Since the classification of beta-adrenergic receptors (beta-ARs) into beta 1 and beta 2 subtypes, additional beta-ARs have been implicated in the control of various metabolic processes by catecholamines. A human gene has been isolated that encodes a third beta-AR, here referred to as the "beta 3-adrenergic receptor." Exposure of eukaryotic cells transfected with this gene to adrenaline or noradrenaline promotes the accumulation of adenosine 3',5'-monophosphate; only 2 of 11 classical beta-AR blockers efficiently inhibited this effect, whereas two others behaved as beta 3-AR agonists. The potency order of beta-AR agonists for the beta 3-AR correlates with their rank order for stimulating various metabolic processes in tissues where atypical adrenergic sites are thought to exist. In particular, novel beta-AR agonists having high thermogenic, antiobesity, and antidiabetic activities in animal models are among the most potent stimulators of the beta 3-AR.

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The genes coding for three pharmacologically distinct subtypes of human P-adrenergic receptors (P,AR, P2AR and P,AR) were transfected for expression in Chinese hamster ovary (CHO) cells. Stable cell lines expressing each receptor were analyzed by ligand binding, adenylate cyclase activation and photoaffinity labeling, and compared to PAR subtypes observed in previously described tissues, primary cultures and transfected cell lines. Each of the three receptor subtypes displayed saturable [' 251]iodocyanopindolol-binding activity. They showed the characteristic rank order of potencies for five agonists, determined by measuring the accumulation of intracellular CAMP. These recombinant cell lines express a homogeneous population of receptors and display the known pharmacological properties of PIAR and P2AR, in human tissues. It is therefore likely that the pattern of ligand binding and adenylate cyclase activation, mediated by the new P3AR in CHO cells, also reflects the yet-undetermined pharmacological profile in humans.For the past few years, considerable effort has been placed on understanding the physiological role of the catecholamine P-adrenergic receptors (PAR). These receptors belong to a family of transmembrane proteins which, upon binding hormones and synthetic agonists, stimulate adenylate cyclase via coupling to the stimulatory guanine nucleotide binding regulatory protein [l]. On the basis of their binding properties, the PAR subtypes were initially classified into the PIAR and P2AR subtypes. A third subtype (P,AR), for which a physiological role has not yet been established, was recently identified by molecular cloning [2]. The discovery of this new PAR requires the re-evaluation of existing drug-binding studies and may present the opportunity to develop new ligands which may become useful tools for further research on PAR subtypes.In recent reports, we have shown that human PAR subtypes expressed in Escherichia coli bacteria constitute a powerful system for the screening of specific ligands [3]. A single receptor type can be studied easily, quickly and at low cost, and therefore this approach appears more attractive than classical procedures which involve tedious preparation of tissue homogenates or cell lines. Eukaryotic expression systems present the additional advantage that the receptors can be coupled to the transducing proteins and catalytic units (adenylate cyclase), allowing evaluation of agonistic or antagonistic effects of ligands. Therefore, the proteins encoded by the human PIAR, P2AR and j3AR genes were expressed in CHO cells using the dihydrofolate reductase (DHFR) selection marker [4], and three independent cell lines, CHO-b1, CHO-P2 and CHO-P, were obtained. Expression of each of the three subtypes of PAR in the same host cell had the advantage that an identical background activity was obtained in each case. The PIAR and P2AR, expressed in CHO cells, display the rank order of potency for adenylate cyclase stimulation and the ligand-binding affinities expected for each respective subtyp...