Effect of Repeated Stimulation by Thyrotropin-Releasing Hormone (TRH) on Thyrotropin and Prolactin Secretion in Perfused Euthyroid and Hypothyroid Rat Pituitary Fragments

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Abstract
The previously reported refractoriness of pituitary response to TRH stimuli was investigated here in an in vitro perfusion system using pituitary tissue from euthyroid and hypothyroid rats. TSH and prolactin (PRL) responses to TRH (28 pmol) were significantly greater in hypothyroid tissue compared with euthyroid. Hypothyroid tissue showed a reduction in response to 2 consecutive stimuli in both TSH and PRL, however, the TSH decline in response was more marked that PRL. Euthyroid tissue showed no significant decline in response to TRH. An increase in the dose of TRH (112 pmol), administered to euthyroid tissue, resulted in increased TSH and PRL response, but no decline in response to sequential stimuli was observed. Three consecutive stimuli by TRH (28 pmol) of hypothyroid tissue resulted in a consistent decline in TSH response. The decline in PRL response only reached statistical significance by the 3rd stimulation. Euthyroid and hypothyroid pituitary tissue was subjected to sequential depolarizing stimulation with KCl (50 .mu.mol). Euthyroid tissue showed no decline in response in either TSH or PRL. In hypothyroid tissue only, the decline in TSH resposne reached statistical significance. This decline in TSH response was significantly smaller than the decline in response observed in hypothyroid tissue stimulated wiht TRH. Refractoriness of hypothyroid pituitary tissue to repeated TRH stimuli is reported here. The decline in hormonal response apparently cannot be explained solely on the basis of tissue depletion.

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