The role of active transport in potassium reabsorption in the proximal convoluted tubule of the anaesthetized rat.

Abstract

1. We have previously assessed the contributions of diffusion and convection to net potassium reabsorption in the rat proximal convoluted tubule (PCT). The present study was conducted to evaluate the role of active transport in convective potassium reabsorption by measuring the transepithelial potassium reflection coefficient (aK) in the presence and absence of cyanide in anaesthetized rats previously prepared for in vivo microperfusion. 2. Osmotic water permeability (Pf) was measured in double-perfused tubules (lumen and peritubular capillaries) by manipulating the applied transepithelial osmotic gradient between -30 and +40 mosmol (kg H20)-4 using raffinose added to or subtracted from luminal perfusates. Pf was unaffected by the presence of cyanide when the estimated dissipation of osmotic gradients along each tubule were taken into account.3. The proportion of K+ (and Na+) convectively transported with water fluxes (i.e. oJK) was not affected by cyanide. In the absence of active transport and following correction for any diffusive component, 0K was 0-56 + 0-13, indicating substantial solvent drag which probably occurs via the paracellular pathway. 4. However, cyanide caused a reduction in net potassium flux over the entire range of fluid fluxes used in double-perfusion experiments. Subsequent single-perfusion experiments (tubule lumen only) using the specific K+-H+-ATPase inhibitor, SCH28080, failed to reveal any direct evidence for a primary active K+ transporting mechanism involved in K+ reabsorption in the PCT.

Transepithelial electrochemical gradients in the proximal convoluted tubule during potassium depletion in the rat

“…Its function is unknown. In vivo microperfusion experiments using SCH28080 failed to disclose a significant effect on net potassium fluxes, but interpretation of the data was hampered by the finding that the vehicle for SCH28080, dimethyl sulphoxide, itself affected potassium transport (Wilson et al 1997). The latter authors speculated that the K + ‐ATPase might provide a secretory component, in view of the observation that the activity of the pump is virtually abolished during potassium depletion (Younes‐Ibrahim et al 1995).…”

“…Recent evidence suggests that most, if not all, potassium reabsorption in the proximal convoluted tubule is passive (Giebisch & Wang, 1996). It is believed that a substantial amount of this reabsorption takes place by solvent drag through the paracellular route (Kibble et al 1995; Wilson et al 1997), but a possible second component is diffusion down an electrochemical gradient. Although the transepithelial potential difference (PD) is lumen negative in the very early (S 1 ) segment, in most of the proximal convoluted tubule (S 2 segment) the transepithelial PD is lumen positive, thus favouring potassium reabsorption.…”

Directional Fluid Transport across Organ–Blood Barriers: Physiology and Cell Biology

“…The remaining fluid flux occurs paracellularly via TJ expressing “leaky” claudins such as claudin-2 (Muto et al 2010) and possibly claudin-10 and claudin-17 (Krug et al 2012), which are also selective to cations, e.g., Na + , K + , Ca 2+ and Mg 2+ (Bomsztyk et al 1984; Wilson et al 1997; Wilson et al 1998; Muto et al 2010). …”

Potassium transport in the maturing kidney

“…1). Most K + reabsorption in the PCT is passive and occurs by solvent drag via the paracellular pathway [16,17]. The positive transepithelial voltage of the late proximal tubule provides a favorable driving force for diffusion of K + out of the lumen [18].…”