Influence of Thyroglobulin on Basal and Stimulated Human Thyroid Adenylate Cyclase Activity*

Abstract
The interaction of thyroglobulin (Tg), thyroidstimulating immunoglobulins (TSI), and TSH on human thyroid plasma membranes from nontoxic goiter was studied in vitro by an adenylate cyclase assay system using human thyroid homogenate. Purified.Tg [3 × 1010 M (0.2 µg⁄ml) to 3 × 1(T8 M (20 µg⁄ ml)] exerted a dose- and time-dependent inhibitory influence on basal adenylate cyclase activity. The inhibition was prevented by preincubatidri with Tg antibody in excess. Tg (3 × 108 M) caused a significant reduction in the TSH- and TSI-stimulated adenylate cyclase activities, but did not influence stimulation with NaF (8 mM) Fractions of thyroid homogenates were obtained by differential centrifugation, and the maximal inhibitory influence of Tg was located in the 5000 × g fraction. Thus, Tg is an efficient inhibitor of basal and TSH- or TSIstimulated adenylate cyclase activities, and might be involved in a short loop counterregulation of thyroid adenylate cyclase sensitivity in vivo.

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