| Abstract: | Many macrophage functions such as chemotaxis, phagocytosis, enzyme secretion, and cytotoxicity are influenced by intracellular cyclic nucleotide levels, but the regulatory mechanisms involved are poorly defined. We have developed methods that allowed us to study the activation of AC in isolated guinea pig (g.p.) macrophage membranes. AC in these membrane preparations could be stimulated approximately twofold by guanine nucleotides. We could not obtain any hormonal activation of membrane-bound AC in the absence of guanine nucleotides. In the presence of GTP, however, the hormones isoproterenol and PGE1 elicited an additional threefold rise in AC activity, which subsided after approximately 15 min. As little as 10(-8) M concentrations of these two hormones induced significant elevations of AC activity. Replacement of GTP by its nonhydrolyzable analogue Gpp(NH)p resulted in a persistent hormone-independent activation of AC, and addition of hormones enhanced this level of activation. Thus, GTP-ase activity is present in macrophage membrane preparations and serves to regulate AC activation. Hormonal stimulation of AC was receptor mediated, because the effect of the beta-adrenergic agonist isoproterenol, but not PGE1, was inhibited by the beta-adrenergic blocker propranolol. In addition, the potency series of PG corresponded to that observed for stimulation of cAMP production in intact g.p. macrophages, i.e., PGE1 = PGE2 greater than PGA1 greater than PGF2 alpha. AC activation by PG in the membrane preparation was inhibited by an alpha-adrenergic agonist, thus demonstrating one means for down regulating cAMP production in g.p. macrophages. Our studies also showed that certain hormones (e.g., beta-adrenergic agonists, PG) can exert their effect on cAMP production by stimulation of membrane-bound AC, whereas other agents such as lectins or arachidonic acid require additional intracellular components to elevate cAMP levels in macrophages. The mechanism of activation of AC by hormones in g.p. macrophage membranes appears to fit the model of a ternary complex, the components of which include the hormone receptor, AC, and guanine nucleotide regulatory protein, which transmits the signal from the receptor to AC. |
