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 i s usually studied by experiments in which radioactively labeled substr ates Pre used and the production of (CO2)-C-14 is measured In vivo, su bstrate oxidation by an organ has, up to now, not been determined by m eans of stable isotopes. Problems in the determination of the concentr ation 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 b lood was combined with the determination of the isotope ratio (C-13/C- 12) Of CO2 by isotope ratio mass spectrometry, The intra-assay relativ e 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 C O2 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 ar terial and venous blood, substrate oxidation by various organs an be c alculated. 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.