AJP - Regulatory, Integrative and Comparative Physiology, Vol 268, Issue 5 1303-R1309, Copyright © 1995 by American Physiological Society
Seasonal influences on the control of plasma sex hormone-binding globulin by T4 in male little brown bats
D. A. Damassa, A. W. Gustafson, G. G. Kwiecinski and G. A. Gagin
Department of Anatomy and Cellular Biology, Tufts University School of Medicine, Boston, Massachusetts 02111, USA.
Male little brown bats showed a well-defined seasonal cycle of plasma
thyroxine (T4). Total T4 concentrations increased during hibernation,
reached peak levels in May at spring emergence, and subsequently declined
to lowest levels in September. Thyroxine-binding globulin (TBG) activity
increased only in summer, and seasonal changes in non-TBG-bound T4
generally paralleled those of total T4. During the fall and early
hibernation period, bats studied under feral conditions or following
photoperiod manipulation showed coincident changes in plasma T4 and sex
hormone-binding globulin (SHBG) levels. In contrast, males studied during
late hibernation or following spring arousal exhibited declining plasma T4
levels during periods of increased SHBG activity. Additionally, the
prehibernation decline in SHBG did not depend on changing environmental
conditions. Bats maintained under summerlike conditions during the fall
showed normal seasonal reductions in SHBG and T4. These studies indicate
that T4 is the likely physiological regulator of plasma SHBG activity
during the fall and early hibernation, but that other factors modulate the
timing of SHBG induction in the spring.
