, Volume 465, Issue 2, pp 233-245
Date: 22 Nov 2012

Downregulation of the renal outer medullary K+ channel ROMK by the AMP-activated protein kinase

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Abstract

The 5′-adenosine monophosphate-activated serine/threonine protein kinase (AMPK) is stimulated by energy depletion, increase in cytosolic Ca2+ activity, oxidative stress, and nitric oxide. AMPK participates in the regulation of the epithelial Na+ channel ENaC and the voltage-gated K+ channel KCNE1/KCNQ1. It is partially effective by decreasing PIP2 formation through the PI3K pathway. The present study explored whether AMPK regulates the renal outer medullary K+ channel ROMK. To this end, cRNA encoding ROMK was injected into Xenopus oocytes with and without additional injection of constitutively active AMPKγR70Q (AMPKα1-HA+AMPKβ1-Flag+AMPKγ1R70Q), or of inactive AMPKαK45R (AMPKα1K45R+AMPKβ1-Flag+AMPKγ1-HA), and the current determined utilizing two-electrode voltage-clamp and single channel patch clamp. ROMK protein abundance was measured utilizing chemiluminescence in Xenopus oocytes and western blot in whole kidney tissue. Moreover, renal Na+ and K+ excretion were determined in AMPKα1-deficient mice (ampk −/− ) and wild-type mice (ampk +/+ ) prior to and following an acute K+ load (111 mM KCl, 30 mM NaHCO3, 4.7 mM NaCl, and 2.25 g/dl BSA) at a rate of 500 μl/h. As a result, coexpression of AMPKγR70Q but not of AMPKαK45R significantly decreased the current in ROMK1-expressing Xenopus oocytes. Injection of phosphatidylinositol PI(4,5)P2 significantly increased the current in ROMK1-expressing Xenopus oocytes, an effect reversed in the presence of AMPKγR70Q. Under control conditions, no significant differences between ampk −/− and ampk +/+ mice were observed in glomerular filtration rate (GFR), urinary flow rate, serum aldosterone, plasma Na+, and K+ concentrations as well as absolute and fractional Na+ and K+ excretion. Following an acute K+ load, GFR, urinary flow rate, serum aldosterone, plasma Na+, and K+ concentration were again similar in both genotypes, but renal absolute and fractional Na+ and K+ excretion were higher in ampk −/− than in ampk +/+ mice. According to micropuncture following a K+ load, delivery of Na+ to the early distal tubule but not delivery of K+ to late proximal and early distal tubules was increased in ampk −/− mice. The upregulation of renal ROMK1 protein expression by acute K+ load was more pronounced in ampk −/− than in ampk +/+ mice. In conclusion, AMPK downregulates ROMK, an effect compromising the ability of the kidney to excrete K+ following an acute K+ load.

Balasaheb Siraskar and Dan Yang Huang contributed equally to this work and thus share first authorship.