Cardiac Ryanodine Receptor (Ryr2)-mediated Calcium Signals Specifically Promote Glucose Oxidation via Pyruvate Dehydrogenase

Cardiac ryanodine receptor (Ryr2) Ca²⁺ release channels and cellular metabolism are both disrupted in heart disease. Recently, we demonstrated that total loss of Ryr2 leads to cardiomyocyte contractile dysfunction, arrhythmia, and reduced heart rate. Acute total Ryr2 ablation also impaired metabolism, but it was not clear whether this was a cause or consequence of heart failure. Previous in vitro studies revealed that Ca²⁺ flux into the mitochondria helps pace oxidative metabolism, but there is limited in vivo evidence supporting this concept. Here, we studied heart-specific, inducible Ryr2 haploinsufficient (cRyr2Δ50) mice with a stable 50% reduction in Ryr2 protein. This manipulation decreased the amplitude and frequency of cytosolic and mitochondrial Ca²⁺ signals in isolated cardiomyocytes, without changes in cardiomyocyte contraction. Remarkably, in the context of well preserved contractile function in perfused hearts, we observed decreased glucose oxidation, but not fat oxidation, with increased glycolysis. cRyr2Δ50 hearts exhibited hyperphosphorylation and inhibition of pyruvate dehydrogenase, the key Ca²⁺-sensitive gatekeeper to glucose oxidation. Metabolomic, proteomic, and transcriptomic analyses revealed additional functional networks associated with altered metabolism in this model. These results demonstrate that Ryr2 controls mitochondrial Ca²⁺ dynamics and plays a specific, critical role in promoting glucose oxidation in cardiomyocytes. Our findings indicate that partial RYR2 loss is sufficient to cause metabolic abnormalities seen in heart disease.

Cardiomyocyte ATP production, metabolic flexibility, and survival require calcium flux through cardiac ryanodine receptors in vivo

Ca(2+) fluxes between adjacent organelles are thought to control many cellular processes, including metabolism and cell survival. In vitro evidence has been presented that constitutive Ca(2+) flux from intracellular stores into mitochondria is required for basal cellular metabolism, but these observations have not been made in vivo. We report that controlled in vivo depletion of cardiac RYR2, using a conditional gene knock-out strategy (cRyr2KO mice), is sufficient to reduce mitochondrial Ca(2+) and oxidative metabolism, and to establish a pseudohypoxic state with increased autophagy. Dramatic metabolic reprogramming was evident at the transcriptional level via Sirt1/Foxo1/Pgc1α, Atf3, and Klf15 gene networks. Ryr2 loss also induced a non-apoptotic form of programmed cell death associated with increased calpain-10 but not caspase-3 activation or endoplasmic reticulum stress. Remarkably, cRyr2KO mice rapidly exhibited many of the structural, metabolic, and molecular characteristics of heart failure at a time when RYR2 protein was reduced 50%, a similar degree to that which has been reported in heart failure. RYR2-mediated Ca(2+) fluxes are therefore proximal controllers of mitochondrial Ca(2+), ATP levels, and a cascade of transcription factors controlling metabolism and survival.

beta-Agonist stimulation produces changes in cardiac AMPK and coronary lumen LPL only during increased workload

Given the importance of lipoprotein lipase (LPL) in cardiac and vascular pathology, the objective of the present study was to investigate whether the beta-agonist isoproterenol (Iso) influences cardiac LPL. Incubation of quiescent cardiomyocytes with Iso for 60 min had no effect on basal, intracellular, or heparin-releasable (HR)-LPL activity. Similarly, Iso did not change HR-LPL in Langendorff isolated hearts that do not beat against an afterload. In the intact animal, LPL activity at the vascular lumen increased significantly in the Iso-treated group, together with a substantial increase in rate-pressure product. This LPL increase was likely via mechanisms regulated by activation of AMP-activated protein kinase (AMPK) and inactivation of acetyl-CoA carboxylase (ACC280). In glucose-perfused hearts, simply switching from Langendorff to the isolated working heart (that beats against an afterload) induced increases in AMPK and ACC280 phosphorylation and enhanced HR-LPL activity. Provision of insulin and albumin-bound palmitic acid to the working heart was able to reverse these effects. In these hearts, introduction of Iso to the buffer perfusate duplicated the effects seen when this beta-agonist was given in vivo. Our data suggest that Iso can influence HR-LPL only during conditions of increased workload, mechanical performance and excessive energy expenditure, and likely in an AMPK-dependent manner.

