NONINVASIVE OBSERVATION OF HEPATIC GLYCOGEN FORMATION IN MAN BY C-13 MRS AFTER ORAL AND INTRAVENOUS GLUCOSE-ADMINISTRATION

The formation of glycogen in the liver of normal volunteers was follow ed noninvasively with C-13 magnetic resonance spectroscopy (MRS) under two different conditions: a) intravenous infusion of [1-C-13]glucose under hyperglycemic and hyperinsulinemic clamp conditions, and b) oral intake of glucose in the form of a bolus. For the intravenous infusio n, [1-C-13]glucose with an enrichment level of 99% was employed. The C 1 signals of alpha- and beta-glucose could be detected in the human li ver already after an infusion period of 8 min. However, an increase in the glycogen signal was observed only after a prolonged infusion of a bout 60 min. Changes in the glycogen signal correlated well with the t ime course of insulin and glucagon during the measurement. Experiments showed also that liver glycogen formation in man can be followed noni nvasively by C-13-MRS using nonlabeled glucose or [1-C-13]glucose with a low level of enrichment (6.6%). The use of nonlabeled glucose may t herefore simplify the quantitation of net liver glycogen synthesis sin ce it can be based directly on changes in the natural abundance C-13 M RS glycogen signal, avoiding label dilution through the various metabo lic pathways of glucose. The glucose uptake, estimated from the increa se in the glycogen signal, was consistent with findings from more comp lex and invasive studies of glucose uptake in the liver. The average l iver glycogen concentration in 12 h overnight fasted volunteers (n = 1 8) without any special dietary preparation was assessed to be 229 +/- 34 mM (minimum = 160 mM; maximum = 274 mM).