Gonadotrophin secretion revisited. How many ways can FSH leave a gonadotroph?

In summary, gonadotrophs make and secrete many biologically active proteins, some of which participate in autocrine regulation of gonadotrophin, and particularly FSH, secretion. This essay focuses on different vehicle(s) for secretion of protein products as one way that the gonadotroph might control secretion of the two gonadotrophins. Gonadotrophs phasically secrete LH and (proportionately less) FSH via secretory granules in response to an increase in intracellular Ca2+, but also secrete FSH without LH by a distinct pathway. The latter is tonically regulated by not only activin, but also inhibin and follistatin, at least partly at the level of FSH synthesis. Thus GnRH-independent secretion of FSH may masquerade as a second form of 'regulated' secretion through its linkage with FSH synthesis. Three major types of vacuole that possibly mediate gonadotrophin secretion have been identified in gonadotrophs: the small, dense core secretory granule that is rich in LH; a larger, more diffuse granule that is rich in FSH; and the much smaller, synaptic vesicle-like vacuole that contains no identifiable gonadotrophin. Which of these subserves the tonically regulated, GnRH-independent pathway for secretion of FSH without LH has not yet been determined. It is unlikely to be the small granule, because of its preponderance of LH. It may be a form of synaptic vesicle-like vacuole, which is immunocytochemically 'silent' with respect to FSH, or the FSH-rich larger form of secretory granule, for which a specialized, constitutive-like secretory function has not yet been assigned.(ABSTRACT TRUNCATED AT 250 WORDS)