Radioimmunoassay for Ovine Luteinizing Hormone. Secretion of Luteinizing Hormone During Estrus and Following Estrogen Administration in the Sheep

Abstract

A solid-phase radioimmunoassay for ovine luteinizing hormone (LH) has been developed, utilizing antibody-coated polystyrene tubes for incubation of the assay and counting of the bound tracer. Tubes were coated with equine antiserum to bovine LH (Snook), and purified ovine LH (Papkoff) was used for iodination with ¹²⁵I and assay standards. The procedure is simple and rapid, being completely performed in the assay tubes and giving results after a period of 48 hr. Basal levels of plasma LH were 2.9 ± 0.9 ng/ml in the cycling ewe, 1.2 ± 0.9 ng/ml in the ram, 12.6 ± 5.5 ng/ml in the oophorectomized ewe, and 1.2 ± 0.9 ng/ml in the pregnant ewe. These values obtained from jugular venous plasma are 25% higher than those measured in peripheral venous plasma. The sheep does not appear to produce a placental gonadotrophin comparable to human chorionic gonadotrophin in regard to immunological crossreaction with pituitary LH. Plasma levels of LH rose sharply 4–16 hr after the onset of estrus to levels of 80-200 ng/ml, over a total period of only 10 hr. Frequent blood sampling is necessary to delineate the estrous LH release in the sheep, and daily estimations carry a greater than 50% chance of completely missing the LH peak. Administration of estradiol-17β to anestrous sheep by intramuscular injection and intravenous infusion was regularly followed by a typical estrous peak of LH secretion. The latent period of the estrogen stimulus was approximately 9 hr, and the dose required to produce an ovulatory LH peak was 6–10 μg, an amount similar to that secreted by the ovary at estrus. It is likely that estrogen secretion by the ovary is a major factor in stimulating the LH release accompanying estrus in the sheep (Endocrinology85: 133, 1969)