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+ permeabi lity of surface proximal convoluted tubules (PCT) in the kidney of ane sthetized 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 techn iques, we found this was not significantly different from the permeabi lity 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 conclu de that fluid reabsorption does not affect the apparent permeability o f the proximal tubule to potassium. The apparent permeability to Rb-86 , measured following its addition to luminal perfusates, was not signi ficantly different from the value obtained for K+, suggesting that rub idium is a useful marker for net potassium movements in the PCT of the rat.