Desensitization of the thyroid cyclic AMP response to thyroid stimulating immunoglobulin: comparison with TSH

Studies were conducted to examine the characteristics of thyroid cell cAMP stimulation by thyroid stimulating immunoglobulins (TSI) and to compare the cAMP response to TSI and TSH in desensitized human thyroid cells. In terms of cAMP production, preexposure (eight hours) of the cells to TSI induced a desensitization very similar to TSH-induced desensitization: both TSH- and TSI-desensitized cells showed a normal response to cholera toxin and forskolin stimulation; TSH and TSI desensitization was interchangeable in that desensitization by either stimulator affected the action of the other; the time of recovery from either TSH and TSH desensitization was identical; the cycloheximide (10(-4) mol/L) prevented both TSI- and TSH-induced desensitization; preexposure of the cells to iodine, which affects mainly the adenylate cyclase catalytic unit, or to epinephrine, which activate the inhibitory regulatory protein Ni by the alpha 2-adrenergic stimulation, induced a similar inhibition of the subsequent stimulation by both TSH or TSI. The remarkable similarities between TSH and TSI in stimulating and desensitizing thyroid cells strongly support the concept that TSI activates thyroid adenylate cyclase by interacting with the TSH receptor and not through an allosteric mechanism.