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Rabbit antiserum to human seminal plasma inhibin (hSPI) was administered subcutaneously to developing male rats of 5, 10, 14, 17 and 24 days of age and the size of the endogenous FSH rise in serum was measured. The FSH levels were threefold higher on day 9 and 1·5-fold higher on days 14 and 18 when compared with levels in control rats treated with normal rabbit serum. Furthermore, the in-vitro binding capacity of pituitary plasma membrane to 125I-labelled hSPI declined with increase in age of the rats. Thus, the results of the present study suggest that the sensitivity of the testicular inhibin–FSH feedback relationship is related to age-dependent changes in pituitary binding of inhibin.
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ABSTRACT
A previously described in-vitro rat granulosa cell plasminogen activator bioassay for FSH has been modified and applied in the assay of human serum. This modified method consists of exposing the diethylstilboestrol-stimulated granulosa cells from 25- to 26-day-old rats to FSH or test substance for 3·5 h in wells coated with 125I-labelled fibrinogen and treated with thrombin. Following stimulation with FSH, the dose-related production of plasminogen activator was measured as the degree of 125I-labelled fibrinolysis in the presence of added plasminogen. Using the urinary FSH/LH bioassay reference preparation as the assay standard, the useful range of the assay was 0·3–15IU/l, with an assay sensitivity of 0·3 IU/l. As determined using purified glycoprotein hormone preparations, the assay was highly specific for FSH. The minor degree of FSH bioactivity measured in some of the hormone preparations was accounted for by the amount of FSH contamination in these preparations. To abolish interference caused by unknown serum factors, we heat-treated the serum samples for 15 min at 56 °C before the assay. The results indicated that neither immunoreactivity nor bioactivity was affected by this treatment. Furthermore, heat-treated human sera gave responses parallel to the standard curve at the three dose levels (2, 4 and 8 μl) studied. We used this bioassay to estimate the FSH-like bioactivity in 15 human serum samples. The estimates of immunoreactive FSH in these samples correlated well with the corresponding FSH bioactivity (r = 0·745, n = 15 and P < 0·05).
The results indicate that with this sensitive and rapid (completed within 24 h) bioassay, it should be possible to measure FSH bioactivity in heat-treated human serum samples.
Journal of Endocrinology (1990) 126, 159–168
