In this study, the benefits of the application of inosine during reperfusion were assessed after cardiac arrest in a canine model of extracorporal circulation. We have shown that inosine improves left ventricle and endothelial function recovery after cardioplegic arrest.
Until recently, inosine was considered an inactive purine metabolite in most biological systems, but several recent studies have shown that it has immuno-modulatory [10
], neuroprotective [11
], cardioprotective [12
] and overall cytoprotective effects. Furthermore, some other studies reported that extracellular inosine has powerful cellular protective effects. For example, inosine decreases the release of intracellular enzymes from hypoxic lymphocytes [13
], improves renal function during ischemia [14
] and inhibits inflammatory cytokine production [7
]. Administration of inosine has also been shown to improve myocardial function during acute left ventricular failure [15
] and improve myocardial and endothelial function after heart transplantation [12
]. Despite the growing evidence of protective effects of inosine, the use of inosine for prevention of reperfusion injury in the context of cardiopulmonary bypass has not yet been investigated.
The most cardiac surgical procedures require cardiopulmonary bypass and cardioplegia, which causes ischemia-reperfusion injury. Ischemia-reperfusion injury initiates pathophysiological cascades including an inflammatory response with liberation of cytokines and free radicals. Triggered by peroxynitrite-induced DNA single-brand breaks, PARP catalyzes an energy-consuming polymerization of ADP-ribose, resulting in NAD depletion, inhibition of glycolysis, and the reduction of intracellular high-energy phosphates in the reperfused heart. In various types of ischemia-reperfusion, the prevention of PARP activation results in better preservation of the high-energy phosphates, resulting in an improved energy status [17
]. It is also demonstrated, that PARP activation occurs during the reperfusion but not during the ischaemia [20
]. Our group demonstrated previously, that inosine inhibits PARP activation in vivo and therefore modulates oxidant-induced cell death [12
]. To the best of our knowledge, this was the first study that showed the effectiveness of inosine during the reperfusion in a clinically relevant large animal model of cardiopulmonary bypass. Administration of inosine during the reperfusion improved both systolic and diastolic indices of left ventricular contractility. In the current study, the load-independent indices such as preload recruitable stroke work (PRSW) and the slope of end-systolic pressure-volume relationship (ESPVR) of the left ventricle in the inosine group remained practically unchanged when compared with the baseline to 60-minutes reperfusion values. It is also well known, that the active phase of ventricle relaxation is an energy consuming phase of the cardiac circle, much like that of contraction. In the present study, we demonstrated that reperfusion injury is associated with impaired cardiac relaxation and diastolic dysfunction, as reflected by prolonged time constant of pressure decay (Tau) and increased LVDP. The decreased value of Tau in the inosine group clearly showed that inosine may significantly improve left ventricular diastolic function after CPB.
The increase of coronary blood flow during reperfusion contributes also to the improvement of cardiac function. Previous studies demonstrated that inosine increased coronary flow dose-dependently and, as a consequence, the function of the reperfused heart [21
]. Our present data is in direct correspondence with these studies showing significant decrease of the coronary blood flow in the control group and unchanged CBF-values in the inosine group after reperfusion. This effect is comparable to those of with application of nitric oxide donors [23
], endothelin receptor antagonists [24
], or PARP-inhibitors during CPB [9
]. How inosine protects the endothelium remains not completely understood. Previous data suggest that energy depletion severely impairs endothelial function [19
]. It also demonstrated that adenosine and inosine breakdown may present an energy source to be preferred over extracellular glucose under hypoxia, as they delay the accumulation of NADH+ H+
, thereby maintaining some vital cellular functions. Inosine may exert some of its cytoprotective effects under ischemia by providing an emergency energy source, when glucose is insufficient to support cellular functions. The above hypothesis is supported both by several reports that cellular ATP levels were elevated in ischaemic or hypoxic cells treated with adenosine or inosine [26
] As inosine restores ATP levels, this may contribute to improved endothelial function. If inosine has a direct effect on nitric oxide synthesis remains to be clarified.