Progressive Alterations of Mitochondrial Function in the Kidney and Heart During the Development of Salt‐Induced Hypertension
Authors: Tomar Namrata , Kandel Sunil M. , Zheleznova Nadezhda , Audi Said H. , Cowley Allen W. , Dash Ranjan K. 
Published: April 2020

Introduction

Hypertension is a multi‐factorial disease and a leading cause of morbidity and mortality globally. Nearly half of these individuals are sensitive to dietary salt intake with the kidney and oxidative stress playing a key role in this disease leading to eventual failure of both the kidney and heart. Although oxidative stress and mitochondrial dysfunction in the kidney and heart have been studied in various models of hypertension, progressive alterations of mitochondrial function and development of oxidative stress in these two organs have not been studies in parallel during the development of hypertension. The goal of the present study was to investigate progressive alterations in mitochondrial energy metabolism in both the kidney and heart concurrently in the Dahl salt‐sensitive (SS) rats during the development of hypertension when rats were switched from a 0.4% NaCl (LS) to a 4% NaCl (HS) diet.

Method

Mitochondria were isolated from the ventricles of the heart and outer medulla (OM) and cortex of the kidney of SS rats fed a LS diet since weaning and studied at days 3, 7, 14 & 21 of a HS diet feeding. Oxygen consumption rates (OCR) were measured in mitochondria energized with pyruvate + malate as substrates for 3 different respiratory states using an Oroboros Oxygraph‐2k respirometer. This includes a) leak state (energized mitochondria in the absence of ADP), b) ADP‐stimulated state, and c) uncoupled state (energized mitochondria in the presence of an uncoupler FCCP).

Results

A biphasic pattern of ADP‐stimulated OCR with progressive uncoupling was observed in both the kidney (OM & cortex) and heart mitochondria. In kidney, mitochondrial efficiency for ATP synthesis was increased in the early phase of hypertension (3 & 7 days) but was severely compromised in the established phase of hypertension (14 & 21 days). Cardiac mitochondria also showed a similar pattern, however the reduced mitochondrial efficiency for ATP synthesis occurred at a later time, specifically at day 21. This decreased mitochondrial efficiency was hypothesized to be associated with higher levels of uncoupling, ROS production, and oxidative stress in both organs due to mitochondrial ROS‐cellular ROS crosstalk. To test this, experiments were performed in SS rats with double knock out (DKO) of the Nox family of NADPH oxidases: Nox4 and p67phox cytosolic subunit of Nox2 (SSp67phox−/−/Nox4−/−) and determined the role of mitochondrial ROS in stimulating cellular ROS production, particularly by membrane Noxs, and vice versa. Specifically, the DKO SS rats were fed a HS diet and isolated mitochondrial OCR was studied at days 7 and 14. DKO SS rats fed a HS diet showed no significant differences in mitochondrial OCR as compared to the control group (LS diet) in the kidney (OM & cortex) and heart.

Conclusion

Our data suggest that a HS diet in SS rats results initially in increased efficiency of renal cortical and medullary mitochondrial energy production during the 1st week of a HS diet followed by an enhanced ROS production with mitochondrial uncoupling and reduced efficiency of ATP production by the 3rd week. Similar changes occurred in the heart but 1–2 weeks following the changes observed in the kidney.

Support or Funding Information

P01‐GM066730, P01‐HL116264