AJP - Endocrinology and Metabolism, Vol 250, Issue 2 121-E124, Copyright © 1986 by American Physiological Society
Pituitary gland: one site of ultrashort-feedback regulation for control of thyrotropin
T. Kakita and W. D. Odell
Studies from our laboratory have previously demonstrated sensitive and
specific autoregulatory control systems for thyrotropin (TSH), luteinizing
hormone (LH), and follicle-stimulating hormone (FSH) in the rabbit. Because
our studies of LH autoregulation showed the feedback regulation acted
directly at a pituitary level, the current studies were designed to
investigate whether the TSH control system also acted at the pituitary
level. Two species-specific TSH assays were employed; a rabbit TSH
radioimmunoassay which showed little or no reaction to human TSH, and a
human TSH radioimmunoassay which showed little or no reaction to rabbit
TSH. Both in vivo and in vitro studies were performed. TRH
(thyrotropin-releasing hormone) in doses of 2, 10, and 50 micrograms was
injected as an intravenous bolus into thyroidectomized hypothyroid rabbits
during continuous perfusion with highly purified human TSH (hTSH) or with
saline. In these in vivo studies, TRH-stimulated rabbit TSH (rTSH)
secretion was suppressed by hTSH perfusion compared with control saline
perfusion. The effect of hTSH was studied in vitro by employing short-term
cultured rabbit pituitary cells. When hTSH was added to the incubation
medium, TRH-stimulated rTSH secretion was inhibited. From these studies, we
conclude that one site of the autoregulatory control for TSH in the rabbit
is at the pituitary level. These studies do not exclude a possible
additional short-loop feedback control at an hypothalamic level, but such a
site of action is not required to explain the autoregulatory phenomenon.
