Bioactive follicle-stimulating hormone release in nutritionally growth-retarded ovariectomized lambs: regulation by...

The nutritionally growth-retarded ovariectomized lamb provides a model system to investigate the regulation of immunoreactive and bioactive FSH release in the absence of feedback effects of ovarian products. We examined the acute effects of nutritional repletion on FSH release in this model. Five March-born lambs were weaned at 7 weeks of age and placed on a limited diet. All were ovariectomized at 31 weeks. At 37 weeks, they were fed ad libitum for 14 days. Blood samples were collected at 12 min intervals for 4 h on day -1 (restricted) and days 2, 7, and 14 of ad libitum feeding, and serum FSH concentrations were measured by both RIA and a Sertoli cell aromatase in vitro bioassay. Mean concentrations of serum immunoreactive FSH (I-FSH) and the bioactive FSH (B-FSH) increased 2- and 3-fold, respectively, after ad libitum feeding for 14 days. Pulse analyses by two objective computerized programs, DETECT and CLUSTER, revealed the presence of FSH pulses (predominantly B-FSH) during the restricted phase, as well as ad libitum fed phases, at times where I-LH pulses were not detected. In contrast to the increases in I-LH pulse frequency during ad libitum fed states, FSH pulse frequency remained stable whereas pulse amplitude increased. In addition, the overall B-FSH pulse amplitudes were 57 +/- 7% (mean +/- SE of three different pulse analyses) greater than I-FSH pulse amplitudes, implying preferential enrichment of FSH bioactivity. The results suggest that the mechanisms governing both the quantitative and qualitative changes in circulating FSH concentrations are extremely sensitive to changes in level of nutrition. The asynchronous occurrence of FSH pulses in the absence of I-LH pulses suggests that FSH may be more sensitive to small changes in GnRH secretion than I-LH, or that a hypothalamic releasing factor specific for FSH may be released.