Influence of Cholera Toxin on in Vitro Refractoriness to Thyrotropin of Thyroids from Rats Fed Propylthiouracil*

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Abstract
Using an increase in the concentration of cAMP as an index of stimulation, we previously reported that in vitro thyroid tissue of rats fed a goitrogenic diet (0.1% propylthiouracil in Purina) were unresponsive to TSH. We now show that the addition of cholera toxin (0.1-1.0 μM) to fragments of normal thyroid enhanced the concentration of cAMP. Subsequent exposure of the tissue to TSH resulted in a further increase in the concentration of cAMP, and there was statistical evidence of interaction or potentiation between the effects of TSH (20 mU/ ml) and cholera toxin (1.0 μM). With fragments of thyroid from animals fed propylthiouracil (i.e. unresponsive in vitro to TSH), an increase in the concentration of cAMP was effected by 1 or 9 μM cholera toxin, and prior exposure to 9μM toxin caused the tissue to be responsive to TSH. Unresponsiveness of the goitrous tissue was associated with a reduced capacity of the membrane to bind [125I]iodo-TSH although the affinity of the binding sites was unaffected. Cholera toxin at 0.1 μM enhanced and at 1 μM diminished the binding of [125I]iodo-TSH to membranes from either normal or goitrous glands, and these effects also reflected influences on the capacity rather than on the affinity of the binding sites. It is postulated that the unresponsiveness to TSH of thyroid tissue from rats fed propylthiouracil represents a down-regulation of receptor capacity, and this is effected by the binding sites becoming relatively inaccessible, rather than nonexistent.

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