Nitric oxide (NO) is an endogenous vasodilator and inhaled NO is a promising therapeutic agent for the treatment of pulmonary hypertension. However, NO's mechanism of action is not completely understood. Previous studies have shown that NO increases intracellular levels of cyclic guanosine 3',5'-monophosphate (cGMP) and that leads to activation of calcium-gated potassium channels in vascular smooth muscle cells. Resulting cell membrane hyperpolarization causes vasorelaxation. The potassium channel activation by NO is inhibited by a blockade of cyclic nucleotide-dependent protein kinases, suggesting a key role of these enzymes in NO-induced vasodilation. To further examine this mechanism, we tested the hypothesis that pharmacological stimulation of the cGMP-dependent protein kinase will simulate the activating effect of NO on potassium channels. Indeed, we found that (Sp)-guanosine cyclic 3’,5'-phosphorothioate (1 /¿M), a selective activator of the cGMP-dependent protein kinase, dramatically increased potassium currents measured by the whole-celi patch clamp technique in freshly dispersed pulmonary artery smooth muscle cells. These currents were inhibited by an inhibitor of calcium-gated potassium channels, charybdotoxin. Our results support the hypothesis that the effect of NO on potassium channels is mediated by the cGMP-dependent protein kinase.
Pneumonia was induced in rats by instillation of carrageenin (0.5 ml of 0.7 % solution) into the trachea. Three or four days after instillation, the lungs were isolated, perfused with blood of healthy rat blood donors, and ventilated with air + 5 % C02 or with various hypoxic gas mixtures. Pulmonary vascular reactivity to acute hypoxic challenges was significantly lower in lungs of rats with pneumonia than in lungs of controls. The relationship between 02 concentration in the inspired gas and Po2 in the blood effluent from the preparation was shifted significantly to lower Po2 in lungs with pneumonia compared to control ones. These changes were not present in rats allowed to recover for 2- 3 weeks after carrageenin instillation. We suppose that blunted hypoxic pulmonary vasoconstriction may contribute to hypoxaemia during acute pulmonary inflammation. Decreased Po2 in the blood effluent from the isolated lungs with pneumonia implies significant increase of oxygen consumption by the cells involved in the inflammatory process.
A common problem in management of polytrauma – a simultaneous injury to more than one organ or organ system, at least one of them lethal without intervention – is a discrepancy between a relatively good initial state and a serious subsequent development. Since nitric oxide (NO) is produced in high quantities during tissue injury, we assumed that serum levels of NO (and its oxidation products, NOx) might serve as a prognostic marker of polytrauma severity. However, we found recently that NOx was increased in polytrauma, but not in the most severe cases. The present study was undertaken to test the hypothesis that serum NOx is reduced in severe polytrauma by concomitant overproduction of reactive oxygen species (ROS). Polytrauma was induced in rats under anesthesia by bilateral fracture of femurs and tibiae plus incision of the right liver lobe through
laparotomy. Serum NOx was measured by chemiluminescence after hot acidic reduction. The role of ROS was assessed by treatment with an antioxidant, N-acetyl-L-cysteine (NAC). Experimental polytrauma elevated NOx from 11.0±0.7 to 23.8±4.5 ppb. This was completely prevented by NAC treatment (9.1±2.2 ppb). Serum NOx is elevated in severe polytrauma, and this is not reduced by ROS. On the contrary, ROS are necessary for the NOx elevation, probably because ROS produced by inflammatory cells activated by the polytrauma induce massive NO production.