GTPγS Assay for Measuring Agonist-Induced Desensitization of Two Human Polymorphic Alpha_2B-Adrenoceptor Variants

α₂-Adrenergic receptors (ARs) mediate many cellular actions of epinephrine and norepinephrine, including inhibition of their secretion (sympathetic inhibition) from adrenal chromaffin cells. Like many other G protein-coupled receptors (GPCRs), they undergo agonist-dependent phosphorylation and desensitization by GPCR kinases (GRKs), a phenomenon recently shown to play a major role in the sympathetic overdrive that accompanies and aggravates chronic heart failure. A three-glutamic acid deletion polymorphism in the human α_2B-AR subtype gene (Glu301-303) causes impaired agonist-promoted receptor phosphorylation and desensitization, resulting in enhanced signaling to inhibition of cholinergic-induced catecholamine secretion in adrenal chromaffin cells. One of the various pharmacological assays that can be used to quantify and quantitatively compare the degrees of agonist-dependent desensitization, i.e., G protein decoupling, of these two polymorphic α_2B-AR variants (or of any two GPCRs for that matter) is the guanosine-5'-O-3-thiotriphosphate (GTPγS) assay that can directly quantify heterotrimeric G protein activation.