Agonist interactions at hepatic alpha 1- and beta-adrenergic receptors: affinity-state regulation by guanine nucleotides and temperature

• Published in: Biochemistry (Easton) Vol. 25; no. 23; pp. 7782 - 7788
• Main Authors: Schwarz, K R, Carter, E A, Homcy, C J, Graham, R M
• Format: Journal Article
• Language: English
• Published: United States 18-11-1986
• Subjects: Adrenergic alpha-Agonists - pharmacology; Adrenergic beta-Agonists - pharmacology; Animals; Cell Membrane - metabolism; Female; Guanosine Triphosphate - analogs & derivatives; Guanosine Triphosphate - pharmacology; Guanylyl Imidodiphosphate - pharmacology; Kinetics; Liver - metabolism; Rats; Rats, Inbred Strains; Receptors, Adrenergic, alpha - drug effects; Receptors, Adrenergic, alpha - metabolism; Receptors, Adrenergic, beta - drug effects; Receptors, Adrenergic, beta - metabolism; Thermodynamics
• ISSN: 0006-2960

We investigated the binding characteristics of agonists to alpha 1- and beta-adrenergic receptors of intact liver cells, broken rat liver cell membranes, and detergent-solubilized preparations under varying experimental conditions, focusing on the different "states" of the receptor for agonists and the regulation of these states by temperature and guanine nucleotides. While only low-affinity binding of agonists to both receptor subtypes was evident in studies performed at 37 degrees C with solubilized preparations, biphasic competition curves for agonists were observed in both intact cells and membrane preparations; the majority of sites were of low affinity. In membrane preparations, the nonhydrolyzable GTP analogue Gpp(NH)p caused a rightward shift of agonist competition curves and a loss of high-affinity binding. These results are consistent with the involvement of guanine nucleotide binding proteins in both alpha 1- and beta-adrenergic transduction pathways. When competition studies were performed at 4 degrees C, receptor sites existed predominantly in the high-affinity configuration, in intact cells and membranes, as well as in soluble preparations. In contrast to the studies conducted at 37 degrees C, no Gpp(NH)p-induced conversion to the lower affinity state could be demonstrated in studies performed with membrane preparations at 4 degrees C. Thus, the high-affinity state of alpha 1- and beta-adrenergic receptors is stabilized at 4 degrees C in intact cells, membranes, and soluble preparations. After incubations had been performed at 37 degrees C, high-affinity binding of agonists could not be restored by subsequent incubation at 4 degrees C.