Evaluation of activation of G proteins in response to thyroid stimulating hormone in thyroid gland cells from euthyroid and hyperthyroid cats

To evaluate alterations in ligand-stimulated activity of G proteins in thyroid gland cells of hyperthyroid cats.

Membranes of thyroid gland cells isolated from 5 hyperthyroid cats and 3 age-matched euthyroid (control) cats immediately after the cats were euthanatized.

Isolated thyroid cell membranes were treated with thyroid-stimulating hormone (TSH), and activation of G protein was quantified by measurement of the binding of guanosine triphosphate gamma labeled with sulfur 35 (GTPgamma(35)S). The separate effects of G-protein inhibitory (G(i)) and G-protein stimulatory (G(s)) proteins were determined by the use of pertussis toxin and cholera toxin, respectively.

Thyroid cell membranes from hyperthyroid cats had higher basal GTPgamma(35)S binding than did thyroid cell membranes from euthyroid cats. Thyroid cell membranes from hyperthyroid and euthyroid cats had a concentration-dependent increase in TSH-stimulated GTPgamma(35)S binding over the TSH range of 0 to 100 mU/mL, with maximal activity at 1 to 100 mU/mL for both. The percentage increase in GTPgamma(35)S binding stimulated by TSH was similar in magnitude between the membranes from hyperthyroid and euthyroid cats. The TSH-stimulated activation of G(s) and G(i) was not different between euthyroid and hyperthyroid cats.

Ligand-stimulated activation of G proteins was the same in thyroid cell membranes obtained from hyperthyroid and euthyroid cats. Therefore, alterations in inherent G(s) or G(i) activities did not appear to be part of the pathogenesis of hyperthyroidism in cats.