Platelet activating factor-induced increase in cytosolic calcium and transmembrane current in human macrophages

Platelet-activating factor (PAF) is synthesized and secreted by macrophages in responding to inflammatory stimuli. When exogenously applied to human monocyte derived macrophages (HMDMs), PAF induces a rapid rise in cytosolic free calcium (Cai) believed to be an early triggering event in macrophage activation. We investigated PAF-induced Ca2+ signaling in HMDMs using the calcium indicator Fura-2, combining single cell ratio fluorimetry and digital video imaging with whole-cell recording techniques. Application of PAF (20 ng/ml) to adherent macrophages induced transient increases in Cai that were biphasic, consisting of an initial phase that could be observed in Ca(2+)-free solutions and a second phase that was critically dependent upon Ca2+ entry. When Mn2+ was applied to cells in the presence and absence of Ca2+, PAF increased the rate of Mn2+ entry rate only when Ca2+ was absent. PAF increased the rate of Ba2+ entry even when measured in the presence of external Ca2+. Ca2+ entry was reversibly inhibited in the presence of external La3+ (1 mM). Data obtained from simultaneous voltage-clamp/microfluorimetry experiments demonstrated the activation of a nonselective cation current which closely paralleled the rising phase of the Cai transient. We investigated whether the non-selective cation conductance provided for the bulk of the agonist-induced Ca2+ influx. Changes in Cai following removal of extracellular Ca2+ (Cao) during the agonist-induced Cai response were not associated with changes in whole-cell current. The inability to detect whole-cell current changes correlated with a decrease in Cao suggests that the bulk of the Ca2+ influx was not through the nonselective conductance and either does not occur through a conductance pathway or occurs via a parallel pathway consisting of channels which are both low conductance and highly Ca2+ selective.