Discussion
cardiac IR injury, hypoxia, hypertension, IR injury
ACUTE MYOCARDIAL INFARCTION, MYOCARDIAL INFARCTION, HYPOXIA, HYPOXIC, VASODILATATION, EVENTS, HYPERTENSION
Here, we show that RBCs under hypoxic conditions induce cardioprotection through a mechanism that is dependent on sGC in the RBC and associated with export of cGMP that, via a paracrine effect, is involved as a mediator of the cardioprotection through activation of PKG in the heart. This cardioprotective effect is boosted by a dietary intervention with inorganic nitrate. The strict involvement of RBC sGC in the cardioprotective effect was demonstrated using both pharmacological and gene-targeted approaches. Finally, we demonstrate that this cardioprotective signaling by RBCs also occurs in humans following intake of dietary nitrate in amounts readily achieved through a rich intake of green leafy vegetables.It has been suggested that RBCs are capable of inducing hypoxic vasodilatation through the generation of a signal during deoxygenation of hemoglobin (We next hypothesized that dietary supplementation with inorganic nitrate — a precursor of nitrite that is further metabolized in blood (An interesting issue is how hypoxia leads to sGC activation in the RBC with downstream cGMP signaling resulting in cardioprotection. RBCs are known to be a major source of nitrite (A major strength of the present study is that we were able to translate these findings to the clinical setting with validation of the results in RBCs from patients with hypertension. RBCs collected from patients on a high nitrate diet afforded marked sGC-dependent cardioprotection compared with RBCs from the group receiving a low-nitrate diet. The cardioprotection was of comparable magnitude irrespective of whether nitrate was given in the form of nitrate pills or nitrate-rich vegetables, strongly supporting that the effect was mediated by nitrate per se. The observation that dietary nitrate induces RBC-mediated protection against cardiac IR injury in this patient group is of interest considering the importance of hypertension as a risk factor for myocardial infarction. It should be emphasized that the amount of nitrate was not more than what can readily be consumed with a normal diet. To study if this cardioprotective effect of nitrate is seen also in vivo in humans will obviously require a prospective clinical trial. Nevertheless, it is interesting that green leafy vegetables (the major dietary nitrate source) constitute a central part of diets associated with reduction in cardiovascular events (This study, along with our earlier studies (This study has certain limitations that need to be considered. First, the cardioprotective effects of the RBCs are demonstrated using RBCs ex vivo in an isolated heart model. It remains to be demonstrated to what degree NO bioactivity derived from hypoxic RBCs may mediate cardioprotection in vivo. Earlier studies have shown that oral administration of nitrite and nitrate protects the heart from IR injury (In conclusion, the present results reveal a fundamental endogenous protective mechanism mediated by RBCs when exposed to hypoxic conditions. To the best of our knowledge, this is the first demonstration of intercellular or paracrine signaling by cGMP exported from hypoxic RBCs. The abundance and ubiquitous presence of RBCs make them highly suitable for the purpose of tissue protection, suggesting that this may represent a physiological defense mechanism designed to minimize IR injury. This finding may provide an explanation to the recently observed cardioprotective effect of RBCs collected from patients with acute myocardial infarction (