Control of plasma sex steroid-binding protein (SBP) in the little brown bat: effects of thyroidectomy and treatment with L- and D-thyroxine on the induction of SBP in adult males.

The male little brown bat is a seasonally reproductive mammal that exhibits dramatic increases in plasma concentrations of sex steroid-binding protein (SBP) in the spring, following arousal from hibernation. Adult male bats, aroused prematurely from hibernation, were found to exhibit increases in plasma SBP titers that were comparable to those observed during normal spring arousal. To evaluate the role of the thyroid gland in the control of SBP in this species, plasma SBP concentrations were determined at weekly intervals in adult male bats that were either thyroparathyroidectomized (TRX) or sham operated (SHAM) after arousal from hibernation. Plasma SBP titers in SHAM males increased markedly within the first week after arousal and by 3 wk had reached levels 20-fold higher than those measured in hibernating controls. In contrast, plasma SBP values in the TRX animals did not increase significantly following arousal but were maintained at low basal levels throughout the experiment. The postarousal rise in SBP, which was blocked by TRX, was completely restored by implantation of either L- or D-thyroxine pellets. In male bats, TRX also hindered the normal postarousal atrophy of the sex accessory glands and resulted in attenuation of the postarousal increases in plasma testosterone concentrations. These effects of TRX were also prevented by treatment with thyroxine. Thus, the thyroid appears to play a significant role in the control of the postarousal rise of SBP in the little brown bat and may be an important factor in the regulation of reproductive function in this species.