Bioactive Follicle-Stimulating Hormone

Specific and sensitive in vitro bioassays suitable for measurement of follicle-stimulating hormone (FSH) in small quantities of serum and urine have recently been validated. Comparisons of bioassay (B-FSH) potencies with immunoassay (I-FSH) estimates and chromatofocusing patterns have facilitated new discoveries regarding the regulation of FSH biosynthesis, storage, secretion, serum concentrations and target cell responsiveness, plasma clearance, and the significance of excreted forms. A review of recent studies reveals that luteinizing hormone and FSH, as well as B-FSH and I-FSH, appear to be differentially regulated by GnRH, GnRH agonists and antagonists, sex steroids and gonadal peptides in both sexes, with significant differences between the sexes. The results support the overall hypotheses that the quality as well as the quantity of FSH are significant contributors to target cell signal transduction and that distinct FSH isoforms with different functional capabilities influence the expression of acute (ovulation) and chronic reproductive events. (Trends Endocrlnol Metab 1991; 2: 145-15 1) Research during the last two decades has firmly established that follicle-stimulat-ing hormone (FSH), like other glycopro-teins, is not a unique protein, but rather a family of heterogeneous isoforms of varying immunologic and bioactive po-tencies. Such mixes of isoforms are found within the pituitary, resulting from differential promoter action, alter-native splicing, induction of point muta-tions during biosynthesis, or posttransla-tional changes, including carbohydrate incorporation prior to or during granule formation within the gonadotrope. Upon secretion, these molecular variations de-termine tertiary structure and metabolic clearance. Molecules lacking sialic acid residues (relatively less acidic) have shorter half-lives within the circulation