Mechanisms of rubidium permeation by rabbit cortical collecting duct during potassium restriction

To examine the pathways of K permeation in the cortical collecting duct (CCD) from K-restricted rabbits, we studied the effects of the following three maneuvers: 1) luminal amiloride addition; 2) luminal Ba addition; and 3) luminal Na removal. Luminal addition of amiloride (1 mM) significantly increased the ⁸⁶Rb lumen-to-bath efflux coefficient (K_Rb), and this effect was fully blocked by the presence of 2 mM luminal Ba. Addition of 2 mM luminal Ba reduced K_Rb both in the absence of luminal Na and in the presence of luminal Na. In contrast to the effect of amiloride addition, removal of luminal Na significantly increased K_Rb, but neither 2 mM luminal Ba nor 4 mM luminal Ba totally abolished this effect. However, simultaneous addition of luminal Ba and Sch 28080 (10 μM) fully inhibited the increase in K_Rb upon luminal Na removal, indicating that luminal Na removal enhances Rb efflux in part via H-K-adenosmetriphosphatase (H-K-ATPase). To test whether Na acts as a partial agonist for cation efflux via the H-K-ATPase we examined the effect of Sch 28080 on the ²²Na lumen-to-bath efflux coefficient (K_Na). These studies were conducted in the presence of 0.1 mM luminal amiloride to block fully apical conductive Na efflux, and the effect of Sch 28080 on X_Na was examined at two different ambient K concentrations. In the presence of 0.5 mM K, Sch 28080 (10 μM) significantly inhibited K_Na from 47.6 ± 4.8 to 35.0 ± 6.8 nm/s (P < 0.005). In contrast, the effect of Sch 28080 on K_Na was abolished when similar experiments were performed in the presence of 20 mM K. Moreover, 20 mM K did not abolish the effect of Sch 28080 on K_Rb. From these data we conclude that 1) an apical Ba-sensitive K conductance mediates the stimulation of K_Rb by amiloride; 2) both a Ba-sensitive K conductance and the H-K-ATPase mediate the enhancement of K_Rb after luminal Na removal; and 3) Na can compete with K for transport via the H-K-ATPase.