Macrophage-Epithelial Sodium Channel (ENaC) Interactions Regulate Kidney Sodium Handling and Salt-Sensitive Hypertension

Background: A third of people are salt-sensitive, showing an exaggerated blood pressure (BP) response to high salt intake. Salt-sensitivity is an independent cardiovascular risk factor, but its underlying causes are unresolved. Immune cells, including monocytes and macrophages (MΦ), are emerging as important regulators of both salt homeostasis and blood pressure physiology. Here, we investigated the impact of macrophage depletion on salt-sensitive hypertension in the mouse. Methods: Male transgenic mice, with myeloid cell-specific expression of the human diphtheria toxin receptor (CD11b-DTR mice) were fed a high salt diet (8% NaCl) (HSD) for 4 weeks to develop hypertension. During the final (4th week) mice received diphtheria toxin (DT) to deplete tissue MΦ. In another group, liposomal clodronate (LC) was used as a second depletion strategy. We measured renal function, BP (by radiotelemetry) and collected kidneys for sodium transporters gene expression analysis. Renal MΦ number was assessed by immunohistochemistry. Results: HSD increased systolic and diastolic BP by ~8mmHg and ~6mmHg from baseline. DT significantly reduced the MΦ count in renal cortex and induced complete depletion in the renal medulla. Although LC reduced the number of cortical MΦ, there was an increase in medullary MΦ count. There was also a differential effect on BP and renal Na+ handling. DT amplified salt-sensitive hypertension by a further ~15mmHg (p<0.0001 vs. no depletion); LC had no additional effect. DT mice showed reduced urinary Na+ excretion and hypernatremia; LC treated animals had no abnormalities in Na+ status. High salt intake supressed aldosterone in all groups but DT depletion caused an inappropriate activation of ENaC at the mRNA and functional levels. ENaC blockade with benzamil significantly increased Na+ excretion and reduced salt-sensitive hypertension in DT-treated animals. Conclusion: We show that tissue MΦ distribution influences ENaC regulation and renal Na+ handling independent of aldosterone, which can contribute to the development of salt-sensitive hypertension.