Single-dose dexamethasone induces whole-body insulin resistance and alters both cardiac fatty acid and carbohydrate metabolism

Glucocorticoids impair insulin sensitivity. Because insulin resistance is closely linked to increased incidence of cardiovascular diseases and given that metabolic abnormalities have been linked to initiation of heart failure, we examined the acute effects of dexamethasone (DEX) on rat cardiac metabolism. Although injection of DEX for 4 h was not associated with hyperinsulinemia, the euglycemic-hyperinsulinemic clamp showed a decrease in glucose infusion rate. Rates of cardiac glycolysis were unaffected, whereas the rate of glucose oxidation following DEX was significantly decreased and could be associated with augmented expression of PDK4 mRNA and protein. Myocardial glycogen content in DEX hearts increased compared with control. Similar to hypoinsulinemia induced by streptozotocin (STZ), hearts from insulin-resistant DEX animals also demonstrated enlargement of the coronary lipoprotein lipase (LPL) pool. However, unlike STZ, DEX hearts showed greater basal release of LPL and were able to maintain their high heparin-releasable LPL in vitro. This effect could be explained by the enhanced LPL mRNA expression following DEX. Our data provide evidence that in a setting of insulin resistance, an increase in LPL could facilitate increased delivery of fatty acid to the heart, leading to excessive triglyceride storage. It has not been determined whether these acute effects of DEX on cardiac metabolism can be translated into increased cardiovascular risk.

Inhibition of nitric oxide synthesis protects the isolated working rabbit heart from ischaemia-reperfusion injury

OBJECTIVES

Nitric oxide (NO) exerts both protective and detrimental actions in a variety of biological systems. During acute reperfusion following myocardial ischaemia, a rapid overproduction of free radicals, including NO, may occur. We investigated the effects of the NO synthase inhibitors NG-nitro-L-arginine methyl ester (L-NAME) and NG-monomethyl-L-arginine (L-NMMA), and the substrate for NO synthesis, L-arginine, on heart function during ischaemia and reperfusion injury.

METHODS

Spontaneously beating, isolated working rabbit hearts, perfused with modified Krebs-Henseleit buffer containing 1.2 mM palmitate bound to 3% bovine serum albumin, were subjected to 15 min of aerobic perfusion followed by 35 min of global, no-flow ischaemia and 30 min of aerobic reperfusion.

RESULTS

Throughout the reperfusion period there was a marked impairment in the recovery of mechanical function, measured as the product of heart rate x peak systolic pressure (rate-pressure product). Addition of L-NAME (3 microM) prior to the onset of ischaemia, but not at reperfusion, caused an immediate and significant increase in the recovery of mechanical function throughout the reperfusion period. The protective action of L-NAME was abolished by L- (but not D-) arginine (100 microM). L-NAME did not cause ischaemia as it did not alter glycogen or lactate content of aerobically perfused hearts. Furthermore, it did not prevent glycogen loss or lactate accumulation during 35 min of ischaemia, suggesting that the effects of L-NAME were not due to metabolic alterations during ischaemia itself. L-NMMA (30 microM) added prior to ischaemia, but not at reperfusion, also had a protective effect which was seen later in the reperfusion period. Addition of L- (but not D-) arginine (100 microM) prior to the onset of ischaemia resulted in an improved recovery of mechanical function only at 15 min of reperfusion.

