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

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Abstract
We investigated the binding characteristics of agonists to .alpha.1- and .beta.-adrenergic receptors of (a) intact liver cells, (b) broken rat liver cell membranes, and (c) detergent-solubilized preparations under varying experimental conditions, focusing on the different "states" of the receptor for agonist and the regulation of these states by temperature and guanine nucleotides. While only low-affinity binding of agonists both receptor subtypes was evident in studies performed at 37.degree. 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.degree. 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.degree. C, no Gpp(NH)p-induced conversion to the lower affinity state could be demonstrated in studies performed with membrane preparations at 4.degree. C. Thus, the high-affinity state of .alpha.1- and .beta.-adrenergic receptors is stabilized at 4.degree. C in intact cells, membranes, and soluble preparations. After incubations had been performed at 37.degree. C, high-affinity binding of agonists could not be restored by subsequent incubation at 4.degree. C. As previously reported for the .beta.-adrenergic receptor [Weiland, G. A., Minnemann K. P., and Molinoff, P. B. (1979) Nature (London) 281, 114-117], the magnitude of the temperature shift in agonist affinity correlated with the functional efficacy of adrenergic agents at .alpha.1-adrenergic receptors (agonist > partial agonist > antagonist) and thus seems to reflect an agonist-induced conformational change at the receptor binding site, per se.