K(Ca)3.1-Dependent Hyperpolarization Enhances Intracellular Ca(2+) Signaling Induced by fMLF in Differentiated U937 Cells

Formylated peptides are chemotactic agents generated by pathogens. The most relevant peptide is fMLF (formyl-Met-Leu-Phe) which participates in several immune functions, such as chemotaxis, phagocytosis, cytokine release and generation of reactive oxygen species. In macrophages fMLF-dependent responses are dependent on both, an increase in intracellular calcium concentration and on a hyperpolarization of the membrane potential. However, the molecular entity underlying this hyperpolarization remains unknown and it is not clear whether changes in membrane potential are linked to the increase in intracellular Ca(2+). In this study, differentiated U937 cells, as a macrophage-like cell model, was used to characterize the fMLF response using electrophysiological and Ca(2+) imaging techniques. We demonstrate by means of pharmacological and molecular biology tools that fMLF induces a Ca(2+)-dependent hyperpolarization via activation of the K(+) channel K(Ca)3.1 and thus, enhancing fMLF-induced intracellular Ca(2+) increase through an amplification of the driving force for Ca(2+) entry. Consequently, enhanced Ca(2+) influx would in turn lengthen the hyperpolarization, operating as a positive feedback mechanism for fMLF-induced Ca(2+) signaling.