Muscarinic cholinergic receptors in rat heart. Effects of thyroidectomy

The effects of thyroid status on muscarinic cholinergic receptors in the rat myocardium were investigated. The potent muscarinic antagonist [3H]quinuclidinyl benzilate was used to identify muscarinic cholinergic receptors in rat heart particulate fractions from control, hypothyroid, and hyperthyroid rats. Thyroidectomy increased specific [3H]quinuclidinyl benzilate binding to heart particulate fractions by about 60% as compared to euthyroid rat cardiac preparations. Administration of triiodothyronine to euthyroid rats decreased specific binding to muscarinic cholinergic receptors by about 20%. Scatchard analysis revealed that the cardiac particulate fraction from thyroidectomized rats contained 134 fmol of [3H]quinuclidinyl benzilate binding sites per mg of protein, as compared with 85 fmol/mg of protein found in the heart preparation of thyroidectomized rats chrinically treated with triiodothyronine. The equilibrium dissociation constant for the interaction of receptors with quinuclidinyl benzilate was the same (1 nM) in the heart particulate fractions derived from these two groups of rats. The results of this study demonstrate that thyroid hormone can regulate the number of cardiac muscarinic cholinergic receptors. Thus, the parasympathetic nervous system may participate in the cardiovascular abnormalities in different thyroid states.