PT - JOURNAL ARTICLE AU - James E. Porter AU - Stephanie E. Edelmann AU - David J. Waugh AU - Michael T. Piascik AU - Dianne M. Perez TI - The Agonism and Synergistic Potentiation of Weak Partial Agonists by Triethylamine in α<sub>1</sub>-Adrenergic Receptor Activation: Evidence for a Salt Bridge as the Initiating Process AID - 10.1124/mol.53.4.766 DP - 1998 Apr 01 TA - Molecular Pharmacology PG - 766--771 VI - 53 IP - 4 4099 - http://molpharm.aspetjournals.org/content/53/4/766.short 4100 - http://molpharm.aspetjournals.org/content/53/4/766.full SO - Mol Pharmacol1998 Apr 01; 53 AB - α1-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 α1-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 α1b-AR and in Rat-1 fibroblasts stably transfected with the human α1a-AR subtype. TEA was also able to synergistically potentiate the second messenger production by weak partial α1-AR agonists and this effect was fully inhibited by the α1-AR antagonist prazosin. However, this synergistic potentiation was not observed for full α1-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 α1-ARs with a K i of 28.7 ± 4.7 mm. In addition, the site of binding by TEA to the α1-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 α1-AR activation.