Neural elements behind the hepatoprotection of remote perconditioning

Abstract

Background

The ability of remote ischemic perconditioning (RIPER) to protect the liver from ischemic–reperfusion (IR) injury has been reported before; however, the mechanism behind the positive effects of RIPER remains unrevealed. Therefore, we aimed to investigate the potential role of neural elements to transfer protective signals evoked by perconditioning.

Materials and methods

Male Wistar rats were randomly allocated into six groups (sham, IR, RIPER ± denervation; n = 7 per group). Half of the animals underwent left femoral and sciatic nerve resection. In IR and RIPER groups, normothermic, partial (70%) liver ischemia lasting for 60 min was induced; parallel animals in the RIPER groups received perconditioning treatment (4×5–5 min IR, left femoral artery clamping). Hepatic microcirculation and systemic blood pressure were monitored during the first postischemic hour. After 24 h of reperfusion, liver samples were taken for histology and redox-state analysis. Automated image analysis software was used for necrosis quantification. Serum alanine aminotransferase, aspartate aminotransferase, and bilirubin levels were measured.

Results

Microcirculation and blood pressure showed significant improvement during reperfusion after perconditioning. This phenomenon was completely abolished by nerve resection (P < 0.05; RIPER versus IR, IR + denervation, and RIPER + denervation). Results of necrosis quantification showed similar pattern. Besides noncharacteristic changes in aspartate aminotransferase levels, alanine aminotransferase values were significantly lower (P < 0.05) in the RIPER group compared with the other IR groups. Mild but significant alterations were observed in liver function assessed by total bilirubin levels. Further supporting results were obtained from analysis of redox homeostasis.

Conclusions

Perconditioning was able to reduce liver IR injury in our model via a mechanism most probably involving interorgan neural pathways.

Introduction

Liver ischemic–reperfusion (IR) injury is an important cause of remnant liver and/or graft damage and one of the most influential factors regarding outcome in patients undergoing major liver surgeries, such as major liver resection and liver transplantation [1], [2]. Since the first report in 1975 by Toledo-Perayra et al. [3] regarding liver reperfusion injury after transplantation of canine livers, several methods were developed to reduce liver IR injury in different experimental and clinical settings.

The concept of ischemic preconditioning was introduced by Murry et al. [4] in 1986. Thanks to persistent research work over the past three decades, a huge amount of scientific data have accumulated concerning local ischemic preconditioning and postconditioning [5], [6]; in parallel, an increasing demand emerged for more feasible methods, for techniques which can be easily translated to clinical practice.

The seminal study of Przyklenk et al. [7] demonstrated that brief IR intervals on coronary arteries can reduce myocardial necrosis caused by the sustained occlusion of a coronary branch supplying a different, distant myocardial area. This basic notion of intraorgan conditioning was then extended beyond the heart, and several studies demonstrated that target organ protection can similarly be achieved by conditioning on non-vital organs—for example, lower limbs or arms—resulting in a feasible, low-risk remote ischemic preconditioning (RIPC) procedure [8]. The shortcomings of preconditioning urged researchers to search for unrevealed ways to attain organ protection. The term “remote ischemic perconditioning” (RIPER) first occurred in the literature in 2007 [9]. The RIPER approach refers to an endogenous protective mechanism, which is achieved by brief episodes of ischemia and reperfusion of a distant organ or tissue, applied after the onset of target organ ischemia, but before reperfusion.

Several experimental and clinical studies with different target organs have shown that strong protection can be achieved using limb RIPER [10], [11], [12], [13], [14], whereas the underlying mechanisms behind this phenomenon remain unclear [15]. According to the prevailing hypothesis today, a certain neural-facilitated pathway might be one of the possible connective mechanisms providing transfer of the protective signal to the target organ [15], [16]. Nevertheless, only a few reports are available, which investigate the mechanistic background of RIPER, and there are no data so far about the role of neural elements behind the effect of this conditioning technique [15].

Presumably, different mediators (adenosine, bradykinin etc.) released from the remote organ during the short IR episodes and stimulating local neural elements are responsible for the triggering of the complex neural mechanisms [15]. It has been proposed as a suspected trigger that these mediators could activate capsaicin-sensitive sensory neurons to release calcitonin gene related peptide and thus induce a subcellular protection with the participation of kinase cascades [17]. Based on previous reports [8], [15] in regards of other conditioning techniques, in the present study we assumed that severing these neural connections of the remote organ before the conditioning treatment may have an effect on perconditioning-induced hepatoprotection.

Our laboratory was the first to report the favorable effects of RIPER on the IR injury of the liver [13], [18]. In the present study, our aim was to investigate the effects of lower limb ischemic perconditioning treatment, to confirm or disprove the role of intact remote organ innervation in transferring protective signals.