Mechanism of desensitization of neutrophil response to N-formylmethionylleucylphenylalanine by slow rate of receptor occupancy. Studies on changes in Ca2+ concentration and phosphatidylinositol turnover

Previous studies on the regulation of responses of neutrophils to fMet-Leu-Phe have demonstrated the relevance of the role of the rate of occupation of the receptors by the stimulant. When this rate is decreased by presenting the peptide to neutrophils over a period of time by means of an infusion pump, the activation of the respiratory burst and of the secretion is greatly depressed or is absent. This paper deals with further investigations on the mechanisms of this desensitization, which previous results have shown to consist of an uncoupling between the ligand-receptor complexes and the target for cell responses, caused by the deceleration of the initial rate of occupation of the receptors. The data presented here demonstrate that this desensitization is not linked to the formation of a negative intermediate such as cAMP, but is associated with: (i) a depression of the rate and magnitude of the phosphatidylinositol response (activation of phospahtidylinositol turnover measured as modification of incorporation of ³²P]P_i and ³H]glycerol into phosphatidylinositol and phosphatidic acid); (ii) a deceleration of the rate of the release of bound Ca²⁺, without a decrease in the total quantity of Ca²⁺ liberated (measured as fluorescence changes of chlorotetracycline treated neutrophils); (iii) a slower rise of cytosolic free Ca²⁺ concentration Ca²⁺]_i, without a decrease in the magnitude of the final increase of Ca²⁺]_i (monitored with Quin 2). These findings, which are discussed in relation to the recent hypotheses on the transduction reactions of receptor-mediated stimuli for neutrophil responses, are consistent with a mechanism of desensitization involving decreased production of diacylglycerol by the hydrolysis of phosphatidylinositol and deficient activation of Ca²⁺-phospholipid-dependent protein kinase C.