Abstract
Summary
In rats, blood was withdrawn through one of two indwelling atrial can-nulas while saline or TRH in saline was infused at a constant rate for 5 hr through the other. Plasma TSH and prolactin were measured by radioimmunoassay. Infusion of TRH at 100 ng or 1 μg/hr in ovariecto-mized and male rats as well as 4-day cyclic rats on proestrus and the first day of diestrus caused a rapid rise in plasma TSH which remained at a plateau for 5 and 2 hr, respectively. In the 1 μg/hr group, plasma TSH declined progressively during the last 3 hr of infusion. No increases in plasma TSH were measured during the 5 hr period after infusion of saline or TRH at 1 or 10 ng/hr. Plasma prolactin rose a small but significant amount only in cyclic rats infused at the 1 μg/hr rate but it decreased to basal values after 30 min of infusion even though infusion was continued. The results suggest that TRH does not prime the pituitary to release greater quantities of TSH or prolactin in response to additional stimulation by TRH. They also suggest that a pituitary refractory mechanism for TRH-induced TSH release exists, but it remains to be determined whether any physiological significance can be attributed to this refractory mechanism.
The author thanks the Rat Pituitary Hormone Program, NIAMDD, for the generous gifts of materials used in the radioimmunoassay procedures in this study, and Sue Zurek for the typing of the manuscript. This study was supported by a grant from the University of Nebraska Medical Center Foundation.
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