Radiorespirometric patterns of 14C-substrates in rats. I. Differences in kinds of 14C-substrates and their structural positions labelled with 14C.

Experiments were conducted by radiorespirometry to elucidate the relation between the respiratory 14CO2 wave pattern derived from intravenously injected 14C-substrates and the dispositional partition of these substrates in rats. Two types of 14C-methionine and 4 types of 14C-glucose molecules were used as the 14C-substrates. The respiratory 14CO2 wave patterns obtained in experiments using 14C-methionine substrates revealed different patterns based on differences in the labelled position and the amount of expired 14C-radioactivity as well as the wave height. The experiments using 14C-glucose substrates reveals that the carbon atoms participate equally in forming the respiratory 14CO2 wave pattern, regardless of their position in the glucose molecule. Moreover, differences in wave height as a result of differences in position of the labelled carbon atom were not observed. Results of 90-minute 14CO2 radioactivity recovery rate substantiated the explanation of the above-described relationship. The experiments also clarified that wave height is determined by rate of partition to the CO2 pathway of intermediate metabolites arising in the process of labelled 14C atom disposition.