Journal of Hypertension:
Ouabain decreases reactive oxygen species and salvages nitric oxide: or is it the other way around?
We all know that blocking the electrogenic Na+/K+ ATPase pump, ouabain depolarizes the vascular smooth muscle, increases peripheral resistance and induces hypertension. Why? Because in physiological conditions, three Na+ are pumped out of the cell in exchange of only two K+, which maintains membrane potential in the low negative values. The consequences of hypertension, at least in the long-term and irrespectively of the origin, are however rather unfriendly to the cardiovascular system and shorten life span . If one only considers the vasculature, hypertension leads to endothelial dysfunction, increases free radical production and ultimately promotes atherosclerosis.
In the current issue of the Journal, Hernanz et al.  demonstrate that ouabain-induced hypertension is associated with an increased basal nitric oxide bioavailability, leading to better endothelium-dependent relaxation of isolated rat basilar arteries. The authors concluded that a reduction in superoxide production was responsible for this favourable response, even though there is no change in the total antioxidant status in these animals. These data are surprising because they are counterintuitive at first.
The reason for a reduced superoxide production after 5 weeks of ouabain treatment is not known, but two hypotheses can be put forward. One is based on the well known fact that nitric oxide and the endothelium-derived hyperpolarizing factor (EDHF) compete and/or compensate for the failure of each other. Inhibition of nitric oxide production reveals the dilatory effect of EDHF and vice versa [3,4]. As very well demonstrated by a study by Taddei et al.  of hypertensive patients, a ouabain-sensitive dilation compensates for the loss of nitric oxide-mediated dilation, whereas vitamin C acutely restores the nitric oxide-dependent dilation. Thus, ouabain may block the EDHF pathway and favour the efficacy of nitric oxide. Decreases in superoxide may be secondary to the increase in nitric oxide, the latter being an excellent antioxidant in physiological conditions .
A second hypothesis is that by increasing intracellular calcium in the smooth muscle, Ca2+ may be transferred to the endothelium by gap junctions, and so increase nitric oxide production. This mechanism is considered as a negative feedback that may counteract excess in tone . The use of a gap junction inhibitor could validate this hypothesis.
Cerebral arteries , as well as arteries from other districts [9,10], depend on free radicals for normal vasomotor tone regulation, and a rigorous regulation of the redox environment is likely to be essential. This raises a methodological concern: at a physiological pO2 value, eNOS cycling produces superoxide that leads to H2O2-dependent dilation in mouse cerebral arteries , suggesting that at least in mice, eNOS activity can produce physiologically relevant levels of H2O2 rather than nitric oxide. Using a 95%O2 gas mixture raises the pO2 to more than 400 mmHg, which is highly pro-oxidative and drives eNOS to produce nitric oxide , whereas it is known that free radicals block the EDHF targets on the smooth muscles [12,13]. This would be a cause for a ‘perceived’ increase in nitric oxide activity in the presence of ouabain in these experimental conditions. If the same experiment was to be performed at a normal pO2 value, would nitric oxide still be increased by 5-week treatment with ouabain?
The study by Hernanz et al.  is of interest because it confirms how plastic the cardiovascular system is and how it can adapt to short-term challenges. But most importantly, it is likely that the changes observed are not sustainable because of the haemodynamic stress induced by the increase in peripheral resistance, not to mention the direct effect on cardiac function. The next step would be to investigate the vascular impact of a long-term inhibition (≫ 5 weeks) of the EDHF pathway in hypertensive conditions. It is known that there is more than one EDHF , but by clamping membrane potential, ouabain is likely to prevent the action of all EDHF. Because dilation is essential to match metabolic demand, and because the endothelium is an important player of this response, what will be its long-term adaptation after the likely loss of nitric oxide associated with the endothelial damage triggered by hypertension? There is evidence that the loss of nitric oxide is first compensated by EDHF as in the study by Taddei et al. . But in this model, would dilatory prostanoids compensate? These are potential perspectives for a future interesting study.
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