Neuronal nitric oxide inhibition attenuates the protective effect of HBO2 during cyanide poisoning

Experiments have shown that hyperbaric oxygen (HBO2) therapy reduces cyanide-induced cerebral distress. The exact mechanism behind HBO2's neuroprotective effect is unknown, but has been proposed to be mediated by an increased neuronal nitric oxide (NO) bioavailability, which may compete with cyanide for the active site of cytochrome oxidase in the mitochondrial respiratory chain. We hypothesized that the ameliorating effect of HBO2 is caused by an increased bioavailability of NO, which can be attenuated by injection of the selective neuronal NO synthase inhibitor, 7-nitroindazole, preceding the HBO2 procedure.

A total of 41 anesthetized female Sprague-Dawley rats were allocated to four groups: 1) vehicle [1.2 ml isotonic NaCl via intra-arterial administration]; 2) cyanide [5.4 mg/kg potassium CN (KCN) intra-arterial] plus 7-nitroindazole [25 mg/kg 7-nitroindazole via intraperitoneal injection]; 3) cyanide plus 7-nitroindazole plus HBO2 [284 kPa for 90 minutes]; 4) cyanide plus 7-nitroindazole plus normobaric oxygen [101.3 kPa for 90 minutes]. Cerebral interstitial lactate, glucose, glycerol and pyruvate were evaluated by means of microdialysis.

HBO2 during inhibition of nNOS worsened cerebral metabolism compared to both solely CN-intoxicated animals and normobaric oxygen-treated animals. This was indicated by elevated lactate (in mM; 0.85 vs. 0.63 and 0.42, P=0.006 and P ⟨ 0.001, respectively), glycerol (in mM; 46 vs. 17 and 14, both P ⟨ 0.001), glucose (in mM; 0.58 vs. 0.31 and 0.32, both P ⟨ 0.001).

The results indicate that a specific nNOS inhibition offsets the ameliorating effect of HBO2 during cerebral CN intoxication. However, other factors might contribute to this neuroprotective effect as well.