Competition binding experiments performed with 125 I-sarafotoxin (SRTX)-b and SRTX-b, SRTX-c and endothelin (ET-1 and ET-3) using homogenates of rat right and left atria, aorta, uterus, cerebellum and candate putamen indicated heterogeneity of the ET/SRTX receptor. The evidence pointed to the existence of three receptor subtypes: a high-atfmity ET-1/ SRTX-b subtype typical of smooth muscle (E-S~ receptor), a high-affinity SRTX-c subtype typical of the cerebellum (E-S# receptor), and a less selective subtype typical of the caudate putamen that binds all of these peptides with high affinity (E-S r receptor).

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The effects of four peptides of the endothelin/sarafotoxin (ET/SRTX) family on the motility of the rat uterus were examined during the different stages of the estrous cycle. ET-1, ET-3, SRTX-b and SRTX-c showed similar effects on the contraction of the uterus: a slight increase in the maximum tension of the spontaneous rhythmic contractions, a suppression of the relaxation phase of these contractions and an increase in their rate. All three effects were concentration dependent. Of the four peptides, ET-1 and SRTX-b showed the highest potency and efficacy, suggesting that among the various peptides of this family so far studied, ET-1 and SRTX-b are the two full agonists. The rank order of susceptibility of the different stages was, in most cases: proestrus greater than estrus greater than metestrus. Freshly excised diestrus uteri showed no spontaneous contractions and did not respond to any of the peptides. The binding potency of ET-1 and SRTX-b to uterine membranes was similar at the various estrous stages, but their maximal binding decreased gradually from proestrus to diestrus. All four peptides induced phosphoinositide (PI) hydrolysis in uterine slices at all four different stages, with ET-1 and SRTX-b again being more potent than ET-3 or SRTX-c. The maximal PI hydrolysis correlated with the increased rate of the rhythmic contractions. It is suggested that the reaction of the rat uterus to the ET/SRTX peptides depends on its hormonal status and that ET may act in concert with steroid hormones in the modulation of the estrous cycle.

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