Ketones, mainly β-hydroxybutyrate (β-HB or β-OHB) and acetoacetate (AcAc), are important alternative fuels to glucose for the brain, the heart, kidneys, skeletal muscle, and tumours. Cerebral metabolic rate of glucose increases almost in proportion to the rise in plasma ketones, independent on the concentration of glucose in plasma (Hasselbach et al., 1996; Blomqvist et al., 2002; Courchesne-Loyer et al., 2017). Increased plasma concentration of ketone bodies also decreases myocardial glucose uptake (Gormsen et al., 2017). In addition, β-HB is an endogenous inhibitor of histone deacetylases (HDACs) and a ligand for cell surface receptors HCAR2 and GPR41 (FFAR3, free fatty acid receptor 3) (Newman and Verdin, 2014).
In normal conditions small quantities of the ketone bodies are produced in the liver as the break-down product of fatty acids in mitochondrial β-oxidation, but during fasting and exercise the production of ketones increases. In humans the β-HB plasma concentrations are normally in the µM range, but can reach 1-2 mM after fasting (Newman and Verdin, 2014). Hepatocytes do not metabolize acetoacetate but release it into the circulation. In other tissues, acetoacetate can be converted to acetoacetyl-CoA and acetyl-CoA in cytosol and mitochondria, and used in energy production or synthesis of several products including amino acids, fatty acids, and sterols (Bentourkia et al., 2009; Croteau et al., 2014).
β-HB, acetoacetate, pyruvate, lactate, and α-keto acids are transported across the BBB by passive and active diffusion. MCT1 is a common carrier for these molecules in endothelial cells and pericytes, and MCT2 in neurons and astrocytes (Bentourkia et al., 2009).
Ketone bodies labelled with positron emitting radionuclides allows in vivo studies of ketone metabolism (Bouteldja et al., 2014). R-β-[1-11C]hydroxybutyrate (Blomqvist et al., 1995), [1-11C]acetoacetate, and [11C]acetate follow the same transport and metabolism pathway through acetyl-CoA, enabling the use of the same models as are used to analyze [11C]acetate PET studies (Bentourkia et al., 2009). R-β-[1-11C]hydroxybutyrate and [1-11C]acetoacetate even equilibrate rapidly in vivo, and a mixture of these tracers will be present in blood after administration of either of those (Blomqvist et al., 2002; Bentourkia et al., 2009). In rats, the concentration of [14C]acetoacetate is much lower than the concentration of [14C]β-HB (Cremer and Heath, 1974; Hawkins and Mans, 1991), and if that is true also in humans, as could be expected by the ratio of unlabelled compounds in humans (Hasselbach et al., 1996), then the radioactivity uptake in R-β-[1-11C]hydroxybutyrate study mainly represents the utilization of β-HB (Blomqvist et al., 2002).
The first 10 min of the brain [11C]β-HB PET data can be analyzed using Patlak plot or irreversible one-tissue compartment model (Blomqvist et al., 1995 and 2002). Metabolic rate of ketones can be calculated from Ki or compartmental model K1 as:
, where CK is the concentration of ketones (AcAc and β-HB) in plasma, and LC is the lumped constant (LC). LC=1.0, because the tracer is chemically identical to the native molecule.
Arterial input function
Arterial sampling is recommended, but blood TAC collected using automated blood sampling system need to be converted to plasma TAC. Blomqvist et al. (2002) used a straight line to represent the plasma-to-blood ratio, with intercept 1.21 and slope 0.00016 for nondiabetic subjects and intercept 1.19 and slope -0.00004 for IDDM patients, during the first 10 min p.i.
The main radioactive metabolite in [11C]β-HB study is [11C]CO2. Blomqvist et al. (1995) estimated that during the 10 min study the loss of [11C]CO2 from tissue leads to about 6% underestimation of the metabolic rate, which was taken into account in the 1995 publication, and not corrected in the later publication (Blomqvist et al., 2002).
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Updated at: 2018-07-30
Created at: 2017-06-28
Written by: Vesa Oikonen