A double–antibody radioimmunoassay utilizing a heterologous system specific for porcine luteinizing hormone (PLH) has been developed with rabbit antibovine LH serum as a first antibody. Porcine LH (LER-786-3) was used for iodination and as a reference standard. Reliable estimates of LH could be obtained in 200 pi of serum from immature pigs at any time. Porcine follicle-stimulating hormone (FSH), porcine thyroid- stimulating hormone, porcine adrenocorticotropic hormone or serum components per se did not affect measurements of LH. Ten prepubertal female pigs (mean wt 12.8 kg) were assigned randomly in equal numbers to a treated or control group. Treatment consisted of 16 im injections of 25 ng of pure synthetic LH and FSH releasing hormone (LH-RH/FSH-RH) in one ml of acidified saline or acidified saline only at 6—hr intervals over a period of 96 hr for treated and control animals respectively. Blood samples were collected via indwelling jugular catheters, at various times following each injection. Serum LH concentrations were observed to peak at IS or 30 min following each LH-RH/FSHRH injection and return to basal levels between 120 and 240 min post-treatment. The mean peak LH level of treated animals was 6.4 ± 0.4 ng/ml compared with the mean basal level of 1.6 ± 0.1 ng/ml for control animals. The LH responses were significantly greater following the first and second injections (16.2 and 9.2 ng/ml) compared with the mean peak LH response of subsequent injections (5.5 ng/ml). No significant differences were observed either in the mean pituitary LH content (90.0 ± 34.4 vs 121.2 ± 36.4 μg/pit.) or concentration (5.5 ± 1.6 vs 7.0 ± 1.8 μg/mg) between the treated and the control animals. Chronic treatment with LH-RH/FSH-RH did not result in any significant difference in the LHreleasing activity of pig stalk median eminence 24 hr post–treatment when tested in estrogenprogesterone pretreated ovariectomized rats. There was no significant difference in the /mean diameter of the largest non–atretic ovarian follicle between treated and control animals (1.9 ± 0.6 vs 1.3 ± 0.6 mm).
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