Role of arginase in vascular function

Abstract

Background: Nitric oxide (NO) is central for the integrity of the cardiovascular system, the maintenance of endothelial function and the protection against ischaemic heart disease. The enzyme arginase is up-regulated during ischaemia-reperfusion and by hypoxia in cell culture and animal models which might be of pathophysiological relevance since it competes with NO synthase for their common substrate arginine. The aim of the studies was to clarify the role of arginase in cardiovascular disease related to ischaemia and hypoxia including myocardial ischaemia and reperfusion injury, heart failure and following resuscitation after cardiac arrest by investigating the therapeutic effect of arginase inhibition and its association to increased NO bioavailability.

Studies I-II: To study the relevance of arginase in the context of myocardial ischaemia and reperfusion two different animal models were used. In a rat model, the animals were treated with an arginase inhibitor (Nω-hydroxy-nor-L-arginine, nor-NOHA) alone or together with substances inhibiting NO or its production intravenously before the onset of ischaemia. The infarct size was reduced by 50 % following administration of the arginase inhibitor. The cardioprotective effect was completely dependent on NO synthase activity and NO activity. Ischaemia and reperfusion was associated with increased expression of arginase I in the ischaemic myocardium. Arginase inhibition induced a 10-fold increase in the citrulline/ ornithine ratio as an indirect enzyme activity measure, indicating a shift in arginine utilization from arginase towards NO synthase. In a subsequent study this concept was investigated in a large animal (pig) model of myocardial ischaemia and reperfusion with intracoronary drug administration in connection with reperfusion. Administration of nor-NOHA resulted in a profound cardioprotection comparable to that observed in rats. Parallel groups confirmed that the cardioprotective mechanism was dependent on NO production.

Studies III-IV: Circulating levels of arginase I were determined in patients with heart failure and following cardiopulmonary resuscitation as well as in healthy volunteers after global hypoxia in an normobaric hypoxia chamber. These conditions were all associated with increased levels of arginase I. In addition, the effect of topical application of nor-NOHA on the sublingual mucosa on microvascular perfusion was studied using a sidestream darkfield microcirculation camera. The impaired microcirculation in heart failure and in patients following resuscitation was improved by local nor-NOHA incubation via a NO-dependent mechanism.

Conclusions: Inhibition of arginase protects from myocardial ischaemia and reperfusion injury by a mechanism that is dependent on NO production and increased bioavailability of NO by shifting arginine utilization towards NO production. In addition, we showed that heart failure, global hypoxia and cardiopulmonary resuscitation lead to increased plasma levels of arginase I. Impaired microcirculatory perfusion in these patients is improved following topical arginase inhibition by a NO dependent mechanism. Inhibition of arginase is a promising potential treatment target for protection against myocardial ischaemia and reperfusion injury and to ameliorate microcirculatory dysfunction in critically ill patients.