-adrenergic receptor: Characterization of signal transduction pathways and mammalian tissue heterogeneity
We recently reported the cloning of a novel α1-adrenergic receptor (AR), the α1CAR. By transient and stable expression of the α1CAR and the previously cloned α1BAR in COS-7 and HeLa cells, respectively, we have now compared their ability to interact with major signal-transduction pathways (including polyphosphoinositide hydrolysis, intracellular calcium, and cAMP metabolism), as well as their mammalian tissue localization. Both α1C-and α1BARs primarily couple to phospholipase C via a pertussis toxin-insensitive GTP-binding protein, leading to the release of calcium from intracellular stores. Even though α1C- and α1BARs activate polyphosphoinositide hydrolysis by similar biochemical mechanisms, the α1CAR couples to phospholipase C more efficiently than does the α1BAR; activation of the α1CAR results in a 2-3-fold greater increase in inositol phosphates, compared with the α1BAR. Both α1AR subtypes can also increase intracellular cAMP, by a mechanism that does not involve direct activation of adenylyl cyclase. In agreement with ligand binding data, the agonist methoxamine and the antagonist WB4101 are 10-fold more potent in activating or inhibiting, respectively, the ability of the α1CAR to stimulate phospholipase C, compared with the α1BAR. In addition, methoxamine is almost a full agonist at the α1CAR, whereas it can only weakly activate the α1BAR. Tissue localization, using Northern blot analysis of total and poly(A)+-selected RNA from rabbit tissues, revealed striking mammalian species heterogeneity. As previously described, the α1BAR is present in several rat tissues, including heart, liver, brain, kidney, lung, and spleen, whereas the α1CAR is not present in any rat tissue studied. The α1BAR is also present in rabbit aorta, heart, spleen, and kidney (and absent in rabbit liver), whereas the α1CAR is present in rabbit liver. Our results indicate that the cloning and expression of different α1AR subtypes represents a valuable tool to elucidate functional correlates of α1AR heterogeneity.
Schwinn, DA; Page, SO; Middleton, JP; Lorenz, W; Liggett, SB; Yamamoto, K; Lapetina, EG; Caron, MG; Lefkowitz, RJ; Cotecchia, S
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