Stimulus-secretion coupling in rabbit neutrophils is not mediated by phosphatidylinositol breakdown

In common with other cells which use intracellular Ca2+ to mediate specific cell function, when rabbit neutrophils are stimulated with specific agonists the rate of metabolism of phosphatidylinositol (PI) increases. This is normally measured as the incorporation of radioactive phosphate or inositol into PI, but these reactions are presumed to be secondary processes following the initial breakdown of pre-existing PI to diacylglycerol. The radioactive labels are incorporated during the stepwise resynthesis of PI via phosphatidic acid (PA). It has been suggested that in the sequence of biochemical events, starting with the binding of the ligand to a receptor, and finally resulting in the expression of cellular activity, the breakdown of PI is an early event immediately directed by activation of the receptor. This could then control the increase in cytoplasmic Ca2+ and other processes dependent on this. Here we report an analysis of the temporal relationship between these phospholipid changes and cell stimulation. Our evidence suggests that in neutrophils, PI breakdown and PA labelling are both consequences and not causes of a rise in intracellular Ca2+.