Acidification in the medullary collecting duct following ureteral obstruction

Ureter obstruction alters expression of renal acid-base transport proteins in rat kidney

Urinary tract obstruction impairs renal function and is often associated with a urinary acidification defect caused by diminished net H+ secretion and/or HCO3- reabsorption. To identify the molecular mechanisms of these defects, protein expression of key acid-base transporters were examined along the renal nephron and collecting duct of kidneys from rats subjected to 24-h bilateral ureteral obstruction (BUO), 4 days after release of BUO (BUO-R), or BUO-R rats with experimentally induced metabolic acidosis (BUO-A). Semiquantitative immunoblotting revealed that BUO caused a significant reduction in the expression of the type 3 Na+/H+ exchanger (NHE3) in the cortex (21 +/- 4%), electrogenic Na+/HCO3- cotransporter (NBC1; 71 +/- 5%), type 1 bumetanide-sensitive Na+-K+-2Cl- cotransporter (NKCC2; 3 +/- 1%), electroneutral Na+/HCO3- cotransporter (NBCn1; 46 +/- 7%), and anion exchanger (pendrin; 87 +/- 2%). The expression of H+-ATPase increased in the inner medullary collecting duct (152 +/- 13%). These changes were confirmed by immunocytochemistry. In BUO-R rats, there was a persistent downregulation of all the acid-base transporters including H+-ATPase. Two days of NH4Cl loading reduced plasma pH and HCO3- levels in BUO-A rats. The results demonstrate that the expression of multiple renal acid-base transporters are markedly altered in response to BUO, which may be responsible for development of metabolic acidosis and contribute to the urinary acidification defect after release of the obstruction.

Potassium transport in the mammalian collecting duct

The mammalian collecting duct plays a dominant role in regulating K(+) excretion by the nephron. The collecting duct exhibits axial and intrasegmental cell heterogeneity and is composed of at least two cell types: collecting duct cells (principal cells) and intercalated cells. Under normal circumstances, the collecting duct cell in the cortical collecting duct secretes K(+), whereas under K(+) depletion, the intercalated cell reabsorbs K(+). Assessment of the electrochemical driving forces and of membrane conductances for transcellular and paracellular electrolyte movement, the characterization of several ATPases, patch-clamp investigation, and cloning of the K(+) channel have provided important insights into the role of pumps and channels in those tubule cells that regulate K(+) secretion and reabsorption. This review summarizes K(+) transport properties in the mammalian collecting duct. Special emphasis is given to the mechanisms of how K(+) transport is regulated in the collecting duct.

Electrical properties of the rabbit cortical collecting duct from obstructed and contralateral kidneys after unilateral ureteral obstruction

Electrophysiological techniques were used to determine the electrical properties of the collecting duct (CD) cell in the isolated cortical collecting duct from obstructed (UUOOK) and contralateral (UUOCK) kidneys in rabbits 24 h after unilateral ureteral obstruction (UUO); results were compared with those from sham-operated kidneys. The lumen-negative transepithelial voltage and the basolateral membrane voltage (VB) were decreased in the UUOOK, and increased in the UUOCK. The transepithelial conductance (GT) was decreased in parallel with an increase in the fractional apical membrane resistance (fRA) and a decrease in apical membrane conductance in the UUOOK. By contrast, the GT was increased in parallel with increases in apical and basolateral membrane conductances in the UUOCK. The amiloride-sensitive changes in apical membrane voltage (VA), GT and fRA were lower in the UUOOK, but greater in the UUOCK. The changes in VA and GT upon raising the perfusate K+ concentration and upon addition of luminal Ba2+ were decreased in the UUOOK, and increased in the UUOCK. Addition of ouabain to the bath resulted in a smaller depolarization of VB in the UUOOK, but in a greater depolarization in the UUOCK. Upon lowering bath Cl-, the change in basolateral membrane electromotive force (delta EMF) was increased in the UUOOK, and decreased in the UUOCK. Reversely, upon raising bath K+, the delta EMF was decreased in the UUOOK, and increased in the UUOCK. We conclude: (a) the conductances of Na+ and K+ in the apical membrane, and active Na(+)-K+ pump activity and relative K+ conductance in the basolateral membrane are decreased in the UUOOK, and increased in the UUOCK; (b) the relative basolateral membrane Cl- conductance was increased in the UUOOK, and decreased in the UUOCK.