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) F SH measured in peripheral plasma by radioimmunoassay does not always r eflect the level of bioactive (B) hormone capable of eliciting a biolo gical response (e.g. oestradiol synthesis by Sertoli cells in vitro). The aim of this study was to measure both B-FSH and I-FSH concentratio ns in male and female sheep during the first year of life, and to rela te 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 onse t of puberty. Eight ewe lambs and eight ram lambs were blood sampled f ortnightly from 2 to 52 weeks of age. All samples were assayed for B-F SH and I-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 an; from plasm a testosterone concentrations. Mean I-FSH concentrations varied signif icantly with time after birth, in both females and males (P < 0.01). I n contrast, B-FSH was found to vary with time in females only (P < 0.0 1). Around the expected time of puberty in ram lambs (i.e. at 30-40 we eks of age), and thereafter, I-FSH concentrations were undetectable (< 0.2 ng ml(-1)), whereas the B-FSH concentrations were measurable at c oncentrations up to twice the assay detection limit (0.8 ng ml(-1))unt il 38 weeks of age. In ewe lambs, but not ram lambs, there was a signi ficant 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 t o reach puberty. No differences for either B-FSH or I-FSH between pube rtal and non-pubertal ram lambs were noted. In summary, B-FSH was ofte n 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 changes in B-FSH that could be directly related to specific pubertal events. II is conc luded that although FSH may be a prerequisite For prepubertal testicul ar development and/or ovarian follicular growth, it is not a critical factor in determining whether puberty is attained during the first yea r of life in this seasonally breeding species. (C) 1998 Elsevier Scien ce B.V.