The effects of thyroid status on the modulation of fat cell β-adrenergic receptor agonist affinity by guanine nucleotides

The influence of thyroid status in vivo on the ability of guanine nucleotides to affect the affinity of fat cell β-adrenergic receptors for an (-)agonist is explored in the present study. In euthyroid rat fat cell membranes, 100 μM GTP or Gpp(NH)p induces a reduction in the affinity of specific (-)[³H]dihydroalprenolol binding sites for isoproterenol, but not propranolol. The kinetics of the effect of 100 μM Gpp(NH)p were rapid, achieving near-steady-state levels within 10 min at 22°C. One micromolar Gpp(NH)p or 5 μM GTP (in the presence of a nucleotide regenerating system) induced half-maximal reduction in the affinity of specific (-)[³H]dihydroalprenolol binding sites for isoproterenol. One hundred micromolar concentrations of either guanine nucleotide produced a maximal effect. The ability of Gpp(NH)p to reduce agonist affinity of the binding sites was shown to be readily reversed by simple washing of the membranes. The affinity of specific (-)[³H]dihydroalprenolol binding sites of fat cell membranes for (-)isoproterenol was reduced in the hypothyroid state. Half-maximal inhibition of specific (-)[³H]dihydroalprenolol binding (at 10 nM radioligand) occurred at 7 μM isoproterenol in hypothyroid, as compared to 1 μM isoproterenol in euthyroid, rat fat cell membranes. In hyperthyroid rat fat cell membranes, only 0.4 μM isoproterenol was required to half-maximally inhibit the specific binding of (-)[³H]dihydroalprenolol. In the presence of 50 μM Gpp(NH)p the concentration of isoproterenol required for half-maximal inhibition of specific binding in euthyroid rat fat cell membranes was increased from 1 to 7 μM. Neither 100 μM GTP nor Gpp(NH)p influenced the affinity of specific (-)[³H]dihydroalprenolol binding sites for isoproterenol in fat cell membranes obtained from hypo- or hyperthyroid rats. These data suggest that thyroid hormones can modulate β-adrenergic receptor affinity for agonists (but not antagonists) and the ability of guanine nucleotides to regulate agonist (but not antagonist) affinity in fat cells.