Effects of Experimental Hyperketonemia on Myocardial Metabolism

Overview

Starvation and metabolic stress increase circulating ketone bodies, potentially providing the heart with an alternative oxidative fuel. Hyperketonemia reduces myocardial fatty acid consumption. It is unclear whether this is due to inhibited peripheral lipolysis or diminished uptake per se. Aim: To test whether infusion of 3-hydroxybutyrate (BHB) inhibits myocardial glucose and fatty acid uptake. Methods: Randomized, single blinded, cross-over interventional study in 8 healthy volunteers. Myocardial glucose and fatty acid metabolism studied by 11C-palmitate and 18F-FDG PET/CT. Experimental elevation of circulating ketone bodies by infusion of β-hydroxy-β-methylbutyrate.

Background: Ketone bodies are produced by the liver in conditions of increased fatty acid oxidation, serving as important fuel sources during fasting and starvation. They are metabolized to acetyl-CoA which enters the tricarboxylic acid cycle, enabling ATP production independently of glycolysis and resulting in lower oxygen consumption per mole of produced ATP compared to glucose \[ref\]. Their primary physiological function appears to be as an alternative protein-sparing source of energy for extrahepatic tissues in times of reduced carbohydrate availability, preventing muscle wasting. The principal ketone bodies in humans are beta-hydroxybutyrate (BHB) and acetoacetate. Increased ketogenesis is a feature common to fasting, starvation and diabetes mellitus. Ketones have been shown to have a number of neuroprotective effects including anticonvulsant activity, improving cognitive function in Alzheimer's disease and decreasing the effects of acute brain injury and ischemic damage \[ref\], as well as antitumoral effect in gliomas. This has led to the suggestion that ketones could be used therapeutically for a number of diseases though currently the only recognized therapeutic use of ketones is in the form of ketogenic diets for the treatment of epilepsy. There are limited in vivo studies on the effect of ketones on the heart. It is known that fatty acids are the preferred myocardial fuel substrate and that this shifts to increased use of glucose, and to a lesser extent ketones, in times of acutely increased demand. Interestingly, acute ketone infusion in pigs appears to inhibit myocardial fatty acid oxidation. In vitro studies suggest ketones decrease myocardial glucose uptake and affect myocardial contractility, with either increased or decreased contractility when ketones are the only energy source. This has not been further investigated in vivo. It is therefore unclear to what extent ketones can contribute to myocardial metabolism in conditions of hyperketonemia, and how this affects contractility. The present project thus proposes to address the issues outlined above, by measuring human cerebral and cardiac uptake of energy substrates, together with functional parameters, using PET imaging and appropriate radiotracers, under experimental hyperketonemia. Hypotheses: 1\. An acute increase in blood ketone concentration without previous ketoadaptation will decrease cardiac palmitate and glucose uptake in healthy humans. Materials and methods Effect of acute ketone infusion on cardiac perfusion and 18F-FDG and 11C-palmitate uptake in healthy subjects: Study population: 10 healthy volunteers. All study subjects will be instructed to follow a standardised diet for 1 week before the study. On the study day, they will undergo a baseline dynamic cardiac PET scan with 15O-water followed by 11C-palmitate and 18F-FDG tracers, together with baseline blood samples, muscle biopsy and subcutaneous fat biopsy to assess peripheral metabolic status. An intravenous infusion of sodium betahydroxybutyrate will then be initiated at a concentration and rate sufficient to achieve 1-2 mM ketonemia after 30 minutes (assessed by blood sample). A second dynamic PET scan identical to the first will then be performed under continuous ketone infusion at a constant rate. Finally, a second set of blood samples, muscle and subcutaneous fat biopsies will be taken after the scan before stopping the ketone infusion. Perspectives: The results of this research are expected to provide insights into how human heart metabolism respond to increased ketone bodies, and whether there are significant functional improvements. It should contribute to further understanding the possible therapeutic benefits of both exogenous ketone administration and of fasting in relation to cardiac function, with implications for the treatment of various diseases such as diabetes and heart failure. Knowledge of ketones' effects on the kinetics of various radionuclide tracers also has importance for the appropriate clinical use of diagnostic PET scans in patients with elevated blood ketone levels. In addition, the implementation and validation of a ketone PET tracer will allow further future non-invasive studies that directly measure ketone metabolism in various tissues and disease states.