Estimated potassium reflection coefficient in perfused proximal convoluted tubules of the anaesthetized rat in vivo.

Abstract

1. As yet there is no definitive description of the mechanism and route by which K+ reabsorption is achieved in the proximal convoluted tubule (PCT). We have assessed the contribution of convective K+ transport to net potassium ion flux (JK) by estimating the reflection coefficient of K+ (sigma K) in the proximal tubule of anaesthetized rats previously prepared for in vivo microperfusion. 2. Alterations in the luminal concentration of the impermeant solute raffinose in single-perfused (lumen only) and double-perfused (lumen and capillaries) PCTs were found to change fluid reabsorption in a predictable fashion. 3. Net potassium ion flux (JK) in single- and double-perfused tubules was significantly correlated with net fluid flux (Jv), suggesting that convective K+ transport may be a significant factor in overall K+ transport by the PCT. 4. Estimates of sigma K in single- and double-perfused tubules were very similar (0.14 +/- 0.06 and 0.13 +/- 0.05, respectively), even though K+ diffusion was not strictly controlled in the former group. The maximum effect of 'pseudo-solvent' drag in double-perfused tubules was estimated to give a sigma K of 0.40. This low value for sigma K suggests that true convection/solvent drag may be an important driving force for the reabsorption of K+ from the PCT of the rat.