K+ deprivation potentiates the renal acid excretory effect of mineralocorticoid: obliteration by amiloride

ENaC activity in the cortical collecting duct of HKα1 H+,K+-ATPase knockout mice is uncoupled from Na+ intake

Modulation of the epithelial Na⁺ channel (ENaC) activity in the collecting duct (CD) is an important mechanism for normal Na⁺ homeostasis. ENaC activity is inversely related to dietary Na⁺ intake, in part due to inhibitory paracrine purinergic regulation. Evidence suggests that H⁺,K⁺-ATPase activity in the CD also influences Na⁺ excretion. We hypothesized that renal H⁺,K⁺-ATPases affect Na⁺ reabsorption by the CD by modulating ENaC activity. ENaC activity in HKα₁ H⁺,K⁺-ATPase knockout (HKα₁^-/-) mice was uncoupled from Na⁺ intake. ENaC activity on a high-Na⁺ diet was greater in the HKα₁^-/- mice than in WT mice. Moreover, dietary Na⁺ content did not modulate ENaC activity in the HKα₁^-/- mice as it did in WT mice. Purinergic regulation of ENaC was abnormal in HKα₁^-/- mice. In contrast to WT mice, where urinary [ATP] was proportional to dietary Na⁺ intake, urinary [ATP] did not increase in response to a high-Na⁺ diet in the HKα₁^-/- mice and was significantly lower than in the WT mice. HKα₁^-/- mice fed a high-Na⁺ diet had greater Na⁺ retention than WT mice and had an impaired dipsogenic response. These results suggest an important role for the HKα₁ subunit in the regulation of purinergic signaling in the CD. They are also consistent with HKα₁-containing H⁺,K⁺-ATPases as important components for the proper regulation of Na⁺ balance and the dipsogenic response to a high-salt diet. Such observations suggest a previously unrecognized element in Na⁺ regulation in the CD.

Effect of mineralocorticoid treatment in mice with collecting duct-specific knockout of endothelin-1

Aldosterone increases blood pressure (BP) by stimulating sodium (Na) reabsorption within the distal nephron and collecting duct (CD). Aldosterone also stimulates endothelin-1 (ET-1) production that acts within the CD to inhibit Na reabsorption via a negative feedback mechanism. We tested the hypothesis that this renal aldosterone-endothelin feedback system regulates electrolyte balance and BP by comparing the effect of a high-salt (NaCl) diet and mineralocorticoid stimulation in control and CD-specific ET-1 knockout (CD ET-1 KO) mice. Metabolic balance and radiotelemetric BP were measured before and after treatment with desoxycorticosterone pivalate (DOCP) in mice fed a high-salt diet with saline to drink. CD ET-1 KO mice consumed more high-salt diet and saline and had greater urine output than controls. CD ET-1 KO mice exhibited increased BP and greater fluid retention and body weight than controls on a high-salt diet. DOCP with high-salt feeding further increased BP in CD ET-1 KO mice, and by the end of the study the CD ET-1 KO mice were substantially hypernatremic. Unlike controls, CD ET-1 KO mice failed to respond acutely or escape from DOCP treatment. We conclude that local ET-1 production in the CD is required for the appropriate renal response to Na loading and that lack of local ET-1 results in abnormal fluid and electrolyte handling when challenged with a high-salt diet and with DOCP treatment. Additionally, local ET-1 production is necessary, under these experimental conditions, for renal compensation to and escape from the chronic effects of mineralocorticoids.