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, hippocampus, 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.