So far, only little is known about the internalization process of the histamine H2 receptor (H2R). One promising approach to study such dynamic processes is the use of agonistic fluorescent ligands. Therefore, a series of carbamoylguanidine-type H2R agonists containing various fluorophores, heterocycles, and linkers (28-40) was synthesized. The ligands were pharmacologically characterized in several binding and functional assays. These studies revealed a significantly biased efficacy (Emax) for some of the compounds, e.g. 32: whereas 32 acted as strong partial (Emax: 0.77, mini-Gs recruitment) or full agonist (Emax: 1.04, [35S]GTPγS binding) with respect to G protein activation, it was only a weak partial agonist regarding β-arrestin1/2 recruitment (Emax: 0.09-0.12) and failed to promote H2R internalization (confocal microscopy). On the other hand, H2R internalization was observed for compounds that exhibited moderate agonistic activity in the β-arrestin1/2 pathways (Emax ≥ 0.22). The presented differently-biased fluorescent ligands are versatile molecular tools for future H2R studies on receptor trafficking and internalization e.g. using fluorescence microscopy.
Keywords: Agonist bias; Carbamoylguanidine; Confocal microscopy; Flow cytometry; Fluorescent ligands; Histamine H(2) receptor.
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