Signal transduction and ligand-receptor dynamics in the human neutrophil. Transient responses and occupancy-response relations at the formyl peptide receptor.

The responses of neutrophils to formyl peptides are initiated and in many cases achieve a maximal level prior to equilibrium receptor occupancy. In order to begin to understand the linkage between receptor occupancy and cell response we have used a pulsed binding procedure to analyze: 1) the number of receptors contributing to three potential signalling events and six functional responses and 2) the evolution of these responses once ligand binding is interrupted. We find that the half-optimal elevations of the potential signals are produced by less than 1% occupancy (Ca2+) or 1-3% occupancy (cAMP, membrane depolarization). In contrast, actin polymerization and a rapid light scatter response are elicited by less than 0.1% occupancy. Half-optimal elastase release and degranulation require approximately 3% occupancy. While half-optimal O2- production and aggregation require approximately 30% occupancy, the half-optimal rate of O2- production requires less than 10% occupancy. To resolve the apparent lack of correlation between the responses and the signals we examined their time courses following the pulse of stimulation. At least four responses and one signal are transient and decay while occupied receptors remain on the membrane surface. These include the Quin 2-Ca2+ signal, actin polymerization, the light scatter response, O2- generation, and aggregation. Ca2+ elevation is correlated with the responses in that: 1) each of these responses is transient unless new receptors are occupied; 2) occupancy of nearly all of the receptors contributes to the time course of these responses; 3) when binding is interrupted, the responses decay with a half-time of 15 s, following a latency of approximately 10 s or less (except for disaggregation where latency is 30-40 s). We discuss evidence in support of the hypothesis that transient cell responses arise from transient receptor activation.