Cytosolic calcium, calcium fluxes, and regulation of alveolar macrophage superoxide anion production

The recently available compound quin‐2, which acts as a high affinity fluorescent indicator for calcium in the cytosol, was used to examine the role of calcium mobilization in the alveolar macrophage during the stimulation of 0 −2 production by the tripeptide N‐formyl norleucyl leucyl phenylalanine (FNLLP). After preloading with quin‐2, the production of 0 −2 was measured in conjunction with the transfer of 45Ca⁺² and changes in quin‐2 fluorescence upon stimultion with FNLLP. When cells were maintained in low (10 μM) extracellular calcium medium the presence of 1.5 mM quin‐2 in the cytosolic space partially inhibited the rate of 0 −2 production upon stimulation by FNLLP. Addition of 1 mM Ca⁺² to the medium prior to stimulation rapidly restored the cell's capability to produce 0 −2 upon stimulation at rates equal to control and extended the duration of stimulated 0 −2 production as well. Quin‐2 fluorescence measurements indicated an increase in cytosolic Ca⁺² upon stimulation with FNLLP. This increase was lowest under conditions in which 0 −2 production was inhibited. The addition of 1 mM Ca⁺² to the medium caused by itself a rapid but transient increase in cytosolic Ca⁺² as measured with quin‐2 without stimulating 0 −2 production. This intracellularly redistributed calcium was determined to be the source of the greater increase in cytosolic calcium during stimulation in the presence of high extracellular calcium. Measurements of 45Ca⁺² transfer demonstrated a buffering of cytosolic Ca⁺² changes by quin‐2, which in low calcium medium could deplete calcium stores. It is suggested that this effect, prior to stimulation, was responsible for the mitigated 0 −2 response for those cells maintained in low calcium medium, wherein calcium stores could not be replenished. These results suggested that the cell's mechanism for regulating cytosolic and bound calcium concentrations may also play an integral role in its normal mechanism for stimulated 0 −2 production. They further support the postulate that the commonly observed rise in the concentration of calcium in the cytosol upon formyl peptide stimulation is a concomitant but nonregulatory event only.