Bioactivity of Gonadotropins

Abstract

It is now certain that both gonadotropins, LH and FSH, are synthesized, stored, and released within the circulation, and excreted as heterologous isoforms that can be distinguished by differences in their bioactive to immunoreactive potential and isoform distribution patterns. The bioactivity (which in this article has been defined as the ability of LH to induce T production in rat interstitial cells and FSH to induce E2 production (via aromatase) in rat Sertoli cells in vitro) results from the ability of gonadotropin isoforms to stimulate postreceptor binding functions upstream from G-protein activation and second messenger stimulation. Within the mix of the heterogeneous isoforms, there could be some that alternatively stimulate Gi protein and inhibit function or some that could stimulate G-protein activation for prolonged periods that extend beyond signal-receptor binding. Methods of separation of isoforms are not yet precise or sophisticated enough to distinguish these isoforms. Therefore, the measurement of in vitro bioactivity measures the sum of stimulatory and inhibitory influences on one defined end result. The immunologic potencies also measure the ability of certain selected antibodies to recognize epitopes on gonadotropin molecules, whether they are biologically active or, by virtue of CHO differences or changes in tertiary structure, biologically inactive. Nonetheless, in many instances, the results have been significantly different, with the bioactivity measurements showing greater excursions in stimulatory or inhibitory paradigms than the immunologic potencies. This has been especially true for FSH. Capitalizing on the usefulness of these methodologic advances, we have reviewed the contribution that measurements of gonadotropin bioactivity have made to our understanding of human puberty, which is a continuum in development from conception to adulthood.(ABSTRACT TRUNCATED AT 400 WORDS)