Bioactive and immunoreactive FSH concentrations in ewe and ram lambs over the first year of life

Several studies suggest that the concentration of immunoreactive (I) FSH measured in peripheral plasma by radioimmunoassay does not always reflect the level of bioactive (B) hormone capable of eliciting a biological response (e.g. oestradiol synthesis by Sertoli cells in vitro). The aim of this study was to measure both B-FSH and I-FSH concentrations in male and female sheep during the first year of life, and to relate this to pubertal development. The hypothesis being tested was that B-FSH is present in both male and female sheep during the prepubertal period and that discrete changes in B-FSH are associated with the onset of puberty. Eight ewe lambs and eight rams lambs were blood sampled fortnightly form 2 to 52 weeks of age. All samples were assayed for B-FSH content. Pubertal development was monitored in ewe lambs from behavioural oestrus and from plasma progesterone concentrations, and in ram lambs from penile and testicular development and from plasma testosterone concentrations. Mean I-FSH concentrations varied significantly with time after birth, in both females and males (P < 0.01). In contrast, B-FSH was found to vary with time in females only (P < 0.01). Around the expected time of puberty in ram lambs (i.e. at 30-40 weeks of age), and thereafter, I-FSH concentrations were undetectable (< 0.2 ng ml-1), whereas the B-FSH concentrations were measurable at concentrations up to twice the assay detection limit (0.8 ng ml-1) until 38 weeks of age. In ewe lambs, but not ram lambs, there was a significant linear relationship between B-FSH and I-FSH values (R = 0.595; P < 0.005). When standardised about the time of puberty, B-FSH (P < 0.05) but not I-FSH was significantly higher in ewe lambs that failed to reach puberty. No differences for either B-FSH or I-FSH between pubertal and non-pubertal ram lambs were noted. In summary, B-FSH was soften measurable in plasma throughout prepubertal development in sheep and the concentrations often differed from those of I-FSH, especially in ram lambs. However, there appeared to be no discrete change in B-FSH that could be directly related to specific pubertal events. It is concluded that although FSH may be a prerequisite for prepubertal testicular development and/or ovarian follicular growth, it is not a critical factor in determining whether puberty is attained during the first year of life in this seasonally breeding species.