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


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



Molecular Pharmacology 18(2): 193-198



ISSN/ISBN: 0026-895X

PMID: 6106887

The influence of thyroid status in vivo on the ability of guanine nucleotides to affect the affinity of fat cell .beta.-adrenergic receptors for an (-)agonist is explored. In euthyroid rat fat cell membranes, 100 .mu.M GTP or Gpp(NH)p [5'-guanylylimidodiphosphate] induces a reduction in the affinity of specific (-)[3H]dihydroalprenolol binding sites for isoproterenol, but not propranolol. The kinetics of the effect of 100 .mu.M Gpp(NH)p were rapid, achieving near-steady-state levels within 10 min at 22.degree. C. One micromolar Gpp(NH)p or 5 .mu.M GTP (in the presence of a nucleotide regenerating system) induced half-maximal reduction in the affinity of specific (-)[3H]dihydroalprenolol binding sites for isoproterenol. Concentrations of 100 .mu.M of either guanine nucleotide produced a maximal effect. The ability of Gpp(NH)p to reduce agonist affinity of the binding sites was 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 .mu.M isoproterenol in hypothyroid, as compared to 1 .mu.M isoproterenol in euthyroid, rat fat cell membranes. In hyperthyroid rat fat cell membranes, only 0.4 .mu.M isoproterenol was required to half-maximally inhibit the specific binding of (-)[3H]dihydroalprenolol. In the presence of 50 .mu.M Gpp(NH)p the concentration of isoproterenol required for half-maximal inhibition of specific binding in euthyroid rat fat cell membranes was increased from 1-7 .mu.M. GTP or Gpp(NH)p (100 .mu.M) did not influence the affinity of specific (-)[3H]dihydroalprenolol binding sites for isoproterenol in fat cell membranes obtained from hypo- or hyperthyroid rats. Thyroid hormones apparently can modulate .beta.-adrenergic receptor affinity for agonists (but not antagonists) and the ability of guanine nucleotides to regulate agonist (but not antagonist) affinity in fat cells.

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Accession: 068626570

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