The NAD
+/NADH ratio regulates metabolic pathways such as glycolysis and oxidative phosphorylation in the mitochondria. Although the protective effects of NAD
+ precursors have been mostly ascribed to de-acetylation by sirtuin activation [
23], it has recently been shown that NMN reduced cardiac IR injury of isolated mouse hearts, primarily through the activation of glycolysis [
34]. NR and NMN are both NAD
+ precursors, but their distribution, metabolism and compartmentation are different [
6,
8,
23], and it was thus unknown whether NR would constitute similar protective effects as NMN in the isolated heart. Here we show that NR affected cardiac IR injury similarly as reported for NMN: it protected hearts perfused with glucose and fatty acids, and protection was associated with glycolysis activation. Although NR was used at lower dosage in our study than NMN in the study by Nadtochij et al. [
34] (0.17 mM NR versus 1 mM NMN for 20–25 min administration), the increase in NAD
+ was larger for NR (two times increase) than for NMN treatment (1.4 times increase), supportive of the observation that NR has a higher bioavailability than NMN [
48]. Glycolysis activation is an established and primary mechanism through which the heart can protect itself against IR injury, playing an important role in the cardioprotective actions of e.g., ischemic preconditioning, metformin, insulin, volatile anesthetics, adenosine, NO donors, Hypoxia-inducible factor 1-a (HIF1α) stabilizers and 5' adenosine monophosphate-activated protein kinase (AMPK) activators [
58]. Glycolysis activation may protect against cell death through (1) facilitating glucose phosphorylation at mitochondrially bound hexokinase thereby lowering mitochondrial potential, mitochondrial ROS production, mitochondrial activation and protection against mitochondrial damage [
7,
18,
45,
60], (2) facilitating ion pumps and exchanger at the plasma membrane knowing that these pumps mainly use glycolytically produced ATP [
25], and (3) maintaining a low pH during early reperfusion thereby inhibiting opening of the mPTP [
20]. In contrast to protection against cell death, increased glycolysis relative to glucose oxidation may be detrimental for the recovery of mechanical function of the heart [
30]. However, this was not observed in the current study. It should thereby also be realized that the cellular mechanisms determining recovery of cardiac function after IR deviate from those cellular mechanisms dictating cell death, explaining the sometimes observed dichotomy between recovery of function and cell death after cardiac IR [
37,
47]. Further experiments with blocked glycolysis in the presence of glucose and fatty acids are necessary to consolidate that NR’s protection is indeed through glycolysis activation, although the loss of protection in hearts without glycolysis or hearts with highly activated glycolysis already indicate a cause-effect relationship between NR and glycolysis activation for protection.