Non-invasive Measurements of Hepatic Glycogen Levels and Glycogen Synthesis Rates Using Chemical Exchange Saturation Transfer and Comparison to 13C NMR Spectroscopy

Abstract

Glycogen is the primary tissue storage form of carbohydrates and plays in integral role in whole body glucose homeostasis, yet remains difficult to measure without invasive biopsies. A novel non-invasive approach to measure liver glycogen using chemical exchange saturation transfer (CEST) was developed and optimized in phantoms, and then applied to isolated perfused mouse livers at 11.7T, and to mice in vivo at 7T. Measurements of total glycogen content and glycogen synthesis rates were performed under various experimental conditions in perfused livers and compared with interleaved measurements using 13C magnetic resonance spectroscopy (MRS). Monte Carlo simulations were used to investigate the correlation between, and the inherent errors in, each measurement. CEST and 13C MRS measurements were highly correlated, with CEST measurements of total glycogen being more precise, and CEST measurements of glycogen synthesis rates being less precise than 13C MRS. This approach was then applied to a novel pharmacological question, the acute effect of AMP-activated protein kinase (AMPK) activation on hepatic glycogen synthesis. In contrast to previous reports of AMPK activation promoting increased glycogen synthesis in muscle, activation of AMPK with the small molecule MK8722 resulted in a decrease in hepatic glycogen synthesis. This CEST approach was then combined with a glucose infusion protocol in mice, and CEST measurements of glycogen synthesis rates were performed in vivo at 7T. Preliminary studies revealed that CEST was capable of detecting increases in glycogen over time, and that high circulating glucose concentrations did not have a large contaminating effect on the glycogen CEST signal. The results of these studies suggest that CEST can be used to accurately measure glycogen levels and glycogen synthesis rates over the physiological range, and lay the groundwork for the translation of this approach into larger animals and humans.