High Number of High-Affinity Binding Sites for (-)-[3H]Dihydroalprenolol on Isolated Hamster Brown-Fat Cells. A Study of the beta-Adrenergic Receptors

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Abstract
The beta‐adrenergic receptors of hamster brown adipocytes have been characterised by binding of the radioactive ligand (–)‐[3H]dihydroalprenolol, directly to isolated intact cells in suspension. The brown fat cell contains 57000 specific and saturable binding sites which have a dissociation constant (Kd) for [3H]dihydroalprenolol of 1.4 nM as determined by Scatchard analysis. The kinetically derived Kd, determined from forward and reverse rate constants, is 5 nM. Both of these values are in agreement with the dissociation constant (Kd= 2.2 nM) for alprenolol, determined from competition studies with [3H]dihydroalprenolol in these cells. Beta‐adrenergic agonists competed for the specific binding sites with a typical β1‐adrenergic specificity. The order of potency of agonists agrees well with the ability of these agents to stimulate respiration in isolated brown adipocytes: 50% stimulation of respiration occurs with apparently less than 10% occupancy of binding sites. Both the high affinity and high number of specific binding sites of [3H]dihydroalprenolol in brown fat cells presumably reflect the generally accepted dominating role of catecholamines in the regulation of brown fat metabolism and non‐shivering thermogenesis.