Nitric oxide generation from L-arginine is required for optimal human peripheral blood lymphocyte DNA synthesis

We examined whether L-arginine is a substrate for nitric oxide (NO) production by peripheral blood mononuclear cells (MNC) in vitro. Minimal extracellular arginine (0.04 mmol/L) is required for maximal lymphocyte proliferation after phytohemagglutinin stimulation. In the absence of arginine, proliferation was 41% of normal without loss of viability. In contrast, MNC total protein synthesis (as assessed by tritiated leucine incorporation) or lymphokine synthesis (interleukin-2, as assessed by cytotoxic lymphoid line (CTLL) proliferation) were not affected by the absence or presence of arginine in the medium. Exogenous nitric oxide provided as sodium nitroprusside could replace L-arginine for maximal blastogenic proliferation. The addition of N(G)-monomethyl-L-arginine (NMMA; 0.1 mmol/L), a specific inhibitor of the NO synthetic pathway, significantly reduced DNA synthesis both at 0 and 0.1 mmol/L arginine concentrations; this effect was reversed to 91% of normal by excess arginine (1.0 mmol/L). Homoarginine (0.1 mmol/L; a known substrate for NO production) partially substituted for arginine, and this effect was also abrogated by NMMA. Nitrite levels (an end product of NO metabolism) were reduced when L-arginine was absent or NMMA was added to L-arginine-containing media. Cytosol from phytohemagglutinin-stimulated MNC-enhanced cyclic guanosine monophosphate production in the presence of L-arginine as substrate. The data suggest that the inductive effects of L-arginine on MNC DNA synthesis are not related to its nutrient requirement for protein synthesis, but rather caused by its role as a substrate for NO production. MNC actively synthesize NO during mitogenic proliferation. NO appears to be a promoter of MNC DNA synthesis, probably by its well-known effect as an activator of guanylate cyclase, which increases cyclic guanosine monophosphate levels.