Thyroid status and adrenergic receptor subtypes in the rat: Comparison of receptor density and responsiveness

The density and functional responsiveness of adrenergic receptor subtypes were determined in tissues from control, hyperthyroid and hypothyroid rats. There was a decrease in sensivity to isoproterenol in spontaneously beating right atria, electrically driven left atria and field-stimulated vas deferens associated with hypothyroisdims, with no change in maximum response. Hyperthyroidism increased the potency of isoproterenol in right atria, but not in left atria or vas deferens. The maximal response to isoproterenol was greatly reduced in hyperthyroid left atria. The potency of procaterol, a partial agonist at beta adrenergic receptors in right atria, was unaltered in hyper- or hypothyroidism, although the maximum stimulation by procaterol was increased in hyperthyroidism. Scatchard analysis of specific [¹²⁵I]pindodol binding showed that beta adrenergic receptor density was greater than in hyperthyroidism in left atria, right atria, ventricles, vas deferens and cerebral cortex, although the proportions of beta-1 and beta-2 adrenergic receptor subtypes did not change. There was no change in the responsiveness of alpha-1 adrenergic receptors mediating contraction of caudal artery and vas deferens or mediating [³H]inositol phosphate accumulation in cerebral cortex in hyperthyroid or hypothyroid rats, although the maximal contraction of caudal artery was significantly reduced in hyperthyroidism. Scatchard analysis of specific [¹²⁵I]BE 2254 binding showed that alpha-1 adrenergic receptor density was significantly decreased in the ventricles from hyperthyroid rats and increased in the ventricles of hypothyroid rats, but was unchanged in vas deferens, caudal aretry and cerebral cortex. Alpha-2 adrenergic receptor density in cerebral cortex, determined by Scatchard analysis of specific [³H]rauwolscine binding, was not altered in hyperthyroid of hypothyroid rats. The results suggest that thyroid status controls both beta-1 and beta-2 adrenergic receptor density and responsiveness in various tissues. Alpha-1 adrenergic receptors in the heart are also regulated by thyroid status, although alpha-1 adrenergic receptors in brain and smooth muscles are not. Non-receptor-mediated changes in responsiveness of some tissues also occur. Overall, changes in receptor density correlate well with changes in functional responsiveness.