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In the heart, detection of hyperpolarized [ 13 C]bicarbonate and 13 CO 2 by magnetic resonance (MR) after administration of hyperpolarized [1- 13 C]pyruvate is caused exclusively by oxidative decarboxylation of pyruvate via the pyruvate dehydrogenase complex (PDH). However, liver mitochondria possess alternative anabolic pathways accessible by [1- 13 C]pyruvate, which may allow a wider diagnostic range for hyperpolarized MR compared with other tissue. Metabolism of hyperpolarized [1- 13 C]pyruvate in the tricarboxylic acid (TCA) cycle was monitored in the isolated perfused liver from fed and fasted mice. Hyperpolarized [1- 13 C]pyruvate was rapidly converted to [1- 13 C]lactate, [1- 13 C]alanine, [1- 13 C]malate, [4- 13 C]malate, [1- 13 C]aspartate, [4- 13 C]aspartate, and [ 13 C]bicarbonate. Livers from fasted animals had increased lactate:alanine, consistent with elevated NADH:NAD + . The appearance of asymmetrically enriched malate and aspartate indicated high rates of anaplerotic pyruvate carboxylase activity and incomplete equilibration with fumarate. Hyperpolarized [ 13 C]bicarbonate was also detected, consistent with multiple mechanisms, including cataplerotic decarboxylation of [4- 13 C]oxaloacetate via phosphoenolpyruvate carboxykinase (PEPCK), forward TCA cycle flux of [4- 13 C]oxaloacetate to generate 13 CO 2 at isocitrate dehydrogenase, or decarboxylation of [1- 13 C]pyruvate by PDH. Isotopomer analysis of liver glutamate confirmed that anaplerosis was sevenfold greater than flux through PDH. In addition, signal from [4- 13 C]malate and [4- 13 C]aspartate was markedly blunted and signal from [ 13 C]bicarbonate was completely abolished in livers from PEPCK KO mice, indicating that the major pathway for entry of hyperpolarized [1- 13 C]pyruvate into the hepatic TCA cycle is via pyruvate carboxylase, and that cataplerotic flux through PEPCK is the primary source of [ 13 C]bicarbonate. We conclude that MR detection of hyperpolarized TCA intermediates and bicarbonate is diagnostic of pyruvate carboxylase and PEPCK flux in the liver.

Elevation of postprandial glucose stimulates release of insulin from granules stored in pancreatic islet β-cells. We demonstrate here that divalent zinc ions coreleased with insulin from β-cells in response to high glucose are readily detected by MRI using the Zn 2+ -responsive T 1 agent, GdDOTA-diBPEN. Image contrast was significantly enhanced in the mouse pancreas after injection of a bolus of glucose followed by a low dose of the Zn 2+ sensor. Images of the pancreas were not enhanced by the agent in mice without addition of glucose to stimulate insulin release, nor were images enhanced in streptozotocin-treated mice with or without added glucose. These observations are consistent with MRI detection of Zn 2+ released from β-cells only during glucose-stimulated insulin secretion. Images of mice fed a high-fat (60%) diet over a 12-wk period and subjected to this same imaging protocol showed a larger volume of contrast-enhanced pancreatic tissue, consistent with the expansion of pancreatic β-cell mass during fat accumulation and progression to type 2 diabetes. This MRI sensor offers the exciting potential for deep-tissue monitoring of β-cell function in vivo during development of type 2 diabetes or after implantation of islets in type I diabetic patients.

The liver plays a crucial role in mobilizing energy during nutritional deprivation. During the early stages of fasting, hepatic glycogenolysis is a primary energy source. As fasting progresses and glycogen stores are depleted, hepatic gluconeogenesis and ketogenesis become major energy sources. Here, we show that fibroblast growth factor 21 (FGF21), a hormone that is induced in liver by fasting, induces hepatic expression of peroxisome proliferator-activated receptor gamma coactivator protein-1alpha (PGC-1alpha), a key transcriptional regulator of energy homeostasis, and causes corresponding increases in fatty acid oxidation, tricarboxylic acid cycle flux, and gluconeogenesis without increasing glycogenolysis. Mice lacking FGF21 fail to fully induce PGC-1alpha expression in response to a prolonged fast and have impaired gluconeogenesis and ketogenesis. These results reveal an unexpected relationship between FGF21 and PGC-1alpha and demonstrate an important role for FGF21 in coordinately regulating carbohydrate and fatty acid metabolism during the progression from fasting to starvation.

To investigate the role of Angptl4, an inhibitor of lipoprotein lipase that is induced by >3-fold in the heart after rosiglitazone treatment, we generated transgenic mice that overexpress Angptl4 in the heart (MHC-Angptl4). We show that MHC-Angptl4 mice exhibit cardiac-restricted expression of the transgene and inhibition of cardiac lipoprotein lipase (LPL) activity. However, LPL activities in other tissues or that released into plasma by heparin are not affected. In addition, MHC-Angptl4 mice also exhibit hypertriglyceridemia after 6 h of fasting. We use echocardiography to show that MHC-Angptl4 mice develop left-ventricular dysfunction. Comparison of the metabolic profiles of isolated working hearts demonstrates that cardiac impairment in MHC-Angptl4 mice is positively associated with decreased triglyceride (TG) utilization. When bred to transgenic mice that overexpress acyl-CoA synthetase in the heart, a strain that exhibits elevated cardiac TG accumulation, cardiac TG content in double transgenic mice is reversed to that of wild-type mice. Taken together, our data support the hypothesis that induction of Angptl4 in the heart inhibits lipoprotein-derived fatty acid delivery. This mouse model will be useful to elucidate the role of reduced fatty acid supply in the pathogenesis of heart failure and related disorders.