High Salt-Induced Changes in Mitochondrial Bioenergetics in the Kidney Proximal Tubules During the Development of Hypertension – Role of mTORC1

Background: Kidney proximal tubules (PTs) reabsorb nearly 65% of Na + from the glomerular filtrate in the cortex (Cx). Under high salt (HS) feeding, this imposes a significant metabolic stress on the PT mitochondria to consume very high O 2 for ATP production to meet the increased energy demand. Although mitochondrial dysfunction associated with salt-sensitive (SS) hypertension in the kidney is well-recognized, there remains a poor understanding of short-term versus long-term chronic changes in the PT mitochondrial bioenergetics and the associated regulatory mechanisms required to meet the increased energy demands in SS hypertension. In this study, we hypothesized that mitochondrial bioenergetics in the kidney PTs is progressively impaired via activation of mTORC1 signaling during the development of SS hypertension. Methods: PTs were isolated from the kidney Cx of Dahl SS rats (9-10 wks old; age-matched) fed a 0.4% NaCl diet (LS) or 4.0% NaCl diet for 7, 14, and 21 days (HS7, HS14, and HS21, respectively), and permeabilized through mechanical stroking. Mitochondrial O 2 consumption rates (OCRs) were measured in the permeabilized PTs, energized with substrates for complex I (pyruvate + malate (PM)) or complex II (succinate) and stimulated with two different ADP perturbations: 1) a saturated dose of ADP and 2) sequentially increasing doses of ADP. The distinct respiratory states such as H + leak state (state 2; S2), ADP-stimulated state (state 3; S3), resting state (state 4; S4), and uncoupled state (state 5; S5) were recorded using an Oroboros Oxygraph-2k instrument to determine the efficiency and coupling of mitochondrial oxidative phosphorylation (OxPhos). To determine the regulatory role of mTORC1 signaling in the HS-induced mitochondrial impairments, the respiratory parameters of PTs were compared between HS14 SS rats chronically treated with mTORC1 inhibitor rapamycin (1.5 mg/kg i.v. infusion per day) and vehicle throughout HS feeding. Results: Permeabilized PTs exhibited significant declines in S3 and S5 OCRs in HS14 and HS21 groups as compared to the LS group when energized with PM, with the corresponding S3 OCR durations significantly increased. The S3 OCRs of permeabilized PTs energized with succinate were also significantly reduced in the late phases of SS hypertension (HS14 and HS21), indicating an overall mitochondrial inefficiency for OxPhos and coupling in the chronic phase of the HS feeding. Since the role of mTORC1 in SS hypertension is well established in previous studies from our lab, we hypothesized that this decline in the efficiency and coupling of OxPhos was associated with activation of mTORC1 signaling. Our preliminary data obtained at HS14 clearly showed that rapamycin treatment of SS rats resulted in a significant improvement of the ADP-induced S3 OCR of permeabilized PTs compared to vehicle treatment when energized with succinate under both normal (single ADP) and stressed (sequential ADP) conditions. Conclusion: In HS fed SS rats, PTs initially are able to sustain their efficiency and coupling of OxPhos for ATP production for about 10 days which is then followed by a significant decline over the late phase of SS hypertension. This ultimate reduction in the efficiency and coupling of OxPhos for ATP production is improved in PTs of SS rats treated with rapamycin, demonstrating the vital causal role of mTORC1 signaling in PT mitochondrial dysfunction during the development of SS hypertension. Funding: NIH R01 HL151587. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.