Studies on the relative in-vitro biological potency of the naturally-occurring isoforms of intrapituitary follicle stimulating hormone.

Abstract

In the present study, we analysed and compared the relative in-vitro biological activity of the various intrapituitary human follicle stimulating hormone (FSH) isoforms employing two different bioassay systems. FSH was fractionated by chromatofocusing (pH range 7.10 to < 3.80) and the several isoforms isolated were quantified at multiple dose levels by three highly specific immunoassay systems: radioimmunoassay (RIA), enzyme-immunoassay (EIA) and immunoradiometric assay (IRMA), as well as by two in-vitro bioassays, one that measures the amount of oestrogen produced by rat granulosa cells in culture and the other that determines the amount of cAMP produced by a human fetal cell line (293) expressing the recombinant human FSH receptor. The relative in-vitro biological activity of each FSH isoform, expressed as the bioassay/ immunoassay (B/I) activity ratio (B/RIA, B/EIA and B/IRMA ratios) varied with its elution pH value. Regardless of the immunoassay or bioassay method employed, less acidic FSH isoforms exhibited higher B/I ratios than their more acidic counterparts [B/RIA, B/EIA and B/IRMA ratios for isoforms with elution pH values > 4.5 = 1.05 +/- 0.13, 0.99 +/- 0.10 and 1.15 +/- 0.08 (rat oestrogen bioassay), and 2.75 +/- 0.34, 2.20 +/- 0.25 and 2.96 +/- 0.35 (human cAMP production bioassay) respectively. Ratios for isoforms with pH values < 4.5 = 0.71 +/- 0.06, 0.47 +/- 0.05 and 0.63 +/- 0.06 (rat oestrogen assay), and 1.80 +/- 0.26, 1.10 +/- 0.09 and 1.44 +/- 0.13 (cAMP assay) respectively (P < 0.05 for isoforms with pH < 4.5 compared with those isoforms with pH > 4.5)]. Furthermore, statistically significant direct relationships between the B/RIA, B/EIA and B/IRMA ratios and elution pH value of each isoform was identified by regression analysis [rat assay: r = 0.844, 0.800 and 0.780 (P < 0.01); human assay: r = 0.730, 0.845 and 0.821 (P < 0.01), for their corresponding B/RIA, B/EIA and B/IRMA ratios respectively]. The finding of significant differences in relative in-vitro biological potency among the various intrapituitary FSH isoforms strongly suggests that the shifts towards the production and secretion of more basic or acidic FSH molecules occurring in certain specific physiological conditions (e.g. puberty and menstrual cycle), may represent an important mechanism through which the anterior pituitary regulates gonadal function.