Regulation of alpha and beta adrenergic receptors by triiodothyronine in cultured rat myocardial cells.

Previously, in this laboratory, we established the presence of alpha and beta adrenergic receptors in primary cultures of neonatal rat heart cells. We now report that exposure to 1-triiodothyronine (10 nM) regulates the binding characteristics of these receptors. As measured by [125I]-I-2-[beta-(4-hydroxyphenyl)ethylaminomethyl]tetralone ([125I]IBE 2254), alpha receptor number (Bmax) decreased by 50% from 37,000 +/- 4,500 to 18,000 +/- 4,300 sites per cell and the equilibrium dissociation constant (KD) decreased from 420 +/- 24 to 150 +/- 37 pM. As measured by [125I]-iodocyanopindolol ([125I]ICYP), beta receptor number increased by 42% from 12,000 +/- 2,600 to 17,000 +/- 4,000 sites per cell with no accompanying change in affinity. An increase in maximal stimulation of adenylate cyclase activity by isoproterenol was also detected under conditions of excess triiodothyronine. No significant changes in agonist or antagonist affinities for either the alpha or the beta adrenergic receptor were detected in thyroid hormone treated cultures. It can be concluded that in cultured myocardial cells thyroid hormone regulates the characteristics of both alpha and beta adrenergic receptors, but in a strikingly different manner.