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1 Department of Physiology and Biophysics, Weill Medical College of Cornell University, New York, NY, USA
* To whom correspondence should be addressed. E-mail: dag2007{at}med.cornell.edu.
Whole-cell K+ current was measured by forming seals on the luminal membrane of principal cells
in split-open rat CCD's. The mean inward, Ba+2-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 Ca+2 and was not inhibited by 3 mM ATP in the pipette. The pKa was ~6.5. Block by extracellular Ba+2 was voltage-dependent with apparent Ki at -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 Ba+2-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.
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