Binding of agonists and antagonists to the porcine adipose tissue beta-adrenergic receptor(s).

1. Affinities of agonists for porcine adipose tissue beta-adrenergic receptors, determined by competitive ligand binding with 3H-dihydroalprenolol to crude adipose tissue membranes in vitro, varied from 50 times > to 25 times < than isoproterenol. Affinities for antagonists varied from 8 times > to 1000 times < propranolol. 2. Receptor affinity was not related to the ability to stimulate or inhibit lipolysis, or to the agonist or antagonist purported receptor subtype specificity. 3. Modeling of ligand-binding data indicated more than one binding site for several ligands. The assignment of beta-adrenergic subtypes to the individual binding sites was unclear because this would depend on the individual ligands used to establish binding sites.

Is the lipolytic response in porcine adipose tissue slices equivalent to the lipolytic response in isolated adipocytes?

1. In most cases, when isolated adipocytes and adipose tissue slices from the same animal were stimulated with various lipolytic agents (adrenergic agonists, theophylline, adenosine deaminase), the qualitative response was similar. 2. There were, however, numerous exceptions; e.g. quinterenol did not affect isolated adipocytes whereas it was a partial agonist for adipose slices from the same animal. 3. The adipocytes present in slices were larger than those isolated from slices by collagenase digestion. 4. Isolated adipocytes were not more sensitive than tissue slices to stimulation by lipolytic agents.

Beta-adrenergic receptor-mediated functions in porcine adipose tissue are not affected differently by saturated vs. unsaturated dietary fats.

Young postweaning pigs were fed a high fat diet containing beef tallow (saturated fat) or corn oil (unsaturated fat). Adipose tissue was used to measure adipocyte size and number of cells per gram of tissue, ligand binding by beta-adrenergic receptors and lipolytic and palmitate esterification rates. Pigs fed the saturated fat diet had more saturated and monounsaturated fatty acids and less polyunsaturated fatty acid in the crude membrane fraction. Adipocytes were larger in pigs fed the saturated fat diet. There was no difference in the binding affinities of the receptors; more binding sites were expressed on a protein or cell basis and fewer sites were expressed per unit surface area in adipocyte ghosts isolated from pigs fed the saturated fat diet. Fatty acid esterification was greater in pigs fed saturated fat diets. Isoproterenol inhibition was marginal in both groups but tended to be greater in pigs fed saturated fat diets. The beta-adrenergic receptor-mediated lipolytic rates were not different; only the theophylline-stimulated rates tended to be greater in the saturated fat-fed group. Thus, a large increase in saturated fatty acid concentration of porcine adipose tissue membranes caused an increase in beta-adrenergic receptor number without any change in receptor affinity. These receptor changes were at best only marginally reflected in beta-adrenergic agonist-mediated functions.