In the present study, rabbit antibodies that possess thyroid stimulation-blocking activity were used to investigate potential mechanisms by which TSH receptor antibodies can inhibit thyroid cell function. The antibodies were produced against two synthetic peptides corresponding to amino acids 357-372 (p357) and 367-386 (p367) of the human TSHr (hTSHr). By enzyme-linked immunosorbent assay, both antisera (alpha 357 and alpha 367) had high titers ( > 1:100,000) of IgG against their respective peptides and recombinant extracellular TSHr protein (ETSHr); alpha 357 had a low IgG titer to p367 (1:800), and alpha 367 had a low IgG titer to p357 ( < 1:200). Based on competitive inhibition studies, alpha 357 and alpha 367 displayed similar relative binding affinities for their respective peptides and for recombinant ETSHr. When tested by commercial RRA, alpha 357 did not block (TSH binding inhibition index, -3.7%), whereas alpha 367 blocked TSH binding to TSHr (TSH binding inhibition index, 53.9%). The blocking effect of alpha 367 could be reversed by incubating the antiserum with p367 before assay. When applied alone to FRTL-5 cells, IgG from alpha 357 inhibited [compared to normal rabbit IgG (NRI); P < 0.01] based cAMP production by the cells, whereas IgG from alpha 367 did not. IgG from both alpha 357 and alpha 367, however, were able to inhibit (P < 0.001) TSH-mediated cAMP production by FRTL-5 cells [bovine (b) TSH, 2.5 x 10(-10) M; cAMP (mean +/- SD; picomoles per ml): NRI, 62.5 +/- 6.1; alpha 357, 12.2 +/- 2.4; alpha 367, 36.2 +/- 3.5]. Alpha 357 continued to inhibit (P < 0.05) cAMP production by FRTL-5 cells in 10(-8) M bTSH, whereas alpha 367 no longer inhibited cAMP production at bTSH concentrations above 5 x 10(-10) M. Compared to NRI, both alpha 357 and alpha 367 were also able to inhibit (P < 0.001) Graves' IgG-mediated cAMP production by FRTL-5 cells. When IgG were tested on FRTL-5 cells in the presence of 10(-7) M forskolin, only alpha 357 inhibited (P < 0.001) cAMP production (NRI, 75.1 +/- 4.8; alpha 357, 52.3 +/- 4.5; alpha 367, 77.2 +/- 1.4). To determine whether the inhibitory effect of alpha 357 on forskolin-mediated stimulation was thyroid cell dependent, IgG were tested on Chinese hamster ovary (CHO) cells transfected with the complementary DNA of the hTSHr (CHO-R). Again, alpha 357 inhibited (P < 0.005) cAMP production mediated by forskolin (at 10(-7) M; NRI, 68.7 +/- 4.4; alpha 357, 36.8 +/- 5.7; alpha 367, 64.6 +/- 8.5). alpha 357 did not inhibit forskolin-mediated cAMP production by untransfected CHO cells (CHO-N), indicating that the inhibitory effect of alpha 357 on forskolin stimulation was TSHr dependent. In addition, alpha 357 inhibited (P < 0.01) basal cAMP production by CHO-R cells, but not by CHO-N cells. alpha 367 had no effect on the basal cAMP production in either CHO-R or CHO-N cells. Neither alpha 357 nor alpha 367 inhibited cholera toxin-mediated cAMP production in FRTL-5 cells. In all relevant bioassays, the inhibitory effects of alpha 357 and alpha 367 could be reversed by preincubating the IgG with the respective peptides. From these data, we conclude that 1) alpha 367 binds to the ETSHr and blocks TSH-mediated cAMP production by inhibiting TSH from binding to its receptor; 2) alpha 357 binds to the TSHr and, without blocking TSH binding, inhibits TSH-mediated cAMP production at a step(s) subsequent to ligand binding that affects adenylate cyclase activity; and 3) forskolin-mediated cAMP production by thyroid cells can be inhibited by IgG that bind directly to the TSHr.