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Am J Physiol Renal Physiol (May 24, 2005). doi:10.1152/ajprenal.00057.2005
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Submitted on February 10, 2005
Accepted on May 16, 2005

Dietary K+ regulates apical membrane expression of maxi-K channels in rabbit cortical collecting duct

Fadi Najjar1, Hao Zhou2, Tetsuji Morimoto2, James B. Bruns1, Hai-Sheng Li1, Wen Liu2, Thomas R. Kleyman1*, and Lisa M. Satlin2

1 Renal-Electrolyte Division, Department of Medicine, University of Pittsburgh, Pittsburgh, PA, USA
2 Division of Pediatric Nephrology, Department of Pediatrics, Mount Sinai School of Medicine, New York, NY, USA

* To whom correspondence should be addressed. E-mail: kleyman{at}pitt.edu.

The CCD is a final site for regulation of K+ homeostasis. CCD K+ secretion is determined by the electrochemical gradient and apical permeability to K+. Conducting secretory K+ (SK/ROMK) and maxi-K channels are present in the apical membrane of the CCD, the former in principal cells and the latter in both principal and intercalated cells. Whereas SK channels mediate baseline K+ secretion, maxi-K channels appear to participate in flow-stimulated K+ secretion. Chronic dietary K+ loading enhances the CCD K+ secretory capacity due, in part, to an increase in SK channel density (Palmer et al, J Gen Physiol 104:693, 1994). Long-term exposure of Ambystoma tigrinum to elevated K+ increases renal K+ excretion due to an increase in apical maxi-K channel density in their CDs (Stoner and Viggiano, J Membr Biol 162:107, 1998). The purpose of the present study was to test whether K+ adaptation in the mammalian CCD is associated with upregulation of maxi-K channel expression. NZW rabbits were fed a low (LK), control (CK), or high (HK) K+ diet for 10-14 d. Real-time PCR quantitation of message encoding maxi-K {alpha} and {beta}2-4 in single CCDs from HK animals was greater than that detected in CK and LK animals (p < 0.05); {beta}1 subunit was not detected in any CCD sample, but was present in whole kidney. Indirect immunofluorescence microscopy revealed a predominantly intracellular distribution of {alpha} subunits in LK kidneys. In contrast, robust apical labeling was detected primarily in {alpha}-intercalated cells in HK kidneys. In summary, K+ adaptation is associated with an increase in steady state abundance of maxi-K channel subunit-specific mRNAs and immunodetectable apical {alpha} subunit, the latter observation consistent with redistribution from an intracellular pool to the plasma membrane.




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