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 S6e 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 mu M, 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-gamma-methyl-leucyl-D-1-methyoxcarbonyltryptophanyl-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 tissue). Preincubation with phosphoramidon (100 mu M) reduced responses to big-endothelin-1, but not endothelin-1. Thus, endotherin-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.