Gtp-Related Difference In Cyclic-Amp Production Between Resident And Inflammatory Human Peritoneal-Macrophages

In macrophages cyclic AMP (c-AMP) plays an important role in regulating many activities such as phagocytosis, migration and tumoricidal activity. High intracellular levels of c-AMP are negatively correlated with these activities. In earlier studies we have shown that c-AMP levels in inflammatory human peritoneal macrophages (IM) were markedly lower when compared to levels in resident macrophages (RM). This is in line with the fact that c-AMP down-regulates macrophage activity. To our knowledge no data are available on the mechanism underlying the difference in c-AMP production between RM and IM. In this study the difference in c-AMP production between RM and IM has been investigated on the level of receptor and G-protein-related mechanisms. Macrophage membranes were incubated with different agents i.e. prostaglandin E2 (PGE2), prostacyclin I2 (PGI2), isoprenalin (ISO) and sodium fluoride (SF). Additionally, the capacity of IM and RM to hydrolyse quanosine triphosphate (GTP) was measured. Only in the presence of GTP (10(-4) M) could the c-AMP difference be detected (RM = 51 +/- 4.4 pmol/mg protein/min +/- S.E., n = 22, IM = 32.8 +/- 5.2 pmol/mg protein/min +/- S.E., n = 10, p < 0. 01). After receptor stimulation with PGE2, PGI2 and ISO, c-AMP levels increased to the same extent in both IM and RM with no effect on the GTP-related difference. After SF stimulation, c-AMP levels in RM and IM increased to the same level (RM = 63 +/- 8 pmol/mg protein/min, n = 14, IM = 58 +/- 11 pmol/mg protein/min, n = 13). RM showed a 38% higher capacity to hydrolyse GTP than IM, (RM = 101, 2 +/- 2.4 mU GTP/mg protein/min. +/- S.E., IM = 62.6 +/- 5.4 mU GTP/mg protein/min. +/- S.E. p < 0.01). We conclude that the c-AMP difference between RM and IM is GTP-related and not influenced by receptor stimulation. No c-AMP difference could be detected after SF stimulation, suggesting that the difference is not directly G -protein-dependent. GTP hydrolysis was lower in IM, suggesting a possible impaired binding of GTP to the G protein thus explaining both lower c-AMP levels and GTP hydrolysis in IM. More research on the GTP-binding capacity to G proteins of IM and RM needs to be done.