Metabolite Exchange between Mammalian Organs Quantified in Pigs

Abstract

Mammalian organs continually exchange metabolites via circulation, but systems-level analysis of this shuttling process is lacking. Here, we compared, in fasted pigs, metabolite concentrations in arterial blood versus draining venous blood from 11 organs. Greater than 90% of metabolites showed arterial-venous differences across at least one organ. Surprisingly, the liver and kidneys released not only glucose but also amino acids, both of which were consumed primarily by the intestine and pancreas. The liver and kidneys exhibited additional unexpected activities: liver preferentially burned unsaturated over more atherogenic saturated fatty acids, whereas the kidneys were unique in burning circulating citrate and net oxidizing lactate to pyruvate, thereby contributing to circulating redox homeostasis. Furthermore, we observed more than 700 other cases of tissue-specific metabolite production or consumption, such as release of nucleotides by the spleen and TCA intermediates by pancreas. These data constitute a high-value resource, providing a quantitative atlas of inter-organ metabolite exchange.

Keywords: circulating metabolite; flux; fuel; inter-organ exchange; isotope tracing; mammalian organ-specific metabolism; metabolomics; pig; tissue; uptake and release.

Fig. 1.. The landscape of metabolite exchange between organs.

(A) AV difference measurements were achieved by sampling blood from the arterial circulation and organ-specific draining veins of fasting, anesthetized pigs. (B) The difference between metabolite production and consumption by an organ (the net flux, F_organ) results in enrichment or depletion of the metabolite in the exiting venous blood relative to entering arterial blood (concentrations C_v and C_a). This concentration difference is measured by LC-MS and reported as a log fold change. (C) Schematic of the circulatory system. Syringes indicate blood sampling sites. (D) Workflow. (E) Venn diagrams of measured metabolites, circulating metabolites, and metabolites with significant AV differences. (F) Number of metabolites with significant AV differences for each organ. (G) Heat map of AV differences of circulating metabolites across organs. Yellow indicates release and blue indicates uptake. Each data point indicates median of three technical replicates, with independent duplicate blood draws from the same pig shown separately (N = 5 pigs and 10 data points for most organs; only a single blood draw per pig was performed in the heart and only 3 pigs were used for the ear vein sampling). Leg, head, and ear primarily reflect skeletal muscle, brain, and skin metabolism, respectively. See also Figures S1, S2, and Data Files 1–3.

Fig. 2.. The kidneys clear most metabolites, with citrate a kidney-specific fuel.

(A) Heat map of metabolite abundance (log₂) in renal vein relative to the arterial blood (left) or urine relative to arterial blood (right) in pigs. Metabolites produced by the kidneys are highlighted on the left. (B) Correlation between a metabolite’s blood concentration and reabsorption efficiency. (C) AV difference of citrate across the indicated organs in pigs. Each data point reflects an independent venous blood draw. (D) Citrate contribution to TCA in mice. The kidneys are unique in using citrate as fuel. Mice were intravenously infused with [U-¹³C]-citrate, and average TCA intermediate carbon labeling was normalized to blood citrate labeling. Data are means and error bars are standard errors (N = 4 mice). See also Figure S3.

Fig. 3.. Glucose production localizes to the liver and kidneys, whereas lactate production is distributed throughout the body.

(A) Metabolite production and consumption across all organs must balance to maintain circulating metabolite homeostasis. (B, D) Glucose and lactate show the opposite exchange patterns. Each data point reflects an independent venous blood draw from pig. (C, E) Production and consumption fluxes of glucose and lactate in pigs. (F) Comparison between sum of organ production fluxes (F_total) and whole body production rate (F_circ) of glucose and lactate in pigs. Note that F_total and F_circ are similar for glucose but different for lactate, indicating simultaneous lactate production and consumption within the same organ. Data are means and error bars are standard errors (N = 5 pigs, except for isotope tracing, which was performed in a single pig). BW, body weight; Viscera, organs feeding into the hepatic portal vein. See also Figure S4.

Fig. 4.. The liver and kidneys release amino acids, which are consumed substantially by the other visceral organs.

(A-C) Exchange fluxes of the indicated amino acids in pigs. Data are means and error bars are standard errors (N = 5 pigs). “Others” refers to Ala, Pro, Arg and Tyr. (D) Newly synthesized proteins in mice. Visceral organs (intestine, pancreas, and spleen) show the most active protein synthesis. Mice were intravenously infused with [U-¹³C]-leucine for 2 h and protein leucine labeling was normalized to blood leucine labeling. Data are means and error bars are standard errors (N = 8 mice). See also Figure S5.

Fig. 5.. The liver preferentially consumes and burns unsaturated as opposed to saturated fatty acids.

(A) Acetate is released from the gut and head. Each data point reflects an independent venous blood draw from pig. (B) LCFAs are released from the visceral organs other than liver, head, and leg, and consumed by the liver and heart. Each data point reflects the mean AV difference for a specific LCFA in pigs, with different fatty acids shown by different colors. (C, D) The liver and leg show the opposite preferences in LCFA uptake. (E) Exchange fluxes of the three most abundant LCFAs in pigs. (F) The liver preferentially oxidizes unsaturated LCFAs in mice. Mice were intravenously infused with the indicated [U-¹³C]-LCFA, and fractions of total labeled carbons in TCA intermediates and 3-hydroxybutyate in the liver were normalized to blood tracer labeling. Data are means and error bars are standard errors. For panels A-E, N = 5 pigs. For panel F, N = 4, 6, and 7 mice for C16:0, C18:1, and C18:2 infusions, respectively. *p<0.01 by two-tailed Student’s t-test. Viscera, organs feeding into the hepatic portal vein. See also Figure S6.