Influence of selective phosphodiesterase inhibitors on human neutrophil functions and levels of cAMP and Cai

Summary Chromatographic analysis of 3,5-cyclic nucleotide phosphodiesterase (PDE) isoenzymes in the cytosol of human neutrophils shows the predominant presence of PDE IV (cAMP specific) and PDE V (cGMP specific). PDE IV is characterized by (1) cAMP selectivity, (2) a K_M for cAMP of 1.2 M and (3) a typical rank order of IC ₅₀-values for PDE inhibitors: 0.13, 0.17, 47 and 9.5 M for PDE IV selective rolipram, PDE III/IV selective zardaverine, PDE III selective motapizone and unselective 3-isobutyl-l-methylxanthine (IBMX), respectively. Functions of polymorphonuclear leukocytes (PMN) such as N-formylmethionyl-leucyl-phenylalanine (fMLP)-stimulated superoxide release and fMLP/thimerosal elicited leukotriene (LT) biosynthesis are inhibited by these PDE inhibitors with the same rank order and even lower IC₅₀-values. Measurements of changes in cytosolic Ca_i in Fura-2 loaded PMN demonstrate a transient Ca_i increase after stimulation with 0.1 M fMLP and an additional sustained elevation of Ca_i levels in the presence of thimerosal. PDE inhibitors suppress this sustained phase of Ca_i release with the same rank order of IC₅₀-values as LT biosynthesis. The correlation between fMLP/thimerosal-induced LT biosynthesis and Ca_i levels reveal a Ca_i threshold of 150 nM for arachidonic acid metabolism. cAMP levels in PMN were elevated by PDE inhibitors alone by less than 2-fold. In the presence of fMLP however, cAMP was increased up to 10-fold and the efficacy of PDE inhibitors to increase cAMP paralleled their potency to inhibit PDE IV. It is concluded that (1) suppression of PMN functions is achieved by PDE IV inhibition, (2) necessary cAMP elevations are within 50% increase, (3) superoxide release was affected by cAMP/protein kinase A (PKA) directly whereas (4) for inhibition of LT biosynthesis a cAMP related reduction of Ca-influx is involved.Send offprint requests to Ch. Schudt at the above address