THE AGONISM AND SYNERGISTIC POTENTIATION OF WEAK PARTIAL AGONISTS BY TRIETHYLAMINE IN ALPHA(1)-ADRENERGIC RECEPTOR ACTIVATION - EVIDENCE FOR A SALT BRIDGE AS THE INITIATING PROCESS

alpha(1)-adrenergic receptor (AR) activation is thought to be initiate d by disruption of a constraining interhelical salt bridge (Porter et al., 1996). Disruption of this salt bridge is achieved through a compe tition for the aspartic acid residue in transmembrane domain three by the protonated amine of the endogenous ligand norepinephrine and a lys ine residue in transmembrane domain seven. To further test this hypoth esis, we investigated the possibility that a simple amine could mimic an important functional group of the endogenous ligand and break this alpha(1)-AR ionic constraint leading to agonism. Triethylamine (TEA) w as able to generate concentration-dependent increases of soluble inosi tol phosphates in COS-1 cells transiently transfected with the hamster alpha(1b)-AR and in Rat-1 fibroblasts stably transfected with the hum an alpha(1a)-AR subtype. TEA was also able to synergistically potentia te the second messenger production by weak partial alpha(1)-AR agonist s and this effect was fully inhibited by the alpha(1)-AR antagonist pr azosin. However, this synergistic potentiation was not observed for fu ll alpha(1)-AR agonists. Instead, TEA caused a parallel rightward shif t of the dose-response curve, consistent with the properties of compet itive antagonism. TEA specifically bound to a single population of alp ha(1)-ARs with a K-i of 28.7 +/- 4.7 mM. In addition, the site of bind ing by TEA to the alpha(1)-AR is at the conserved aspartic acid residu e in transmembrane domain three, which is part of the constraining sal t bridge. These results indicate a direct interaction of TEA in the re ceptor agonist binding pocket that leads to a disruption of the constr aining salt bridge, thereby initiating alpha(1)-AR activation.