Implication of Mitochondrial NO/cGMP/PKG Signaling System in the Activation and Inhibition of Mitochondrial Respiration by L-Arginine and NO Donors

The involvement of the mitochondrial calcium-dependent NO synthase (mtNOS) in the regulation of mitochondrial respiration has not been sufficiently studied. Moreover, the possible functioning of a mitochondrial signaling system involving mtNOS/guanylate cyclase (GC)/protein kinase G (PKG) and the impact of this system (mtNOS/GC/PKG-SS) on mitochondrial respiration have not yet been analyzed. To investigate this issue, we performed experiments on isolated rat liver using specific inhibitors of NOS, GC, and PKG. The high rate of mitochondrial respiration was supported by pyruvate and glutamate or by succinate in the presence of hexokinase, glucose, and ADP. It was shown that L-arginine and the NO donor sodium nitroprusside (SNP) exert concentration-dependent effects on the mitochondrial respiration rate. At low concentrations, L-arginine (up to 200 mu M) and SNP (up to 100 mu M) activated mitochondrial respiration. The inhibitors of NOS, GC, and PKG eliminated this effect indicating that mtNOS/GC/PKG-SS is involved in the activation of respiration. At high concentrations, L-arginine and SNP, on the contrary, inhibited respiration. Under these conditions, the inhibitors of GC and PKG enhanced the inhibition of respiration, which indicates an opposite effect of the excess of NO and PKG on the mitochondrial respiration. The results suggest that the functioning of calcium-dependent mtNOS/GC/PKG-SS can ensure the activation of respiration at low concentrations of L-arginine or SNP in the medium.