CONCLUSIONS

These results suggest that: (1) the recovery of mechanical function of hearts subjected to ischaemia-reperfusion injury can be improved by modulation of myocardial NO synthesis, (2) inhibition of NO synthesis (with L-NAME or L-NMMA) may offer prolonged protection whereas its stimulation (with L-arginine) provides only brief protection, and (3) the reasons for the pharmacological effectiveness of these divergent strategies may be due to the formation of peroxynitrite from NO and superoxide anion during reperfusion.

Islet transplantation improves glucose oxidation and mechanical function in diabetic rat hearts

In poorly controlled diabetes an impairment of glucose use can contribute to a depression in mechanical function of rat hearts. In this study we determined the effects of islet transplantation on glucose use and heart function in streptozotocin-induced diabetic rats. Myocardial function, glycolysis, and glucose oxidation were measured in isolated working hearts obtained from control, diabetic, and islet-transplanted diabetic Wistar-Furth rats. Islets (1200) were transplanted beneath the kidney capsule 2 weeks after a single i.v. dose of streptozotocin (55 mg/kg). The study consisted of three groups: (i) islet-transplanted diabetic rats, (ii) untreated diabetic controls, and (iii) normal controls. Following 11 weeks of monitoring, working hearts were perfused at a 11.5-mmHg (1 mmHg = 133.3 Pa) preload and 80-mmHg afterload, with buffer containing 11 mM [5-3H,14C(U)]glucose, 1.2 mM palmitate, and 100 microU/mL insulin. In untreated diabetic rat hearts, glucose oxidation rates were markedly depressed compared with control hearts (30.4 +/- 4 and 510 +/- 68 nmol.g-1 dry wt..min-1, respectively). Low glucose oxidation rates in diabetic rats were significantly improved in islet-transplanted animals (234 +/- 39 nmol.g-1 dry wt..min-1). The low glucose oxidation rates in untreated diabetic rat hearts were accompanied by an impaired mechanical function compared with control hearts, which was improved by islet transplantation (heart rate x developed pressure x 10(-3) was 10.6 +/- 0.9, 14.8 +/- 1.3, and 14.8 +/- 1.5 beats.mmHg.min-1, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of diltiazem on glycolysis and oxidative metabolism in the ischemic and ischemic/reperfused heart

We have recently demonstrated that calcium channel blockers can protect the ischemic myocardium at concentrations which do not decrease myocardial workload or metabolic demand before ischemia. In this study, we extended these observations by determining what effect the calcium channel blocker, diltiazem, has on overall myocardial energy substrate metabolism in aerobic, ischemic and reperfused ischemic hearts. Isolated working rat hearts were perfused at a 11.5-mm Hg preload, 80-mm Hg afterload, with Krebs-Henseleit buffer containing 11 mM glucose, 1.2 mM palmitate and 500 microU/ml insulin. Glycolysis and glucose oxidation rates were determined in aerobic and reperfused ischemic hearts perfused with [3H]/[14C]glucose, whereas fatty acid oxidation rates were determined under similar conditions in hearts perfused with [14C]palmitate. Addition of diltiazem (0.8 microM) before subjecting hearts to a 30-min period of global no-flow ischemia resulted in a significant improvement in recovery of mechanical function (heart rate x developed pressure during reperfusion recovered to 28 and 53% of preischemic levels, in control and diltiazem-treated hearts, respectively). If diltiazem was added at reperfusion, no improvement of functional recovery was seen. Addition of diltiazem before or after ischemia had no effect on palmitate or glucose oxidation during reperfusion, but did significantly decrease rates of glycolysis during reperfusion. In hearts subjected to low-flow ischemia (coronary flow = 0.5 ml/min), diltiazem significantly decreased glycolytic rates during ischemia (glycolytic rates were 2.09 +/- 0.25 and 1.58 +/- 0.28 mumol/min.g dry wt. in control and diltiazem-treated hearts, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)