A Ba2+-Insensitive K+ Conductance in the Basolateral Membrane of Rabbit Cortical Thick Ascending Limb Cells

Abstract

The nature of the K⁺ exit across the basolateral membrane of microperfused rabbit cortical thick ascending limbs (cTALs) was investigated using the transepithelial and transmembrane potential difference (PD_te, PD_bl) and conductance measurements. An increase in bath K⁺ concentration from 4 to 10, 25, 50 mmol/l depolarized the basolateral membrane in a concentration-dependent manner, accompanied by a decrease in the fractional resistance of the basolateral membrane (FR_bl). The Cl^– channel blocker, 5-nitro-2-(3-phenylpropyl-amino)-benzoic acid (NPPB), did not prevent these effects. The effect of Ba²⁺ on PD_bl was bimodally distributed: paradoxically, in the tubules in which Ba²⁺ largely depolarized, the effects on PD_bl of the bath K⁺ concentration increases were not inhibited by extracellular Ba²⁺, in tubules in which Ba²⁺ moderately depolarized, Ba²⁺ partially inhibited the K⁺ concentration increase-induced depolarization of the basolateral membrane. However, the parallel decrease in FR_bl was Ba²⁺ insensitive, indicating that the K⁺ channel of the basolateral membrane was not modified by extracellular Ba²⁺. The Ba²⁺-induced depolarizations were prevented by furosemide suggesting that Ba²⁺ acts by inhibiting basolateral KCl extrusion. Finally, the K⁺ concentration increase-induced depolarizations were insensitive to tetraethylammonium, charybdotoxin, apamin and verapamil. In conclusion, the present study provides evidence that, in addition to a Ba²⁺-sensitive KCl cotransport system, the basolateral membrane of rabbit cTAL cells possesses a K⁺ conductance which is insensitive to extracellular Ba²⁺.