Basolateral K+conductance in principal cells of rat CCD

Abstract

Whole cell K+current was measured by forming seals on the luminal membrane of principal cells in split-open rat cortical collecting ducts. The mean inward, Ba2+-sensitive conductance, with 40 mM extracellular K+, was 76 ± 12 and 141 ± 22 nS/cell for animals on control and high-K+diets, respectively. The apical contribution to this was estimated to be 3 and 16 nS/cell on control and high-K+diets, respectively. To isolate the basolateral component of whole cell current, we blocked ROMK channels with either tertiapin-Q or intracellular acidification to pH 6.6. The current was weakly inward rectifying when bath K+was ≥40 mM but became more strongly rectified when bath K+was lowered into the physiological range. Including 1 mM spermine in the pipette moderately increased rectification, but most of the outward current remained. The K+current did not require intracellular Ca2+and was not inhibited by 3 mM ATP in the pipette. The negative log of the acidic dissociation constant (p Ka) was ∼6.5. Block by extracellular Ba2+was voltage dependent with apparent Kiat −40 and −80 mV of ∼160 and ∼80 μM, respectively. The conductance was TEA insensitive. Substitution of Rb+or NH4+for K+led to permeability ratios of 0.65 ± 0.07 and 0.15 ± 0.02 and inward conductance ratios of 0.17 ± 0.03 and 0.57 ± 0.09, respectively. Analysis of Ba2+-induced noise, with 40 mM extracellular K+, yielded single-channel currents of 0.39 ± 0.04 and −0.28 ± 0.04 pA at voltages of 0 and −40 mV, respectively, and a single-channel conductance of 17 ± 1 pS.