Functional coupling of renal K+ and Na+ handling causes high blood pressure in Na+ replete mice
Authors: Vitzthum Helga , Seniuk Anika , Schulte Laura Helene , Müller Maxie Luise , Hetz Hannah , Ehmke Heimo 
Published: January 2014

Key points

The adrenal hormone aldosterone can stimulate K+ secretion during hyperkalaemia and Na+ reabsorption during hypovolaemia in the kidney.

Angiotensin II is thought to switch the physiological mode of action from K+ excretion towards Na+ retention, but how the regulation is achieved when angiotensin II levels are suppressed by high Na+ intake remains unknown.

We report that both dietary K+ depletion and dietary K+ loading provoke renal Na+ retention and increase blood pressure in Na+ replete mice, but these occur through different renal kinase signalling and Na+ transport pathways.

An angiotensin II‐ and aldosterone‐independent activation of the sodium‐chloride cotransporter NCC contributes to the blood pressure increase induced by K+ depletion, whereas the hypertensive response to K+ loading is dependent on neither aldosterone nor Na+ transport via the epithelial sodium channel ENaC.

These findings imply a major impact of K+ homeostasis on renal Na+ handling in the Na+ replete state and suggest a mechanism for the hypertensive effect of the Western diet (high Na+ and low K+) in humans.

A network of kinases, including WNKs, SPAK and Sgk1, is critical for the independent regulation of K+ and Na+ transport in the distal nephron. Angiotensin II is thought to act as a key hormone in orchestrating these kinases to switch from K+ secretion during hyperkalaemia to Na+ reabsorption during intravascular volume depletion, thus keeping disturbances in electrolyte and blood pressure homeostasis at a minimum. It remains unclear, however, how K+ and Na+ transport are regulated during a high Na+ intake, which is associated with suppressed angiotensin II levels and a high distal tubular Na+ load. We therefore investigated the integrated blood pressure, renal, hormonal and gene and protein expression responses to large changes of K+ intake in Na+ replete mice. Both low and high K+ intake increased blood pressure and caused Na+ retention. Low K+ intake was accompanied by an upregulation of the sodium‐chloride cotransporter (NCC) and its activating kinase SPAK, and inhibition of NCC normalized blood pressure. Renal responses were unaffected by angiotensin AT1 receptor antagonism, indicating that low K+ intake activates the distal nephron by an angiotensin‐independent mode of action. High K+ intake was associated with elevated plasma aldosterone concentrations and an upregulation of the epithelial sodium channel (ENaC) and its activating kinase Sgk1. Surprisingly, high K+ intake increased blood pressure even during ENaC or mineralocorticoid receptor antagonism, suggesting the contribution of aldosterone‐independent mechanisms. These findings show that in a Na+ replete state, changes in K+ intake induce specific molecular and functional adaptations in the distal nephron that cause a functional coupling of renal K+ and Na+ handling, resulting in Na+ retention and high blood pressure when K+ intake is either restricted or excessively increased.