It was recently suggested that the opening of neutrophil plasma membrane Ca2+ channels by chemotactic agents is mediated by a rise in free cytosolic Ca2+ concentration ([Ca2+]i). This hypothesis was tested in human cells monitoring [Ca2+]i, with the indicator indo-1. In cells loaded with the Ca2+-chelating agent bis(o-aminophenoxy)ethane-N,N,N′,N′-tetraacetate, transmembrane Ca2+ uptake could be stimulated by formyl-methionyl-leucyl-phenylalanine (fMLP) even when [Ca2+]i was at or below the resting level. In contrast, simply elevating [Ca2+]i in unstimulated cells failed to increase transmembrane uptake. It was concluded either that Ca2+ uptake across the plasma membrane is activated directly by the formation of the chemotactic factor-receptor complex or, more likely, that a transduction mechanism distinct from changes in [Ca2+]i is involved.
