High Salt Diet Increases Renal Oxygen Consumption in Sprague‐Dawley Rats

Abstract

The physiological relationships between kidney O2 delivery and consumption and metabolism have been studied only under conditions of anesthesia and stressful surgical procedures. We report here techniques developed in our laboratory enabling determination of kidney O2 utilization over many days in instrumented unanesthetized rats in which the effects of a high salt diet were determined in normal Sprague Dawley (SD) rats. Male SD rats (10 weeks of age) fed a 0.4% NaCl diet were instrumented with a left renal artery flow probe (Transonic) for continuous (24 hr/day) measurement of renal blood flow (RBF). An indwelling intra-renal vein catheter (MRE 025) was inserted through the femoral vein and tip fixed close to the renal pelvis. An aortic catheter inserted though the femoral artery was implanted to sample blood and continuously (24 hr/day) measure arterial blood pressure (BP). Catheters and flow probe cables which exited from the neck were attached to a custom designed swivel and a servo-controlled circular cage system (BASi cage) enabling the rat to move freely throughout the study. Following 7 days of recovery from surgery, continuous monitoring of RBF and BP began and continued throughout the study. Following a 3 day control period with rats fed a 0.4% NaCl diet, renal venous and arterial blood were samples (250 ul) and blood gas was measured immediately by a radiometer (ABL800 FLEX). The diet was switch to 4% NaCl and blood sampling was performed at days 7, 14 and 21 of the high salt diet. O2 content, O2 consumption, and O2 extraction were determined as: O2 content (mL/dL): 0.31 × Hemoglobin (Hb) (g/dL) × HbO­2 (%) + 0.003 × PaO2 (mmHg). O2 consumption (ml/min): RBF (ml/min) × (arterial-renal venous O2 content difference). O2 extraction (% of arterial supply): 1-(renal venous O2 content/arterial O2 content) × 100. Daily average mean arterial pressure (MAP) increased moderately soon after switching to the 4% NaCl diet from 113 ± 2 to 120 ± 2 mmHg by day 3 (p=0.03, n=9) and remain elevated at this level throughout the study (120 ± 3 mmHg at day 13). Renal blood flow also rose soon after switching to the 4% salt diet from 9.4 ± 0.8 to 10.3 ± 0.8 ml/min during day 2 (p=0.03, n=9) . Thereafter, it also remained elevated throughout the study (10.7 ± 0.8 ml/min at day 13). Changes in O2 consumption occurred more gradually increasing from 0.21 ± 0.04 and rising to 0.27 ± 0.03 (ml/min) at day 14 of 4% salt diet (p=0.02, n=5). The calculated O2 extraction ratio remained unchanged ranging from 12.1 ± 0.1 (%) to 12.8 ± 0.1 (%) (p=0.48, n=5) throughout the study. It appears that in normal SD rats, the increased tubular metabolic work associated with a high salt diet is initially accommodated by increased perfusion (RBF) and a gradual increase of O2 consumption while extraction remains unchanged for a two-week period.