The corticotropin-suppressing effects of dexamethasone-21-phosphate given by 2 routes of administration, systemic and intrapituitary, were compared in rats. Direct pituitary injection of dexamethasone in amounts ranging from 10 to 400 ¿ug /kg was no more effective in suppressing adrenal venous corticosterone secretion than an equal amount administered subcutaneously. The findings were extended to include a group of animals pre-tested with a systemi- cally ineffective amount of thyroxine injected into The pituitary. Animals showing suppression of The thyroidal I131release rate in response to this treatment were also studied with respect to The corticotropin-suppressing effects of dexamethasone injected into The same site. Again, The intrapituitary dexamethasone injections proved to be no more effective than systemic dexamethasone injections. The observations suggest that thyroxine and adrenal steroids may act in different sites to produce suppression of Their respective tropic hormones.
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