Participants were fasted for 8 to 15 h and oral glucose loading was used to maximize PDH flux and standardize metabolic conditions in both the rest and stress study [
16,
20,
21]. Adenosine is a validated and widely used pharmacological stress test in cardiac imaging [
10,
22,
23]. We found significant increases in HR, rate pressure product and cardiac output during the adenosine stress. The HP observations and the haemodynamic response must reflect both the systemic and coronary effects of adenosine. Adenosine induces coronary and systemic vasodilatation and reflex tachycardia [
24]. Semi-parametric measures of the large blood pool component of [1-
13C]pyruvate was used to evaluate the hemodynamic response to adenosine. We found a significant reduction in the TTP and FM for [1-
13C]pyruvate in the mid-LV myocardium (inversely correlated to perfusion [
25]) consistent with increased HR and thus pyruvate delivery. The increased myocardial signal of pyruvate during stress, suggest an enhanced myocardial [1-
13C]pyruvate uptake likely due to coronary vasodilation, consistent with previous experimental findings [
26]. However, unfortunately we cannot distinguish how much of the increased [1-
13C]pyruvate signal is derived from increased myocardial uptake and how much is caused by increased vascular signal due to vasodilation. In this regard, it is interesting that we observed a tendency towards increased [1-
13C]alanine signal which has been suggested as a surrogate measure for pyruvate uptake [
16]. We observed a statistically significant increases in PDH flux as well as [1-13C]pyruvate to [1-
13C] lactate exchange during stress. The relative increase in [1-
13C]lactate signal is explained by [1-
13C]pyruvate to [1-
13C]lactate exchange, which depend on the delivery of pyruvate, on the rate of pyruvate uptake and subsequently on the concentration of LDH and its substrate pool sizes. The relative increase in PDH flux was larger, demonstrating that the healthy heart can increase oxidative energy production during moderate stress. This agrees with previous animal studies and invasive human studies, showing activation of PDH and increased pyruvate oxidation in response cardiac stress [
27]. Interestingly, a similar metabolite dynamic pattern (TTP) of the individual metabolites was observed, which seem to contradict the increased metabolic conversion (TTP of the individual metabolite dynamic curve is inversely correlated with the apparent rate constant of the individual metabolites) [
28]. It is important to note that the temporal dynamics of the metabolites are 3 times lower than that of pyruvate and thus, could at least partly explain this surprising finding. Also, given the voxel size, partial volume effects may influence these results. Finally, due to coronary vasodilation, coronary vasculature may be a significant part of the pyruvate signal in the myocardium and thus may influence TTP. Further studies are needed to solve these issues.