In vitro autoradiographic endothelin-1 binding sites and sarafotoxin S6B binding sites in rat tissues.

1. The distribution of binding sites for [125I]-labelled endothelin-1 ([125I]-ET-1) and [125I]-labelled sarafotoxin S6B ([125I]-SRT) was visualized in rat tissues using in vitro autoradiography. 2. A high density of endothelin-1 (ET-1) binding was found in the heart. In the kidney, ET-1 binding occurred in association with glomeruli, proximal tubules, the inner stripe and inner medulla. In the adrenal, a high density of ET-1 binding occurred in the medulla as well as the zona glomerulosa. 3. The binding affinity constant (KA) for ET-1 binding in these sites ranged from 1 to 10 x 10(9)/mol per litre. 4. Although sarafotoxin S6B (SRT) was 10-100-fold weaker than ET-1 in displacing [125I]-ET-1 from these sites, 1 mumol/L unlabelled SRT completely abolished [125I]-ET-1 binding in all sites. Other venom peptides did not affect [125I]-ET-1 binding. 5. The pattern of [125I]-SRT receptor binding in rat tissues by in vitro autoradiography was identical to that for ET-1 receptor binding, and both unlabelled SRT and unlabelled ET-1 fully competed with [125I]-SRT for binding. 6. These results provide evidence that SRT binds to the ET receptor in a range of rat tissues. The results suggest that there may be subclasses of ET receptors which can be distinguished by the relative potencies of ET-1 and SRT at various tissues.