Measurement of Rat Serum FSH by Radioreceptor Assay and Comparison with Radioimmunoassay

Abstract

Follicle stimulating hormone (FSH) in rat serum is successfully measured by a radioreceptor assay system employing PMS-treated immature rat ovary. The nonspecific inhibitory effect of serum was partially overcome by the addition of merthiolate to every component, while the residual effect was compensated for by using FSH-free serum which was prepared by passing the pooled female diestrous rat sera through an immunoadsorbent column packed with anti-ovine FSH-coupled Sepharose 4B. The assay system consisted of 100 μl of Tris-MgCl₂-BSA or standard, 100 μl of FSH-free serum or sample, 100 μl of the receptor preparation and 100 μl of ¹²⁵I-FSH. The incubation was carried out for 4 hr at 37°C and 500μl of cold Tris-MgCl₂-BSA was used for the termination. Serum FSH could be measured within a range of 0.125-16 ng NIAMDD rat FSH I-3/tube. The mean within-assay coefficient of variation was 10.5%. The mean between-assay coefficient of variation was 11.0%. The assay values obtained by RRA showed a good correlation to those by RIA under the same physiological states of the animals. The ratio of the assay values, RRA/RIA, was found to change according to the sex and the physiological states, e.g. around 1.3 in normal males and 1.7 in orchiectomized animals and 2.21 in female rats. Serum FSH levels in female rats obtained by RRA and RIA changed almost in parallel until 20:00 (hr) of proestrous day, but after the first surge of serum FSH they were not parallel. These facts seem to indicate possible changes in the affinity of FSH with its receptor according to the state of animals and lead to the problem of the heterogeneity of FSH.