Vol. 53, Issue 4, 766-771, April 1998

The Agonism and Synergistic Potentiation of Weak Partial Agonists by Triethylamine in ₁-Adrenergic Receptor Activation: Evidence for a Salt Bridge as the Initiating Process

James E. Porter, Stephanie E. Edelmann, David J. Waugh, Michael T. Piascik, and Dianne M. Perez

Department of Molecular Cardiology, The Lerner Research Institute, The Cleveland Clinic Foundation, Cleveland, Ohio 44195 (J.E.P., D.J.W., D.M.P.) and Department of Pharmacology, College of Medicine, University of Kentucky, Lexington, Kentucky 40536 (S.E.E., M.T.P.).

₁-adrenergic receptor (AR) activation is thought to be initiated by disruption of a constraining interhelical salt bridge (). Disruption of this salt bridge is achieved through a competition for the aspartic acid residue in transmembrane domain three by the protonated amine of the endogenous ligand norepinephrine and a lysine residue in transmembrane domain seven. To further test this hypothesis, we investigated the possibility that a simple amine could mimic an important functional group of the endogenous ligand and break this ₁-AR ionic constraint leading to agonism. Triethylamine (TEA) was able to generate concentration-dependent increases of soluble inositol phosphates in COS-1 cells transiently transfected with the hamster _1b-AR and in Rat-1 fibroblasts stably transfected with the human _1a-AR subtype. TEA was also able to synergistically potentiate the second messenger production by weak partial ₁-AR agonists and this effect was fully inhibited by the ₁-AR antagonist prazosin. However, this synergistic potentiation was not observed for full ₁-AR agonists. Instead, TEA caused a parallel rightward shift of the dose-response curve, consistent with the properties of competitive antagonism. TEA specifically bound to a single population of ₁-ARs with a K_i of 28.7 ± 4.7 mM. In addition, the site of binding by TEA to the ₁-AR is at the conserved aspartic acid residue in transmembrane domain three, which is part of the constraining salt bridge. These results indicate a direct interaction of TEA in the receptor agonist binding pocket that leads to a disruption of the constraining salt bridge, thereby initiating ₁-AR activation.