Isotopomer analyses with the tricarboxylic acid cycle intermediates and exchanging metabolites from the rat kidney

Renal metabolism is essential for kidney functions and energy homeostasis in the body. The TCA cycle is the hub of metabolism, but the metabolic activities of the cycle in the kidney have rarely been investigated. This study is to assess metabolic processes at the level of the TCA cycle in the kidney based on isotopomer distributions in multiple metabolites. Isolated rat kidneys were perfused with media containing common substrates including lactate and alanine for an hour. One group of kidneys received [U-¹³C₃]lactate instead of natural abundance lactate while the other group received [U-¹³C₃]alanine instead of natural abundance alanine. Perfused kidneys and effluent were prepared for analysis using NMR spectroscopy. ¹³C-labeling patterns in glutamate, fumarate, aspartate and succinate from the kidney extracts showed that pyruvate carboxylase and oxidative metabolism through the TCA cycle were comparably very active, but pyruvate cycling and pyruvate dehydrogenase were relatively less active. Isotopomer analyses with fumarate and malate from effluent, however, indicated that pyruvate carboxylase was much more active than the TCA cycle and other metabolic processes. The reverse equilibrium of oxaloacetate with four-carbon intermediates of the cycle was nearly complete (92%), based on the ratio of [2,3,4-¹³C₃]/[1,2,3-¹³C₃] in aspartate or malate. ¹³C enrichment in glucose with ¹³C-lactate supply was higher than that with ¹³C-alanine. Isotopomer analyses with multiple metabolites (i.e., glutamate, fumarate, aspartate, succinate and malate) allowed us to assess relative metabolic processes in the TCA cycle in the kidney supplied with [U-¹³C₃]lactate. Data from the analytes were generally consistent, indicating highly active pyruvate carboxylase and oxidative metabolism through the TCA cycle. Different ¹³C-labeling patterns in analytes from the kidney extracts versus effluent suggested metabolic compartmentalization.