Effect of age on second messenger generation in neutrophils

Neutrophils from healthy elderly donors generate significantly less diacylglycerol (DAG) and inositol triphosphate (IP3) than neutrophils from young donors, following stimulation by the chemotactic peptide, formyl-methionyl-leucylphenylalanine (FMLP). The defect in signal transduction occurred at a point proximal to the generation of IP3 and DAG, since the reduction in FMLP-induced superoxide generation was corrected if the intervening signal transduction steps were bypassed, either by priming with a substimulatory dose (1.62 nmol/L) of phorbol myristate acetate (PMA), by ionophore elevation of cytosolic calcium, or by using a stimulatory dose of PMA (1.62 mumol/L). FMLP receptor number and affinity were unaffected by aging. On FMLP activation, neutrophils from old, as compared with young, volunteers showed significantly greater and more long-lasting decreases in the concentrations of phosphatidylinositol (PI), phosphatidylinositol 4-monophosphate (PIP), and phosphatidylinositol 4,5-bisphosphate (PIP2). This indicates a reduction with age in the metabolically active precursor pools responsible for the generation of IP3 and DAG. In contrast, aging had little effect on the production of phosphatidic acid (PA), which has recently been suggested to serve as a major activator of the NADPH oxidase. This may explain why the decrease in IP3 and DAG production was not accompanied by a comparable decrement in superoxide generation, which was only 17% lower in the old than in young donor neutrophils. Thus, aging is associated with reductions in the concentration of critically important phosphoinositides, resulting in diminution in the ability to produce key second messengers. Although the aged neutrophil is largely able to compensate for the decrements in signal transduction, its reserve capacity is compromised, making it particularly vulnerable to external insults that also impair function.