Regulation of the Rat Thyrotropin Receptor in Vitro*

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Abstract
After our recent finding that the TSH receptor of the rat is regulated by endogenous TSH, we studied the system further by investigations in vitro. After exposure of the tissue to TSH, thyroid lobes showed decreased responsiveness to stimulation by the hormone when cAMP accumulation was measured as an end-point. This state of refractoriness was correlated with a decrease in hormone binding, the nature of which was mainly a loss of receptor sites when data were subjected to Scatchard analysis. Dissociation of bound TSH by washing the membrane preparation with NaCl revealed that occupation of the receptor could be responsible for part of the apparent downregulatory effect after 14 h of exposure to TSH, whereas after 24 h of exposure, a true decrease in receptor sites prevailed.