The pubertal process with its multifaceted neuroendocrine control provides an excellent model for the study of the regulation of FSH heterogeneity. We tested the hypothesis that during the pubertal transition in the female lamb 1) an increase in both pituitary and circulating bioactive FSH concentrations occur and 2) that the increase in bioactivity is associated with a change in the distribution pattern of both pituitary and circulating FSH isoforms. Pituitary and serum immunoreactive (I), and bioactive (B, Sertoli cell bioassay) FSH concentrations were measured in six prepubertal lambs (18 +/- 1 weeks, 29.9 +/- 2.8 kg body weight; mean +/- SE) and compared to those of six others (24.2 +/- 2.2 weeks of age, 41.4 +/- 2.5 kg body weight) during the pubertal transition period. Puberty was synchronized by pulsatile iv administration of GnRH (2 ng/kg every 2 h for 24 h and then at hourly intervals for the next 12 h) in a manner mimicking the I-LH pulse patterns observed during the natural transition to adulthood. Blood samples were collected at 12-min intervals for 4 h from both groups of lambs; for the pubertal group this included the final 32-36 h of GnRH administration. At the end of the study, a 25 ml volume of peripheral blood was collected from both prepubertal and pubertal females for the determination of serum FSH distribution patterns; the lambs were then euthanized, and pituitaries were removed for determination of pituitary hormone content and FSH isoform distribution patterns. In addition, the distribution pattern of I-FSH isoforms in the pituitary and serum from both groups of lambs were compared. The pubertal stages of all lambs were verified by measuring the size of follicles, the circulating concentrations of estradiol (E2) and inhibin, and the I-LH pulse patterns. Prepubertal lambs had low frequency I-LH pulses, small (2-3 mm) size ovarian follicles and low circulating concentrations of E2 (4.1 +/- 0.4 pg/ml) and inhibin (38.0 +/- 2.9 U/ml WHO). By contrast, all the pubertal lambs had hourly I-LH pulse frequency (induced with exogenous GnRH), a large (5-6 mm) follicle (in one lamb a 4-mm follicle), follicular phase levels of E2 (7.1 +/- 0.8 pg/ml), and higher concentrations of inhibin (53.2 +/- 3.1 U/ml). Mean (+/-SE) pituitary and serum I-LH concentrations were greater (P < 0.05) in the pubertal group (2.1 +/- 0.3 mg/g pituitary and 8.4 +/- 2.4 ng/ml serum) than in the prepubertal group (1.3 +/- 0.2 mg/g pituitary and 3.2 +/- 0.7 ng/ml serum). Pituitary I-FSH and B-FSH concentrations were reduced by 90 and 87% (P < 0.01), respectively, in the pubertal group compared to the prepubertal group. Circulating I-FSH concentrations were not different between the two groups (4.9 +/- 0.8 vs. 6.6 +/- 1.6 ng/ml for the prepubertal and pubertal group, respectively). In contrast, the circulating B-FSH concentration was greater (P < 0.01) in pubertal lambs (5.4 +/- 0.5 ng/ml) than prepubertal lambs (1.9 +/- 0.5 ng/ml). The percentage of pituitary FSH isoforms eluting at pH greater than 5.4, less than 5.4, and in the salt peak averaged 23.6 +/- 2.1, 67.9 +/- 4.3, and 8.6 +/- 3%, respectively, for the prepubertal lambs; these were not different in the GnRH-induced pubertal lambs (28.6 +/- 9.8, 52.9 +/- 11.1, and 18.5 +/- 6.6%, respectively). Serum FSH isoforms eluting at pH greater than 5.4, less than 5.4, and in the salt peak in the prepubertal group averaged 14.9 +/- 3.3, 40.8 +/- 5.9, 44.3 +/- 5.8%. In striking contrast, a greater proportion of serum FSH isoforms eluted in pH above 5.4 in GnRH-induced pubertal lambs (48.8 +/- 5.7, 20.4 +/- 3.4, and 30.8 +/- 5.2%, respectively, for pH greater than 5.4, pH less than 5.4, and in the salt peak). Thus, during the experimental induction of puberty in the female sheep, an increase in serum B-FSH, but not I-FSH concentration occurs, and this is associated with an increase in the proportion of less acidic FSH serum isoforms. These results suggest that changes in FSH heterogeneity may be involved in the pubertal process.
