Study 3. Freshly collected plasma (25 μL) was pipetted directly into glass test tubes with 2 mL isopropanol : heptane : 1 M acetic acid (40 : 10 : 1 vol). Plasma extraction was performed according to [22] with the exception that sulphuric acid was replaced with 1 M acetic acid. 14C-P and 3H-R-BrP, as well as esterified fatty acids, were quantitatively removed in the upper, heptane phase, while 13C-glucose and 14C-2DG, as well as polar metabolites and 3H2O, were quantitatively recovered in the lower aqueous phase. Upper phase polar lipids (including 14C-P and 3H-R-BrP) were separated from neutral lipids (including endogenously esterified 14C-P) by means of solid phase extraction (200 mg NH2 columns, Isolute, Sorbent AB, Göteborg, Sweden). Retained polar lipids were driven off the columns using 5% glacial acetic acid in methyl-tert-butyl ether. The effluent was mixed with 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
