Influence of thyroglobulin on basal and stimulated human thyroid adenylate cyclase activity

The interaction of thyroglobulin (Tg), thyroid-stimulating 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 X 10(-10) M (0.2 micrograms/ml) to 3 X 10(-8) M (20 micrograms/ml)] exerted a dose- and time-dependent inhibitory influence on basal adenylate cyclase activity. The inhibition was prevented by preincubation with Tg antibody in excess. Tg (3 X 10(-8) 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 X g fraction. Thus, Tg is an efficient inhibitor of basal and TSH- or TSI- stimulated adenylate cyclase activities, and might be involved in a short loop counterregulation of thyroid adenylate cyclase sensitivity in vivo.