<p><b>A)</b> The agonist-occupied FPR activates a G-protein and the second messengers generated activate the electron-transporting NADPH-oxidase that reduces oxygen to superopxide anion. The signaling state of the receptor is fairly short lived. <b>B</b>) The agonist-occupied receptor is desensitized and the functional response is terminated. This non-signaling state is hypothetically achieved through a physical separation of the receptor-ligand complex from the G-protein, made possible by binding of actin polymers and/or arrestin molecules to the receptor. <b>C</b>) The desensitized FPR is reactivated by signals generated when PAF binds to its neutrophil receptor (arrow, 1). Reactivation of the desensitized FPR is achieved also with cytoskeletal inhibitors, (shorter filaments, 2), suggesting a mechanism for reactivation that involves uncoupling of the receptor-ligand complex from the cytoskeleton. The described cross talk is hierarchial and unidirectional.</p
