We have characterized a C3a receptor on guinea-pig macrophages by I-12
5-C3a binding and functional responses. Scatchard analysis applied to
the I-125-C3a binding to guinea-pig macrophages revealed the existence
of two receptor classes; a high-affinity class with approximately 0.6
3 x 10(5) binding sites/cell with a K(d) = 2.7 nM, and a relatively lo
w-affinity class with approximately 1.2 x 10(5) binding sites/cell wit
h a K(d) = 51 nm. The binding of C3a to macrophages was totally blocke
d when there was an excess of C3a. C3a triggered a transient intracell
ular 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 we
re not detected in the pertussis toxin-treated macrophages, suggesting
that G proteins are coupled with the C3a receptors of macrophages. Al
though the C3a-induced O2- generation was inhibited by staurosporine,
it was more resistant to staurosporine than phorbol 12-myristate-13-ac
etate (PMA)-induced O2 generation, suggesting that a protein kinase di
stinct from protein kinase C may be associated with the C3a receptor.