist endothelin‐3 (1 n M –1 μ M ) was much less potent, and the endothelin ET B receptor‐selective agonists sarafotoxin S6c and BQ‐3020 (Ac‐[Ala 11,15 ]‐endothelin‐1 (6–21)), up to 1 μ M , were without effect. The endothelin ET A receptor antagonist BQ‐123 (1 μ M ) markedly inhibited the potentiation induced by endothelin‐1, endothelin‐2 and sarafotoxin S6b while the endothelin ET B receptor antagonist BQ‐788 (1 μ M ) was less effective. While our binding data indicates the presence of ET A and ET B binding sites in the guinea‐pig prostate, the endothelin‐induced facilitation of neurotransmission to the prostatic smooth muscle is mediated largely via activation of endothelin receptors of the ET A subtype.British Journal of Pharmacology (1999) 127 , 1091–1098; doi: 10.1038/sj.bjp.0702644