Potassium transport by medullary collecting tubule of rabbit: Effects of variation in K intake

These studies examine the effect of dietary K content on the rate efflux coefficient (Kk) assessed both isotopically (42K) and by net K flux in segments dissected from the inner stripe of the outer medullary collecting tubule (MCT). Female New Zealand White rabbits were given either a K-replete diet (KCl diet) or one of two K-restricted diets with similar K content. After 7 days, renal K excretion varied with K intake. To determine whether alterations in Kk of MCT could contribute to these changes in urinary K excretion, the MCT from the three groups of rabbits was perfused in vitro and volume reabsorption and Kk were measured. Net K flux in the presence of a 15 mM lumen-to-bath K gradient confirmed a significantly greater K reabsorptive flux and net rate coefficient in either K-restricted group than the K-replete control group. A model used to assess the relative contribution of cortical secretory flux and medullary absorptive flux shows that at physiological flow rates for the MCT such alterations in the apparent K permeability of the MCT could have dramatic effects on the reabsorption of K from the lumen and thereby on K excretion. These observations demonstrate that K movement from lumen to interstitium is influenced by dietary K intake and that the MCT participates in K homeostasis