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

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.