Maintenance of FSH biosynthesis requires ongoing exposure to pulsatile GnRH. Recent data demonstrate that activin also stimulates FSH biosynthesis. We used a perifused pituitary system to examine regulation of FSHβ mRNA levels by pulsatile GnRH and activin. Hourly pulses of 10 nM GnRH increased FSHβ mRNA levels by 3-fold. In the same experiment, continuous infusion of 50 ng/ml activin elicited a 50-fold increase in FSHβ mRNA. This magnitude of response to activin in perifusion was unexpected, as only a 2.7-fold increase in FSHβ mRNA was observed when activin was administered to pituitary cells that were cultured in dishes. Since perifusion columns, unlike culture dishes, are exposed to a continuous supply of fresh medium, we examined the possibility that endogenous factors produced by pituitary cells cultured in dishes were stimulating the cells in a paracrine fashion, thereby precluding the full response to exogenously added activin. The kinetics of FSHβ mRNA expression were examined immediately after pituitary dispersion and at different times after culturing the cells in plates. FSHβ mRNA levels fell rapidly after dispersion to &% of initial levels and remained low over 8 h. Thereafter, FSHβ mRNA levels increased slowly and exceeded initial levels by the second day of culture. In a parallel set of experiments, when medium conditioned by exposure to plated cells was applied to the perifusion system, FSHβ mRNA levels were selectively stimulated (6-fold). These data suggest the removal during dispersion and subsequent accumulation in culture of pituitary-derived factors that are important for the maintenance of FSHβ mRNA levels. We conclude that activin plays a greater role in the regulation of FSHβ mRNA levels than was suggested by previous experiments employing static culture systems in which autocrine or paracrine stimulation may have obscured the effects of exogenously added activin.
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