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

Abstract

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 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 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.