Release of thyrotropin receptor from thyroid plasma membranes: effect of hydrocortisone, propranolol, and adenosine...

Soluble TSH receptors were released into the medium when bovine thyroid plasma membranes were incubated in 0.01 M Tris-HCl, pH 7.5, at 0 or 20 C. This is a conventional hypotonic medium used in binding assays. The characteristics of binding of bovine [125I]TSH to the released TSH receptor were almost the same as those of binding to TSH receptors solubilized by lithium 3,5-diiodosalicylate or to the original plasma membrane. Released TSH receptor had two binding sites with Ka values of 0.7 x 10(10) and 0.1 x 10(8) M-1. T3, T4, KI, methimazole, and propylthiouracil had no effect on spontaneous TSH receptor release or on bovine [125I]TSH binding to solubilized TSH receptor. Hydrocortisone (10(-5)--10(-3) M) and d,l-propranolol (10(-3) M) inhibited receptor release. cAMP increased the release of TSH receptor. Hydrocortisone, d,l-propranolol, and cAMP had no effect on bovine [125I]TSH binding to solubilized or released TSH receptor. d,l-Propranolol and hydrocortisone may act as membrane-stabilizing agents. cAMP stimulation of release suggests that the release mechanism could depend upon a protein kinase-phosphoprotein system. Although these studies were conducted with membranes in an unphysiological medium, receptor release may occur normally and could be a source of circulating antigen related to production of antireceptor antibody in autoimmune thyroid diseases. Release of receptors during incubation in vitro may affect the results of studies of hormone-receptor interaction.