Microperfusion study of distal tubular potassium and sodium transfer in rat kidney

Abstract

The ability of proximal and distal tubules to establish limiting transepithelial electrochemical potential gradients was tested in stationary microperfusion experiments in vivo. Droplets of isoosmotic raffinose solution were injected as initial perfusion fluid. Samples of reaspirated perfusate were analyzed by ultramicroflame photometry for sodium and potassium. Transtubular electrical potential differences of perfused nephron segments were measured by glass microelectrodes. Comparison of potas- sium predicted for electrochemical equilibrium indicates that the distal tubular potassium concentration is always below the equilibrium value and significantly affected by changes in the electrical potential gradient. It is postulated that entry of potassium into the distal tubular lumen is passive and that a component of active reabsorption prevents attainment of electrochemical equilibrium. At uniform electrical potential difference along the distal tubule, the progressive increase in potassium concentration along this nephron segment is believed to be due to shift in the balance between an active reabsorptive potassium pump and passive potassium leak into the lumen.