Analysis of K+ transport by rabbit CCD: conductive pathways and K(+)-K+ exchange by Na(+)-K+ pump

We studied the cellular pathways of K+ transport by the rabbit cortical collecting duct that was stimulated to absorb Na+ and to secrete K+. The vast majority of K+ secretion (into the lumen) was inhibited by benzamil, a blocker of epithelial Na+ channels. The residual K+ secretion was completely inhibited by ouabain. Thus all active K+ secretion was dependent on Na+ transport by the Na(+)-K+ pump. The passive pathways of K+ transport were further examined using tracer and electrophysiological measurements. K+ transfer across the apical membrane was predominantly or exclusively conductive; the apical K+ conductance was 31 mS/cm2. The basolateral membrane contained two pathways for K+ tracer translocation. The (barium-sensitive) conductive pathway accounted for a relatively small (12-20%) portion of the tracer permeation. A larger pathway appeared to be via K(+)-K+ exchange on the Na(+)-K+ pump. The magnitude of the Ba2(+)-sensitive (basolateral) K+ conductance predicted a substantially larger tracer flux than was actually measured. The best explanation for this difference is the presence of single-file diffusion through K+ channels on the apical and basolateral membranes. An analysis of the electrically silent K+ transport from lumen to bath suggests that the Na(+)-K+ pump can vary the ratio of its Na(+)-K+ and K(+)-K+ modes of operation. When the tubule is actively transporting Na+ and K+, the Na(+)-K+/K(+)-K+ turnover ratio is greater than 7. When Na+ transport is limited by inhibiting Na+ entry across the apical membrane, the ratio falls to less than 1. A major factor determining this ratio is probably the availability of Na+ to the cytoplasmic side of the pump.