Effects of different natriuretic peptides on nitric oxide synthesis in macrophages

Atrial natriuretic peptide (ANP) has previously been suggested to inhibit the production of NO in LPS-activated primary macrophages. The aim of the present study was 1) to examine whether ANP elicits this effect also on macrophage cell lines (RAW 264.7, J774), 2) to elucidate whether ANP is the only natriuretic peptide (NP) inhibiting NO synthesis, 3) to look for the expression of natriuretic peptide receptors (NPR) on macrophages, 4) to consequently determine the type of receptor mediating the ANP effect and 5) to obtain first information on the underlying mechanism. Whereas ANP dose dependently (10(-6)-10(-8) M) inhibited NO synthesis (measured as nitrite accumulation, 20h) in all four types of macrophages (bone marrow derived and peritoneal macrophages; RAW 264.7 and J774), urodilatin and atriopeptin I displayed only a weak effect restricted to the highest concentration (10(-6) M) employed. Importantly, C-type natriuretic peptide (CNP) showed no NO-inhibitory effect. The lack of effect of CNP was shown not to be due to its lower stability or its missing receptor. Macrophages were shown to express all three natriuretic peptide receptors (NPR-A, NPR-B, NPR-C) using RT-PCR technique. Furthermore, two types of NPR-B seem to be present in macrophages. The effect of ANP was mediated via the guanylate cyclase coupled NPR-A as shown by experiments employing stable cGMP analogs, the NPR-A antagonist HS-142-1, LY-83583, a cGMP inhibitor as well as C-ANF, a specific ligand of the NPR-C. Reduction of nitrite accumulation by ANP was highest when added simultaneously with LPS and abolished when added 12 h after LPS stimulation. In summary, ANP was shown to inhibit NO production of LPS-activated macrophages via cGMP.