Pharmacological characterization of putative β1-β2-adrenergic receptor heterodimers

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Abstract
In the last few years, significant experimental evidence has accumulated showing that many G protein coupled receptors (GPCRs) are structurally and perhaps functionally homodimers. Recently, a number of studies have demonstrated that many GPCRs, notably GABAB, somatostatin, and δ and κ opioid receptors form heterodimers, as well. Based on these observations, we undertook a pharmacological and functional analysis of HEK 293 cells transiently transfected with the β1AR or β2AR or with both subtypes together. High-affinity binding for subtype-specific ligands (betaxolol and xamoterol for the β1AR, and ICI 118,551 and procaterol for the β2AR) was detected in cells expressing the cognate receptors alone with values similar to those reported in the literature. However, a significant portion of these high-affinity interactions were lost when both receptors were expressed together while nonspecific ligands (propranolol and isoproterenol) retained their normal affinities. When competition assays were performed with each subtype-specific ligand in the presence of a constant concentration of the other subtype-specific ligand, the high-affinity binding site was rescued, suggesting that the two receptor subtypes were interacting in a fashion consistent with positive cooperativity. Our data suggest that the β1AR and β2AR can form heterodimers and that these receptors have altered pharmacological properties from the receptor homodimers.Key words: G protein coupled receptor, signalling, G protein, dimerization, heterodimer, adrenergic.