Early renal response to long-term salt loading: mitochondrial dysfunction, ER stress, and uromodulin accumulation in the kidney medulla.

Kidneys play a critical role in maintaining water and electrolyte balance, but prolonged salt loading can disrupt renal function by inducing osmotic and oxidative stress. Although high salt intake is well-known to contribute to hypertension and kidney damage, the early renal responses to mild, long-term salt intake, particularly in normotensive individuals, remain poorly understood. To address this knowledge gap, we investigated the effects of exposing normotensive Wistar Kyoto (WKY) rats to 1% NaCl over a 3-mo period, focusing on the medullary region and the adaptive cellular mechanisms in response to salt-induced stress. In addition, we examined the acute effects of 4 h of salt exposure on medullary tubules. The long-term salt intake did not significantly alter blood pressure or cause notable kidney damage but did lead to differential expression of proteins associated with mitochondrial dysfunction and endoplasmic reticulum (ER) stress in the renal medulla. Acute 4-h salt exposure triggered a rapid cellular response involving proteins linked to mitochondrial activity and oxidative stress responses. Both acute and chronic settings significantly reduced uromodulin (UMOD) excretion with altered trafficking indicating intracellular accumulation within medullary cells. This provides evidence that chronic salt loading disrupts normal protein handling without immediate renal injury, shedding light on adaptive mechanisms in the kidney to mitigate osmotic stress. These early adaptations provide insights into the mechanisms underlying salt-related renal pathologies and may inform therapeutic strategies for individuals susceptible to the effects of dietary salt. This study reveals that even in normotensive Wistar Kyoto rats, mild long-term salt loading induces early renal stress without overt kidney damage or hypertension. Novel findings include reduced uromodulin (UMOD) excretion and altered intracellular trafficking in the renal medulla, alongside mitochondrial dysfunction and endoplasmic reticulum stress. These data highlight UMOD as a sensitive marker of salt-induced renal adaptation and provide insights into early cellular responses to salt before clinical disease onset.