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Abstract: FR-OR059

Regulated Dephosphorylation of NCC Shapes the Renal Potassium Switch Pathway

Session Information

Category: Fluid and Electrolytes

  • 901 Fluid and Electrolytes: Basic

Authors

  • Grimm, P. Richard, University of Maryland School of Medicine, Baltimore, Maryland, United States
  • Delpire, Eric J., Vanderbilt University Medical Center, Nashville, Tennessee, United States
  • Welling, Paul A., University of Maryland School of Medicine, Baltimore, Maryland, United States
Background

The Renal Potassium Switch Pathway stimulates the thiazide-sensitive sodium chloride cotransporter (NCC) to limit urinary potassium loss at the expense of retaining sodium and elevating blood pressure. It has been established that low extracellular potassium ([K+]o) activates the WNK4-SPAK kinase cascade to drive NCC phosphorylation, but it remains mysterious how NCC is dephosphorylated in response to a rise in [K+]o. It has been assumed that high [K+]o turns off WNK4-SPAK mediated NCC phosphoactivation, allowing phosphatase activity to dephosphorylate WNK4, but this remains unclear.

Methods

Kinase-activating mutations were introduced in SPAK, the terminal kinase in the WNK signaling pathway, and renal expression of the constitutively active (CA) SPAK mutant was specifically targeted in mice to the early DCT. NCC abundance (tNCC) and phosphorylation (pNCC) were evaluated in vivo and in isolated tubules, together with telemetric blood pressure (BP) measurements in response to changes in dietary potassium and [K+]o. CA-SPAK were compared to Control mice.

Results

CA-SPAK mice display NCC hyperphosphorylation and thiazide-treatable hypertension. BP remained elevated in CA-SPAK mice, compared to control, over a wide range of [K+]o, as predicted. However, BP dropped in CA-mice when [K+]o exceeded 5mM, and this was coincident with a reduction in pNCC/tNCC, revealing a [K+]-regulated dephosphorylation mechanism. Acute elevation in [K+]o decreases pNCC in isolated DCT from WT but not CA-SPAK mice. However, chronic potassium loading for 2 or 5 days reduces pNCC abundance with an IC50 of ~4.1mM in WT. pNCC is also reduce in CA-SPAK mice under the same conditions but with a rightward shift in pNCC vs. [K+]o curve (IC50 ~ 5.5mM), suggesting chronic activation of a protein phosphatase (PP). Subsequent RNA profiling studies revealed a network of PP-subunits are regulated to decrease pNCC in response to increased [K+]o.

Conclusion

Because CA-SPAK drives NCC phosphorylation independent and downstream of WNK, these observations provide strong support for a potassium-activated phosphatase that directly dephosphorylates NCC. Thus [K+]o–regulated phosphorylation as well as dephosphorylation of NCC shape the Renal Potassium Switch Pathway. Regulated NCC dephosphorylation provides new mechanism to explain why high dietary K+ consumption alleviates salt-sensitive hypertension.

Funding

  • NIDDK Support