Adenylate cyclase uncoupled β-adrenergic receptors in salamander proximal tubules

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Abstract
The isolated perfused proximal tubule of the neotenic salamander Ambystoma tigrinum responds with either a hyperpolarization or depolarization of both the basolateral cell membrane and transepithelial potentials following the addition of 10−5 M isoproterenol to the bath superfusate. Both responses were blocked by 10−6 M propranolol but neither response was mimicked by 10−4 M cAMP. β-Adrenergic binding studies of individual microdissected proximal tubules using (−)-[3H]CGP-12177 as a hydrophyllic radioligand and (±)-timolol (0.1 mM) as the displacer drug revealed two distinct populations of proximal tubules possessing either low (KD = 153.8 nM; Bmax = 110.2 fM/mm) or high affinity (KD = 12.0 nM; Bmax = 3.9 fM/mm) binding characteristics. Competition studies indicated that the bound (−)-[3H]CGP-12177 behaved as a typical β-adrenergic ligand, being displaced by (−)-isoproterenol but not by (+)-isoproterenol or (−)phenylephrine. However, neither appeared to be coupled to the adenylate cyclase system. These data suggest the presence of functional β-adrenergic receptors that do not appear to be coupled to the adenylate cyclase system.Key words: proximal tubule, β-receptors, adenylate cyclase.