Transient inhibition by chemotactic peptide of a store-operated Ca2+ entry pathway in human neutrophils

Emptying the intracellular calcium stores of fura-2-loaded human neutrophils by treatment with the endomembrane ATPase inhibitor thapsigargin leads to a maintained increase of [Ca2+]i by Ca2+ entry through a store-operated Ca2+ entry pathway. Under these conditions, [Ca2+]i was reduced transiently by N-formyl-methionyl-leucyl-phenylalanine (fMLP) and permanently by phorbol 12,13-dibutyrate (PDB). Platelet-activating factor (PAF) had no effect. The fMLP- and PDB-induced [Ca2+]i decreases were not due to stimulated Ca2+ efflux but to inhibition of store-operated Ca2+ entry pathway. PDB and fMLP, but not PAF, inhibited the entry of Ca2+, Mn2+, and Ba2+ in thapsigargin-treated cells. This inhibition was dependent on [Ca2+]i, barely detectable at [Ca2+]i of 50 nM and increasingly strong and fast to appear at 170 and 630 nM. Inhibition of entry by fMLP was complete within 5-10s, disappeared within 2-3 min, and was partially prevented by staurosporin (100 nM). Inhibition by PDB was equally fast, but no recovery was detected within 5 min, and it was fully prevented by staurosporin. The inhibitory effect of fMLP had similar characteristics when PAF was used instead of thapsigargin to induce the entry of Ca2+ or Mn2+. We conclude that fMLP, but not PAF, is able to produce a transient inhibition of store-operated Ca2+ entry pathway, probably mediated by protein kinase C. This action could be part of a general homeostatic mechanism designed to moderate [Ca2+]i increases induced by some agonists.