scintillant and counted for determination of 14C-P and 3H-R-BrP. Two aliquots of the lower phase were taken: one counted for total 14C-content (used to determine 14C-2DG); a second for determination of 13C-glucose. For the latter aliquot, glucose was derivatized to aldonitrile penta acetate using hydroxylamine in pyridine and acetic anhydride. The glucose derivative was dissolved in ethyl acetate after evaporation to dryness and analysed by gas-chromatography-isotope ratio-mass spectrometry using a Hewlett Packard 6890 gas chromatogram (GC) connected to an isotope ratio mass spectrometer (Finnigan Delta Plus, Finnigan, San Jose, CA) via a Finnigan GC Combustion III interface. The 13C/12C ratio of the formed CO2 was compared with results for a 13C-glucose enriched linear calibration curve and tracer to tracee ratios (TTR) for 13C to 12CO2 were calculated. Tissue samples were homogenized and extracted in ice chilled glass-glass homogenizers, essentially according to [23]. The organic phase was quantitatively recovered for determination of 3H- and 14C-labelled lipids. Aliquots of the aqueous phase were taken for determination of (1) total polar 3H- and 14C-labelled metabolites and (2) 14C-labelled anion, by solid phase extraction (200 mg PE-AX columns, Isolute) assumed to be phosphorylated 14C-2DG. Separate tissue pieces were used for determination of glycogen content
