Effects of changes in electrical potential difference on tubular potassium transport.

To assess directly the role of the transepithelial potential difference (PD) on potassium concentration differences across distal tubular epithelium, continuous and stationary microperfusion experiments were done in tubules voltage-clamped over a wide range of lumen-negative potentials. Potassium was measured either chemically or in situ by potassium-sensitive microelectrodes. Distal cell PD measurements show that most of the potential drop induced by luminal current injection occurred across the luminal cell membrane. Experiments were done in rats either on a control or on a high potassium diet and after amiloride administration. Luminal potassium was highly sensitive to imposed electrical potential changes, attainment of a new steady-state intraluminal potassium concentration was rapid (less than 1 s), and higher luminal potassium concentrations were observed in animals in which potassium secretion had been stimulated. Similar slopes of tubular fluid-to-plasma potassium ratios versus transepithelial potential differences were observed in all experiments. All slopes intersected, at zero PD, at a luminal tubular fluid-to-plasma concentration ratio in excess of unity, indicating the presence of an active component of potassium secretion.