A1 adenosine receptors in human neutrophils: Direct binding and electron microscope visualization

Abstract

By occupying specific surface receptors, adenosine and adenosine analogues modulate neutrophil functions; in particular, functional and biochemical studies have shown that A1 adenosine receptors modulate chemotaxis in response to chemotactic peptides. Until now, the characteristics of the specific agonist binding and the visualization of A1 receptors in human neutrophils have not been investigated. In the present study, we used the agonist [3H] CHA for radioligand binding studies and a CHA-biotin XX probe in order to visualize the A1 binding sites in human neutrophils, ultrastructurally, by conjugation with colloidal gold-streptavidin. [3H] CHA bound A1 adenosine receptors with selectivity and specificity, although with a low binding capacity. Scatchard analysis showed a Kd value of 1.4 ± 0.08 nM and a maximum density of binding sites of 7.1 ± 0.37 fmol/mg of proteins. The good affinity and selectivity of the CHA-biotin XX probe for A1 adenosine receptors allowed us to visualize them, after conjugation with colloidal gold-streptavidin, as electron-dense gold particles on the neutrophil surface and inside the cell. The internalization of the ligand-receptor complex was followed in a controlled temperature system, and occurred through a receptor-mediated pathway. The kinetics of the intracellular trafficking was fast, taking less than 5 min. These data suggest that the CHA-biotin XX-streptavidin-gold complex is a useful marker for the specific labelling of A1 binding sites and to follow the intracellular trafficking of these receptors. J. Cell. Biochem.75:235–244, 1999. © 1999 Wiley-Liss, Inc.