Potassium transport in the early distal tubule of Amphiuma kidney. Effects of potassium adaptation.

Studies were performed to investigate potassium transport in early distal tubule of the doubly-perfused kidney of Amphiuma under control conditions and following K-adaptation. Double barreled K-sensitive microelectrodes were used in stationary microperfusion experiments. Net K-flux was evaluated along with measurements of both cell membrane potential and cell K activity. Net K flux and electrochemical driving forces of K were described over a wide range of peritubular K concentrations. Whereas in control animals, at normal and low peritubular K concentrations K reabsorption occurs, K secretion is induced by elevating peritubular K. In contrast, net K secretion is seen at all peritubular K levels in the K-adapted kidney. Net K secretion approaches saturation at high peritubular K concentrations. Intracellular K activities also approach plateau values which are shifted upward in the state of K-adaptation. In control animals at zero net flux conditions intracellular K is maintained above electrochemical equilibrium across both the peritubular and the luminal cell membrane. After K-adaptation, however, K approaches electrochemical equilibrium across the luminal cell membrane. The results indicate that in control conditions, K is taken up actively into the cell across the peritubular and across the luminal cell barrier. It is likely that both luminal and peritubular transport components (increased luminal K conductance, diminished luminal K cotransport, stimulation of peritubular K-uptake) are responsible for increased K secretion during K adaptation.