Salt-induced exposure of high affinity thyrotropin receptors on human and porcine thyroid membranes.

Thyropin binding to high affinity receptors on human and porcine membranes was studied at pH 7.4, 37 degrees C, in 10 mM Tris-HCl, 150 mM NaCl, 0.1% albumin. By preincubating the membranes in high salt concentration before binding studies, the number of high affinity receptors could be increased 4- to 8-fold. The salt-induced exposure of high affinity TSH receptors was pH- and temperature-dependent and was maximal at pH 5.0, 37 degrees C in the presence of 1 M (NH4)2SO4. Other salts tested, including NaCl, HN4Cl, and Na2SO4, were also able to increase high affinity THS binding. The receptors exposed by salt were indistinguishable from those present on the membranes before such treatment. They had an affinity constant of 0.5 to 1 X 10(10 M-1, and a high TSH specificity with no inhibition of 125I-TSH binding in the presence of a thousandfold excess of gamma-globulin, thyroglobulin, corticotropin, cholera toxin, and gangliosides. Thyrotropin binding to low affinity TSH binding sites (affinity constant 1 to 3 X 10(7) M-1) measured at pH 6.0, 4 degrees C in 10 mM Tris/acetate, 0.1% albumin was unaltered by pre-exposure of membranes to high salt concentrations. These receptors had low TSH specificity and binding was inhibited by gamma-globulin, thyroglobulin, cholera toxin, and gangliosides. The salt-induced selective exposure of high affinity receptors with unaltered number of low affinity sites is further support for the existence of two separate TSH binding sites on thyroid membranes.