COMPARISON OF THE CONTRACTILE EFFECTS AND BINDING-KINETICS OF ENDOTHELIN-1 AND SARAFOTOXIN S6B IN RAT ISOLATED RENAL-ARTERY

1 To date, only two mammalian endothelin (ET) receptors, termed ETA an d ETB. have been cloned, sequenced and characterized. However, several functional studies of isolated blood vessels suggest that ET-l-induce d contractions may be mediated by multiple ETA receptors. In this stud y, the ETA receptors in renal arteries isolated from Wistar rats were characterized by isometric tension recording and radioligand binding t echniques. 2 ET-1, sarafotoxin S6b (StxSGb) and ET-3 produced concentr ation-dependent contraction with similar response maxima in endotheliu m-denuded arteries, whereas the ETB receptor-selective agonist StxS6c was inactive. ET-1 and StxSGb were equipotent and 30 times more potent than ET-3. This agonist profile, together with the findings that the ETA receptor-selective antagonists, BQ-123 and FR-139317 caused concen tration-dependent, rightward shifts of the concentration-effect curves to each agonist indicated that ET-l-induced contractions in rat renal artery were mediated via ETA receptors. 3 BQ-123 and FR-139317 were b oth significantly more potent inhibitors of contractions induced by St xS6b or ET-3 than of responses to ET-l, raising the possibility that a component of ET-l-induced contraction was mediated through atypical, BQ-123 (or FR-139317)-insensitive ETA receptors. However, in competiti on binding studies. specific [I-125]-ET-1 and [I-125]-StxS6b binding t o rat renal artery sections was completely abolished by BQ-123 in a ma nner consistent with an action at a single site. Thus, competition bin ding studies did not provide any supportive evidence of the existence of a BQ-123-insensitive ETA receptor. 4 Additional studies revealed ma rked differences in the kinetics of[I-125]-ET-1 and [I-125]-StxS6b bin ding. Following a 3 h period of association of [I-125]-ET-1 with its r eceptors, no significant dissociation of receptor-bound [I-125]-ET-1 w as observed during a 4 h washout period. In stark contrast, dissociati on studies revealed that specific [I-125]-StxS6b binding to ETA recept ors was reversible (t(0.5diss), 100 min). A series of association bind ing studies were also consistent with the specific binding of [I-125]- ET-1 and [I-125]-StxS6b bring irreversible and reversible processes, r espectively. 5 Thus, differences in BQ-123 potency against ET-I and St xS6b-induced contractions in rat renal arteries might be due to differ ences in the kinetics of agonist binding, rather than due to the exist ence of atypical ETA receptors.