In vivo evidence of impaired solute transport by the thick ascending limb in potassium-depleted rats.

The objective of this investigation was to determine if thick ascending limb (TAL) solute removal is impaired in potassium-depleted rats, in vivo. We estimated TAL NaCl concentration by measuring in situ conductivity of tubular fluid presented to the early distal site after stop-flow periods of 10-60 s, during which a proximal equilibrium solution remained in contact with the reabsorbing epithelium. This allowed us to calculate the rate constant of the decrease in tubular fluid NaCl concentration and to determine equilibrium values for control, potassium-depleted, and potassium-repleted rats. After 60 s of stop-flow, NaCl concentration of TAL fluid decreased to 18.3 +/- 2.73 mM in control rats, while potassium-depleted rats had values almost twice as high (36.5 +/- 2.97 mM, P less than 0.01). The amount of NaCl remaining after 60 s of stop-flow in K-depleted rats was highly correlated with the plasma K concentration. Calculated rates of NaCl efflux from the TAL appeared to be normal in K-depleted rats while the concentration of NaCl achieved at equilibrium was nearly twice that measured in control rats. Acute systemic administration of KCl by gavage or infusion in K-depleted rats was associated with a decrease in TAL NaCl concentration to normal values. Addition of K to the perfusate, however, did not repair the defect. Our results can best be explained by assigning a special role to the peritubular K concentration. We suggest that the defect in TAL solute removal in K-depletion can be rapidly reversed, because decreases in peritubular K concentration limit Na efflux across the peritubular membrane by decreasing the activity of the Na-K-ATPase pump. We recognize that factors such as regional renal blood flow, local angiotensin II levels, and products of the cyclo-oxygenase enzyme system may play a role.