Control of sex steroid-binding protein (SBP) in the male little brown bat: relationship of plasma thyroxine levels to the induction of plasma SBP in immature males.

The seasonally reproductive male little brown bat (Myotis lucifugus lucifugus) exhibits marked increases in plasma concentrations of sex steroid-binding protein (SBP) in the spring following arousal from hibernation. In this species an increase in SBP levels is induced prematurely in male bats aroused during the first half of hibernation and housed under long photoperiods; however, this rise is inhibited in bats housed under short photoperiods. In order to investigate the physiological role of the thyroid gland in the regulation of plasma SBP activity, plasma total thyroxine (T4) and SBP concentrations were determined in immature male little brown bats prematurely aroused from the first half of hibernation and maintained on either a short or long photoperiod. For this purpose a radioimmunoassay for the measurement of total T4 in bat plasma was established and validated. The results showed that immature male little brown bats aroused prematurely from hibernation and housed under a long spring-like photoperiod exhibited marked increases in plasma T4 and SBP concentrations, while animals housed under a short photoperiod showed only marginal increases in SBP, and plasma T4 remained undetectable. These results suggest that the thyroid gland, through the action of T4, may normally play an important role in the control of the post-arousal rise in plasma SBP concentrations in the little brown bat.