Kinetics of prostanoid synthesis by macrophages is regulated by arachidonic acid sources

The dependence of prostanoid synthesis on the nature of free arachidonic ac id (AA) appearance was investigated in mouse peritoneal macrophages. AA del ivery from intracellular sources to the constitutive prostaglandin (PG)H sy nthase was provided by action of calcium-ionophore A23187; and from extrace llular sources by AA addition to the culture medium. It was found that the kinetics of prostanoid synthesis dramatically depends on the sources of AA. Free AA concentration used for prostanoid synthesis is either a constant o r a variable value depending upon the sources. The kinetics of cellular pro stanoid synthesis can be regulated by the following processes: (a) the irre versible inactivation of PGH-synthase in the course of the reaction (k(in)) , (b) prostanoid metabolism (k(r)), and (c) incorporation of exogenous AA i nto cellular membranes (k(a)). From our experiments and mathematical calcul ation these parameters were found to be k(in) = 0.20 +/- 0.02 min(-1), k(r) = 0.17 +/- 0.03 min(-1) in the case of stimulation with A23187, and k(in) = 0.0156 min(-1), k(r) = 0.0134 min(-1), k(a) = 0.0025 min(-1) in the case of exogenous AA addition. The studies of prostanoid biosynthesis by macroph age microsomes led to independent determination of k(in) = 0.20 +/- 0.02 mi n(-1). This value perfectly fits the kinetics of the prostanoid cell synthe sis under endogenous AA supply but shows a 10-fold decrease in the case of exogenous AA supply. Our study on the kinetics of prostanoid synthesis by m ouse peritoneal macrophages clearly demonstrate that AA is able to regulate cellular prostanoid synthesis in the presence of constitutive PGH-synthase only. A regulation mechanism based on the co-operation of the constitutive PGH-synthase isoform and the availability of free AA is proposed and could be confirmed by mathematical modelling.