Electrolyte transport in kidney tubule cells

Abstract

Transtubular movement of Na and K takes place across an electrically negative cell compartment rich in K and poor in Na. Some properties of the luminal and peritubular cell boundaries with respect to ionic pump and leak characteristics are analysed. Sodium enters the tubule cell from the lumen down an electrochemical potential gradient. Peritubular Na-extrusion takes place both by an ouabain-sensitive Na-K exchange pump and by an electrogenic ouabain-insensitive Na pump. Net Na transport can be uncoupled from peritubular K uptake. It is highly likely that peritubular K uptake is pH sensitive. Once sodium has been extruded into the peritubular infoldings net Na transepithelial-transport is further critically affected by physical factors regulating capillary uptake of interstitial fluid. Several lines of evidence indicate that a large, variable intercellular transport pathway is present at the proximal tubular level. Tubular K secretion is controlled at the distal tubular level by: (1) the interplay of luminal and peritubular active K uptake into the tubule cell and (2) by a variable passive leak of K from cell into lumen across the partly depolarized luminal cell membrane. Changes in active peritubular K uptake regulate the size of a relatively small intracellular K transport pool and are critically involved in setting the rate of net tubular potassium secretion.