| Abstract: | The interaction between prostaglandin E1 (PGE1) and chemotactic peptide formylmethionyl-leucyl-phenylalanine (fMLP) in cAMP production in guinea pig neutrophils was investigated. Both PGE1 and fMLP increased the cAMP content in neutrophils. At low concentrations of PGE1 (less than 10 nM), the effects of fMLP and PGE1 in stimulating cAMP accumulation were additive, but at high concentrations of PGE1, their effects were synergistic. The effects of PGE1 and Ca2+ ionophore A23187 instead of fMLP on cAMP accumulation were also synergistic. The synergy did not appear to be related to change in cyclic nucleotide phosphodiesterase activity, because it was still marked in the presence of isobutyl-3-methyl-1-xanthine, a phosphodiesterase inhibitor. Studies on the time course of PGE1-induced cAMP accumulation showed that cAMP production ceased within 5 min after the addition of high concentrations of PGE1. The period of cAMP production could not be prolonged by combined treatment with PGE1 and fMLP or Ca2+ ionophore A23187. The synergy was found to be caused through Ca2+-dependent processes, because depletion of the medium of Ca2+ and addition of the Ca2+ antagonist TMB-8 inhibited the synergistic increase in cAMP. Moreover, the calmodulin antagonist W-7 also effectively inhibited the synergistic increase in cAMP. These results suggest that the potentiation of PGE1-induced cAMP production by fMLP or Ca2+ ionophore A23187 is catalyzed by calmodulin-dependent processes. However, the synergistic increase in cAMP production was not inhibited by arachidonic acid cascade inhibitors such as indomethacin, BW755C, or nordihydroguiaretic acid, and a combination of PGE1 and a protein kinase C activator, tetradecanoyl phorbol acetate (TPA), did not cause synergistic increase in cAMP. Marked increase in cAMP was also induced by a combination of cholera toxin and fMLP or Ca2+ ionophore A23187, but not by a combination of forskolin and fMLP or Ca2+ ionophore A23187. The synergistic increase in cAMP was not sustained in isolated membranes. On the contrary, PGE1-induced cAMP production in isolated membranes was suppressed by their pretreatment with fMLP or Ca2+ ionophore A23187. These data suggest that the synergistic effects of PGE1 and fMLP or Ca2+ ionophore in increasing the cAMP level are due to potentiation of PGE1-induced cAMP production by Ca2+ and calmodulin-dependent processes. |
