Changes in Oxygen Consumption and Metabolomic Profiles in the Kidney of Sprague‐Dawley Rat fed a High‐Salt Diet
Authors: Shimada Satoshi , Hoffmann Brian , Greene Andrew , Liang Mingyu , Dash Ranjan K. , Cowley Allen W. 
Published: May 2022

The metabolism of oxygen and substrates in the kidney under normal conditions and in salt‐sensitive hypertension is still not well understood. A novel method was developed to collect renal arteriovenous blood and urine from conscious freely moving rats, while measuring renal blood flow (RBF) and blood pressure (BP). As such, global renal O2 consumption and metabolic profiles can be studied in response to various stimuli such as changes in salt diet. We report here changes observed in male Sprague‐Dawley rats (SD; 10 wk age) before and after 21 days switching from a 0.4% NaCl (LS) diet to a 4.0% NaCl diet (HS). Rats were surgically instrumented with a renal ultrasonic artery flow probe (Transonic), a femoral arterial catheter, and a renal venous catheter and placed in a movement response caging system. After 7 days recovery, control levels of RBF and BP (24 hr/day) were obtained, and arterial and renal venous blood were sampled. Rats were then switched to the HS diet and blood sampled at days 7, 14 and 21 while RBF and BP were recorded continuously throughout the study. Blood O2 content was measured by radiometer immediately after blood collection and plasma samples frozen for global metabolomic analysis by the Jackson laboratory. The metabolomic analysis (Thermo Q‐Exactive Orbitrap coupled to a Vanquich UPLC system) was performed in 4 modes (C18 positive, C18 negative, HILIC positive and HILIC negative). Average 24 hr mean arterial pressure (MAP) of the SD rats increased slightly over the 21 days of the HS diet from 111 ± 2 to 119 ± 5 mmHg (p<0.05; n=7). Average 24 hr RBF rose from 9.2 ± 0.6 to 11.7 + 0.6 ml/min (p<0.05). O2 consumption increased significantly from 0.17 ± 0.03 to 0.26 ± 0.03 ml/min (p<0.05) as did O2 extraction which increased from 10.8 ± 1.1 % to 15.0 ± 1.2 % by Day 21 of HS diet. Of a total 1205 named metabolites detected, we present here those that differed between arterial and renal venous blood by more than 2‐fold and significantly (p<0.05; Benjamini‐Hochberg corrected). At the time of this report, in rats fed the LS diet, 33 compounds were found more abundant in arterial blood than in venous blood. 10 compounds were found more abundant in venous blood. The HS diet increased the arterial / venous ratio of some compounds progressively throughout the 21 days while others decreased. No evidence of anaerobic respiration was obtained with lactate, pyruvate, glutamate and TCA cycle related metabolites remaining unchanged throughout the study. Interestingly, some gut microbiome generated compounds were found to differentially altered by the HS diet. Despite no changes in arterial concentrations, a progressive reduction was observed in Triethylamine in the renal venous blood at each time point, while a progressive elevation was observed in Hexanoic acid. Venous levels of Cis‐4‐Decenedioic acid and Succinylacetone, known to be altered in some metabolic diseases were significantly increased by the HS diet. Urine measurements and cortical and outer medullary tissue measurements are currently under way which together with the RBF will enable calculation of metabolic fluxes of these various metabolites. We conclude that a high‐salt diet increases oxygen consumption and alters the metabolomic profiles of the kidney.