K activity of CCD principal cells from normal and DOCA-treated rabbits

We used liquid ion exchanger and conventional microelectrodes to evaluate the effects of mineralocorticoids on the intracellular K activity (a(i)(K)) and K transport properties of principal cells (PC) of isolated cortical collecting ducts (CCDs). Hoffman modulation optics and electrophysiological methods were used to identify PC. K activity was measured with two single-barreled electrodes. We found that a(i)(K) of PC from deoxycorticosterone acetate (DOCA)-treated rabbits (97.6 mM) was not different from controls (94.8 mM). The driving forces for K transport across the basolateral membrane favored cell to bath (reabsorption) in PCs from controls and bath to cell (secretion) in PCs from DOCA-treated rabbits. However, the driving force for K secretion across the apical membrane was not significantly different between the two groups. We used the intracellular a(i)(K)s and bath ion substitutions (gluconate for Cl and K for Na) to evaluate the effects of DOCA on the ion-selective properties of the basolateral membrane of PC. DOCA increased P(K)/P(Cl) from 0.33 to 0.89. Our conclusion was as follows: in PC of control rabbits K is above electrochemical equilibrium across the basolateral membrane. However, the basolateral K conductance is probably too small for significant K recycling. In PC of DOCA-treated rabbits the a(i)(K) is below electrochemical equilibrium across the basolateral membrane and the K conductance is increased. These effects enhance K secretion across this border while maintaining cell K constant.