ESTIMATED POTASSIUM REFLECTION COEFFICIENT IN PERFUSED PROXIMAL CONVOLUTED TUBULES OF THE ANESTHETIZED RAT IN-VIVO

1. As yet there is no definitive description of the mechanism and rout e by which Kf reabsorption is achieved in the proximal convoluted tubu le (PCT). We have assessed the contribution of convective K+ transport to net potassium ion flux (J(K)) by estimating the reflection coeffic ient of K+ (sigma(K)) in the proximal tubule of anaesthetized rats pre viously prepared for in vivo microperfusion. 2. Alterations in the lum inal concentration of the impermeant solute raffinose in single-perfus ed (lumen only) and double-perfused (lumen and capillaries) PCTs were found to change fluid reabsorption in a predictable fashion. 3. Net po tassium ion flux (J(K)) in single- and double-perfused tubules was sig nificantly correlated with net fluid flux (J(V)), suggesting that conv ective K+ transport may be a significant factor in overall K+ transpor t 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, respective ly), even though K+ diffusion was not strictly controlled in the forme r group. The maximum effect of 'pseudo-solvent' drag in double-perfuse d tubules was estimated to give sigma(K) of 0.40. This low value for s igma(K) suggests that true convection/solvent drag may be an important driving force for the reabsorption of K+ from the PCT of the rat.