The objective of the present study was to determine how rapidly estradiol (E 2 ) was able to suppress the secretion of LH in ovariectomized (OVX) ewes and to evaluate the ability of conjugated forms of E 2 (E 2 conjugated to BSA [1,3,5(10)estratrien-3,17b-diol-6-one-6-carboxymethyloxime:BSA [E 2 -BSA] and a novel conjugate, E 2 conjugated to a small peptide [E 2 -PEP]) to mimic the actions of E 2 on secretion of LH and FSH. Animals (n ¼ 5-6 per group) were given infusions for 4 h of 50 lg of E

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... or equimolar concentrations of E 2 -BSA or E 2 -PEP. Treatments with E 2 , E 2 -BSA, and E 2 -PEP each induced an acute suppression of LH secretion (,20 min, P , 0.01). In contrast, E 2 , but not E 2 -BSA or E 2 -PEP, induced the characteristic preovulatory-like surge of LH (at 10 h after priming treatment) and decreased secretion of FSH (at 4 h after priming treatment). In conclusion, the acute inhibition of LH secretion induced by E 2 in OVX ewes supports the concept of a nongenomic action as the mechanism underlying the sudden suppression in secretion of LH. In addition, the fact that conjugated forms of E 2 mimicked only the acute suppression of secretion of LH, without inducing the putative genomic actions on secretion of LH or FSH (i.e., a preovulatory-like surge), suggests that the acute effect of E 2 may be mediated via the plasma membrane. estradiol, estradiol receptor, follicle-stimulating hormone, luteinizing hormone, pituitary