Determination of organ substrate oxidation in vivo by measurement of (CO2)-C-13 concentration in blood

Substrate oxidation by various organs in animals as web is in humans is usually studied by experiments in which radioactively labeled substrates Pre used and the production of (CO2)-C-14 is measured In vivo, substrate oxidation by an organ has, up to now, not been determined by means of stable isotopes. Problems in the determination of the concentration of (CO2)-C-13 in blood may have impeded the use of C-13-labeled substrates. For the determination of (CO2)-C-13 concentration in blood a direct method for the determination of total CO2 concentration in blood was combined with the determination of the isotope ratio (C-13/C-12) Of CO2 by isotope ratio mass spectrometry, The intra-assay relative standard deviation of the CO2 concentration (mean: 19.26 mmol l(-1); n = 7) was 0.8%. The inter-assay relative standard deviation of the CO2 concentration in solutions of a weighed amount of Na2CO3 determined over a 5 year period was 0.64% (mean: 21.99 mmol l(-1); n = 22). The intra-assay relative standard deviation of C-13 in CO2 was 0.03% (mean C-13/C-12: 0.0111557; n = 5). From the (CO2)-C-13 concentration in arterial and venous blood, substrate oxidation by various organs an be calculated. as an illustration, the determination of myocardial glucose oxidation in lambs, both at rest and during exercise, is described. (C) 1998 John Wiley & Sons, Ltd.