Adenylate cyclase uncoupled β-adrenergic receptors in salamander proximal tubules

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 hydrophylic radioligand and (±)-timolol (0.1 mM) as the displacer drug revealed two distinct populations of proximal tubules possessing either low (K(D) = 153.8 nM; B(max) = 110.2 fM/mm) or high affinity (K(D) = 12.0 nM; B(max) = 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 adenylated cyclase system.