Desensitization of the actin polymerization response in human neutrophils at low cell density

Many chemoattractant-activated responses in neutrophils show transient kinetics, suggesting that rapid desensitization occurs during the time course of the response. We found that desensitization of the actin polymerization response to N-formyl peptides is, in a large part, due to inhibition by adenosine released from cells to the medium and depletion or a chemical inactivation of the agonist. To reduce the influence of these factors, we stimulated neutrophils in a very diluted suspension, sometimes with continuous replacement of the medium. The actin polymerization response to a high agonist concentration was greatly enhanced and prolonged under these conditions, often without any tendency to subside within 10 min at 25 degrees C. It has previously been shown that the N-formyl peptide receptor converts from a rapidly dissociating to a slowly dissociating and presumably inactive form during activation. Under the conditions of low cell concentration, the conversion to a slowly dissociating receptor still occurred. Thus the prolonged response was not due to prolonged presence of rapidly dissociating receptors. We conclude either that a low number of rapidly dissociating receptors, which we failed to see, is sufficient to maintain actin polymerization or that slowly dissociating receptors can support the actin response. In contrast to responses stimulated by high agonist concentrations, the responses to low concentrations of the agonists were transient. The results of other authors indicate that low concentrations of N-formyl peptides do not desensitize the receptors. Other mechanisms, which are specific for the actin polymerization response, must be involved in response termination to low concentrations of N-formyl peptides. Activation at low cell density will be a useful approach for studying other processes (Ca2+ elevation, oxidant production, etc.) and chemoattractants (leukotriene B4, interleukin 8, etc.) for which an understanding of the kinetics due to desensitization of the components of the receptor-mediated activation pathway is desired.