Coordinated regulation of intracellular K+ in the proximal tubule: Ba2+ blockade down-regulates the Na+,K+-ATPase and up-regulates two K+ permeability pathways.

To avoid large changes in cell K+ content and volume during variations in Na+,K+-ATPase activity, Na+-transporting epithelia must adjust the rate of K+ exit through passive permeability pathways. Recent studies have shown that a variety of passive K+ transport mechanisms may coexist within a cell and may be functionally linked to the activity of the Na+,K+-ATPase. In this study, we have identified three distinct pathways for passive K+ transport that act in concert with the Na+,K+-ATPase to maintain intracellular K+ homeostasis in the proximal tubule. Under control conditions, the total K+ leak of the tubules consisted of discrete Ba2+-sensitive (approximately 65%), quinine-sensitive (approximately 20%), and furosemide-sensitive (approximately 10%) pathways. Following inhibition of the principal K+ leak pathway with Ba2+, the tubules adaptively restored cell K+ content to normal levels. This recovery of cell K+ content was inhibited, in an additive manner, by quinine and furosemide. Following adaptation to Ba2+, the tubules exhibited a 30% reduction in Na+-K+ pump rate coupled with an increase in K+ leak by means of the quinine-sensitive (approximately 70%) and furosemide-sensitive (approximately 280%) pathways. Thus, the proximal tubule maintains intracellular K+ homeostasis by the coordinated modulation of multiple K+ transport pathways. Furthermore, these results suggest that, like Ba2+, other inhibitors of K+ conductance will cause compensatory changes in both the Na+-K+ pump and alternative pathways for passive K+ transport.