High-fat diet improves tolerance to myocardial ischemia by delaying normalization of intracellular PH at reperfusion

Highlights

• HFD increases myocardial tolerance to ischemia in a strain-dependent way.

• HFD inhibits glucose oxidation and induces mild respiratory mitochondrial uncoupling.

• The resulting deficient energy metabolism delays pHi normalization at reperfusion.

• Delayed pHi recovery protects myocardium against reperfusion injury.

• Results could explain the better outcome observed in some obese patients with STEMI.

Abstract

Reports on the effect of obesity on the myocardial tolerance to ischemia are contradictory. We have described that obesity induced by high-fat diet (HFD) reduces infarct size in B6D2F1 mice submitted to transient coronary occlusion. In this study, we analysed the mechanism by which dietary obesity modifies the susceptibility to myocardial ischemia and the robustness of this effect. B6D2F1 (BDF), C57BL6/J (6J), C57BL6/N (6N) male mice and BDF female mice were fed with a HFD or control diet for 16 weeks. In all three strains, HFD induced obesity with hyperinsulinemia and hypercholesterolemia and without hyperglycemia, hypertension, ventricular remodelling or cardiac dysfunction. In obese mice from all three strains PDK4 was overexpressed and HSQC NMR spectroscopy showed reduced ¹³C-glutamate and increased ¹³C-lactate and ¹³C-alanine, indicating uncoupling of glycolysis from glucose oxidation. In addition, HFD induced mild respiratory uncoupling in mitochondria from BDF and 6N mice in correlation with UCP3 overexpression. In studies performed in isolated perfused hearts submitted to transient ischemia these changes were associated with reduced ATP content and myocardial PCr/ATP ratio at baseline, and delayed pHi recovery (³¹PNMR) and attenuated hypercontracture at the onset of reperfusion. Finally, in mice subjected to 45 min of coronary occlusion and 24 h of reperfusion, HFD significantly reduced infarct size respect to their respective control diet groups in male BDF (39.4 ± 6.1% vs. 19.9 ± 3.2%, P = 0.018) and 6N mice (38.0 ± 4.1 vs. 24.5 ± 2.6%, P = 0.017), and in female BDF mice (35.3 ± 4.4% vs. 22.3 ± 2.5%, P = 0.029), but not in male 6J mice (40.2 ± 3.4% vs. 34.1 ± 3.8%, P = 0.175). Our results indicate that the protective effect of HFD-induced obesity against myocardial ischemia/reperfusion injury is influenced by genetic background and appears to critically depend on inhibition of glucose oxidation and mild respiratory mitochondrial uncoupling resulting in prolongation of acidosis at the onset of reperfusion.

Introduction

Obesity has pandemic proportions, its prevalence is steadily increasing and it represents a major independent risk factor for myocardial infarction and heart failure [1]. Yet, the effect of obesity on the outcome of acute coronary syndrome is controversial with clinical studies suggesting a paradoxical favourable effect of obesity [2]. Moreover, different studies have associated obesity with smaller infarct size in patients undergoing primary PCI for STEMI [[3], [4], [5], [6]]. The experimental studies analysing the effect of obesity on the myocardial susceptibility to ischemia/reperfusion (IR) injury show discrepancies, with studies reporting both cardioprotective [[7], [8], [9]] and deleterious effects of obesogenic diets [[10], [11], [12]]. These discrepancies could reflect the association of obesity with other comorbidities including type 2 diabetes mellitus (DM2), hypertension or dyslipidemia that may modify the tolerance to IR. In addition, growing evidence suggests that the genetic differences between strains may impact the response of the myocardium to an obesogenic diet [13,14]. Recently, we have demonstrated in B6B2F1 (BDF) mice fed with a high fat diet (HFD) that obesity in the absence of hyperglycemia, hypertension or altered vascular reactivity, increases the tolerance to transient ischemia [15], but the mechanisms underlying this beneficial effect remains to be elucidated. Since C57BL/6 strain is widely used as a model for diet-induced obesity and most transgenic models are developed in this genetic background, it is necessary to determine whether the same myocardial response to a HFD occurs in this strain. In addition, because it has been described that the gender may affect the myocardial response to a high carbohydrate diet [16] and cardioprotective strategies [17], the impact of the gender on the effect of HFD to the susceptibility to IR needs to be determined.

Under normal conditions, the metabolic machinery of cardiomyocytes is highly flexible to switch its substrate preference from fatty acids to glucose in a variety of stress situations such as myocardial infarction [18]. Obesity alters this tightly regulated and dynamic relationship between FAO and glucose oxidation (GO) that characterizes a healthy myocardium, and different studies propose that the resulting loss of metabolic flexibility may influence the myocardial tolerance to ischemia [19]. HFD feeding increases the rate of FAO and uncouples glycolysis from GO by inhibiting pyruvate dehydrogenase (PDH) impairing cardiomyocytes to switch to increased glucose utilization under stress conditions [20]. The resulting uncoupled glucose metabolism increases H⁺ production, an effect that may modify the recovery of intracellular pH (pHi) during reperfusion [21]. Furthermore, obesity can also uncouple mitochondrial respiration and reduce energy production [22], which is critical to normalize pHi at reperfusion. Sound experimental evidence demonstrates that prolongation of acidosis during the first minutes of reperfusion has cardioprotective effects by preventing the activation of many molecular mechanisms implicated in reperfusion injury [23]. Therefore, based on these previous observations, we hypothesize that obesity induced by HFD increases the tolerance to IR injury by delaying the recovery of pHi at the onset of reperfusion due to a deficient energy metabolism.