Expression of an artificial Cl- channel in microperfused renal proximal tubules

Claudin-2-deficient mice are defective in the leaky and cation-selective paracellular permeability properties of renal proximal tubules

Claudin-2 is highly expressed in tight junctions of mouse renal proximal tubules, which possess a leaky epithelium whose unique permeability properties underlie their high rate of NaCl reabsorption. To investigate the role of claudin-2 in paracellular NaCl transport in this nephron segment, we generated knockout mice lacking claudin-2 (Cldn2(-/-)). The Cldn2(-/-) mice displayed normal appearance, activity, growth, and behavior. Light microscopy revealed no gross histological abnormalities in the Cldn2(-/-) kidney. Ultrathin section and freeze-fracture replica electron microscopy revealed that, similar to those of wild types, the proximal tubules of Cldn2(-/-) mice were characterized by poorly developed tight junctions with one or two continuous tight junction strands. In contrast, studies in isolated, perfused S2 segments of proximal tubules showed that net transepithelial reabsorption of Na(+), Cl(-), and water was significantly decreased in Cldn2(-/-) mice and that there was an increase in paracellular shunt resistance without affecting the apical or basolateral membrane resistances. Moreover, deletion of claudin-2 caused a loss of cation (Na(+)) selectivity and therefore relative anion (Cl(-)) selectivity in the proximal tubule paracellular pathway. With free access to water and food, fractional Na(+) and Cl(-) excretions in Cldn2(-/-) mice were similar to those in wild types, but both were greater in Cldn2(-/-) mice after i.v. administration of 2% NaCl. We conclude that claudin-2 constitutes leaky and cation (Na(+))-selective paracellular channels within tight junctions of mouse proximal tubules.

Effect of trimethoprim-sulfamethoxazole on Na and K+ transport properties in the rabbit cortical collecting duct perfused in vitro

BACKGROUND

In this study, the membrane mechanisms of hyperkalemia caused by trimethoprim-sulfamethoxazole (TMP-SMX) combination antibiotics were assessed in the cortical collecting duct (CCD).

METHODS

We used the microelectrode technique and flux measurements, and examined the effects of TMP and SMX on electrical properties of the apical and basolateral membranes in the rabbit CCD perfused in vitro.

RESULTS

TMP in the lumen caused increases in apical membrane voltage, fractional apical membrane resistance (fRA), and transepithelial resistance (RT), all effects which were completely inhibited by luminal amiloride, but not by luminal Ba2+. The luminal TMP inhibited both net Na+ reabsorption and K+ secretion in the CCD. TMP in the bath slightly but significantly depolarized transepithelial voltage and basolateral membrane voltage without influencing fRA or RT. SMX in the lumen or bath had no effect on barrier voltages or resistances.

CONCLUSION

TMP mainly acts on the apical membrane of the CCD, inhibits the amiloride-sensitive macroscopic Na+ conductance in this membrane, and thereby decreases the net driving force for K+ exit across the membrane, resulting in an inhibition of K+ secretion. SMX in the lumen or bath had no effect on the CCD.