Characterization of C3a anaphylatoxin receptor on guinea-pig macrophages.

We have characterized a C3a receptor on guinea-pig macrophages by 125I-C3a binding and functional responses. Scatchard analysis applied to the 125I-C3a binding to guinea-pig macrophages revealed the existence of two receptor classes; a high-affinity class with approximately 0.63 x 10(5) binding sites/cell with a Kd = 2.7 nM, and a relatively low-affinity class with approximately 1.2 x 10(5) binding sites/cell with a Kd = 51 nM. The binding of C3a to macrophages was totally blocked when there was an excess of C3a. C3a triggered a transient intracellular Ca2+ ([Ca2+]i) mobilization in macrophages, which was accompanied by homologous desensitization. C3a was also capable of generating O2- from macrophages. The C3a-induced Ca2+ response and O2- generation were not detected in the pertussis toxin-treated macrophages, suggesting that G proteins are coupled with the C3a receptors of macrophages. Although the C3a-induced O2- generation was inhibited by staurosporine, it was more resistant to staurosporine than phorbol 12-myristate-13-acetate (PMA)-induced O2- generation, suggesting that a protein kinase distinct from protein kinase C may be associated with the C3a receptor.