The Agonism and Synergistic Potentiation of Weak Partial Agonists by Triethylamine in α1-Adrenergic Receptor Activation: Evidence for a Salt Bridge as the Initiating Process

• Published in: Molecular pharmacology 1998-04, Vol.53 (4), p.766
• Main Authors: James E. Porter, Stephanie E. Edelmann, David J. Waugh, Michael T. Piascik, Dianne M. Perez
• Format: Article
• Language: English
• ISSN: 0026-895X; 1521-0111

α 1 -adrenergic receptor (AR) activation is thought to be initiated by disruption of a constraining interhelical salt bridge ( Porter et al., 1996 ). 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 α 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 α 1b -AR and in Rat-1 fibroblasts stably transfected with the human α 1a -AR subtype. TEA was also able to synergistically potentiate the second messenger production by weak partial α 1 -AR agonists and this effect was fully inhibited by the α 1 -AR antagonist prazosin. However, this synergistic potentiation was not observed for full α 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 α 1 -ARs with a K i of 28.7 ± 4.7 m m . In addition, the site of binding by TEA to the α 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 α 1 -AR activation.