Expression of three α₂-adrenergic receptor subtypes in rat tissues: Implications for α₂ receptor classification

Based on biochemical and ligand binding studies in various tissues and species, evidence for several α₂-adrenergic receptor subtypes has accumulated. The current α₂-adrenergic receptor classification (α_2A, α_2B, α_2C) is based exclusively on pharmacological criteria. The molecular cloning of three distinct genes for human α₂-adrenergic receptors has confirmed the existence of multiple α₂-adrenergic receptor subtypes. According to their localization on different human chromosomes, the receptor genes were termed α₂-C10, α₂-C4, and α₂-C2. The relationship, however, between the pharmacologically characterized α₂-adrenergic receptors and the isolated genes has yet to be clarified. Using Northern blot hybridization, we analyzed the expression of the three cloned α₂-adrenergic receptor genes in 13 rat tissues, as well as in cell lines previously described as model systems for the pharmacologically defined α₂-adrenergic receptor subtypes. The α₂-C10 receptor corresponds to the α_2A subtype and is expressed in rat brainstem, cerebral cortex, hippo-campus, pituitary gland, cerebellum, kidney, aorta, skeletal muscle, spleen, and lung. Messenger RNA coding for the α₂-C4 receptor was detected only in brain regions, not in peripheral tissues, whereas the α₂-C2 message was found only in liver and kidney. Hybridization experiments with RNA derived from tissues and cells from which the pharmacological α₂-receptor classification has been developed lead to the conclusion that the α_2B subtype represents two distinct receptor molecules, the α₂-C4 and a subtype previously undetected by classical ligand binding approaches. Furthermore, our results suggest that the α_2C subtype characterized in opossum kidney cells is an interspecies variation of α₂-C4 rather than a separate subtype. Finally, the cloned α₂-C2 receptor was found to be "α_2B-like" and not covered by the current pharmacological classification.