Zn²⁺ inhibits nitric oxide formation in response to lipopolysaccharides: Implication in its anti-inflammatory activity

There is compelling evidence to indicate an anti-inflammatory action of Zn²⁺. Most inflammatory diseases are associated with an increase of the inducible form of nitric oxide (NO) synthase. Additionally, inflammatory mediators such as histamine or bradykinin stimulate the constitutive NO synthase. Thus, the present study was undertaken to investigate whether Zn²⁺ inhibits production of inducible NO synthase and/or constitutive NO synthase activity to produce NO. Lipopolysaccharide, 5 mg/kg i.v., administered to Zn²⁺-deficient (ZD) rats, rats supplemented with Zn²⁺ sulfate (ZG), 10 mg/kg s.c., or controls resulted in a significant reduction of their serum Zn²⁺. The levels of N(G)-nitro-L-arginine methylester (L- NAME)-sensitive cyclic GMP (cGMP) in aortas isolated from ZD or ZG were significantly lower than those obtained from control animals. Zinc (100-150 μM) produced a dose-dependent inhibition of lipopolysaccharide or interleukin-1β-induced NO formation in isolated rat aortic smooth muscle cells. Compared to cyclohexamide or actinomycin-D, the time course of inhibition of NO formation by 150 μM Zn²⁺ did not suggest an effect of Zn²⁺ on inducible NO synthase protein synthesis. Moreover, Zn²⁺ (150 μM) significantly reduced the rate of conversion of [³H]arginine to [³H]citrulline in lung homogenates from lipopolysaccharide-treated rats. Incubation of rat aortic smooth muscle cells and bovine pulmonary artery endothelial cell co-cultures with Zn²⁺ (150 μM) caused a significant reduction in basal and bradykinin- or A-23187-induced formation of cGMP. Thus, our results indicate that Zn²⁺ is capable of inhibiting lipopolysaccharide- or interleukin-1β-induced NO formation as well as NO formation by constitutive NO synthase basally or in response to bradykinin or A-23187, and may explain the reported anti-inflammatory activity of Zn²⁺.