Evidence for atypical endothelin receptors and for presence of endothelin-converting enzyme activity in the mouse isolated vas deferens

The endothelin receptors controlling sympathetic neurotransmission and the presence of endothelin-converting enzyme were investigated in the mouse vas deferens. Endothelin-1 or endothelin-3 (0.01–100 nM) enhanced contractions evoked by field stimulation, yielding EC 50 (geometric mean and 95% confidence limits) of 0.7 nM (0.4–1.6) and 13.7 nM (10.2–14.1) and E max (mean ± S.E.M. increase in twitch tension, in mg/10 mg wet tissue) of 473 ± 35 and 520 ± 51, respectively. The selective endothelin ET B receptor agonists IRL 1620 (Suc-[Glu 9,Ala 11,15]endothelin-1) and sarafotoxin S6c were inactive up to 100 nM. Responses to endothelin-3 were progressively inhibited by the selective endothelin ET A receptor antagonist BQ-123 (cyclo[ d-Trp- d-Asp-Pro- d-Val-Leu]) (10,30 and 100 nM). At 100 nM, BQ-123 almost abolished the response to endothelin-3 (100 nM). In contrast, at 100, 300 nM and 1 μM, BQ-123 shifted the curve to endothelin-1 to the right only 2-, 5- and 6-fold, respectively. The selective endothelin ET B receptor antagonist BQ-788 ( N-cis-2,6-dimethylpiperidinocarbonyl- l-γ-methyl-leucyl- d-1-methoxycar-bonyltryptophanyl- d-norleucine) (100 nM) did not modify responses to endothelin-1 or endothelin-3 (0.01–100 nM). Big-endothelin-1 (0.3–30 nM) was 10-fold less potent than endothelin-1 in increasing neurogenic responses (EC 50 6.8 nM, 4.7–9.6; E max 457 ± 37 mg/10 mg wet tisue). Preincubation with phosphoramidon (100 μM) reduced responses to big-endothelin-1, but not endothelin-1. Thus, endothelin-1 and endothelin-3 potently enhance sympathetic neurotransmission in the mouse vas deferens via stimulation of BQ-788-insensitive endothelin receptors, which are variably sensitive to blockade by BQ-123. It remains to be clarified if these findings are due to the presence of an atypical population of endothelin receptors in this tissue, or reflect the binding of endothelin-1 and endothelin-3 to distinct subdomains of a single sub-type of endothelin ET A receptor. This tissue also displays pronounced phosphoramidon-sensitive endothelin-converting enzyme activity.