Potassium permeability in the absence of fluid reabsorption in proximal tubule of the anesthetized rat.

A luminal microperfusion technique was used to examine the K+ permeability of surface proximal convoluted tubules (PCT) in the kidney of anesthetized rats. Transtubular potassium concentration ([K+]) gradients were varied by altering the concentration of KCl in luminal perfusates, to which 32 mmol/l of the impermeant solute raffinose was also added to prevent net fluid reabsorption. The arithmetic mean transtubular [K+] gradient was highly predictive of net potassium flux, yielding an apparent K+ permeability of 31.9 +/- 1.7 x 10(-5) cm/s in the absence of fluid reabsorption. When compared using identical calculation techniques, we found this was not significantly different from the permeability derived in a previous study when fluid reabsorption was present [J. D. Kibble, M. Wareing, R. W. Wilson, and R.Green. Am. J. Physiol. 268 (Renal Fluid Electrolyte Physiol. 27): F778-F783, 1995]. We conclude that fluid reabsorption does not affect the apparent permeability of the proximal tubule to potassium. The apparent permeability to 86Rb, measured following its addition to luminal perfusates, was not significantly different from the value obtained for K+, suggesting that rubidium is a useful marker for net potassium movements in the PCT of the rat.