Na+-K+-ATPase activities in renal tubule segments of rat inner medulla.

Recent studies have revealed that both the thin segment of the long loop of Henle and the inner medullary collecting duct (IMCD) are morphologically and functionally nonuniform along their lengths. In the present study, we have carried out measurements of Na+-K+-ATPase activity in thin limb and IMCD segments microdissected from different positions along the axis of the rat inner medulla. Among collecting duct segments, in control rats, Na+-K+-ATPase activity was highest in the initial quarter of the IMCD. The Na+-K+-ATPase activity was lower and relatively uniform along the remaining three quarters of the IMCD. Na+-K+-ATPase activities in cortical and outer medullary collecting duct segments were similar to previously reported values. The Na+-K+-ATPase activity in the initial quarter of the IMCD was increased following in vivo deoxycorticosterone administration (by 140%) and following dietary NaCl restriction (by 100%). These procedures increased Na+-K+-ATPase activity by smaller degrees or not at all in the remaining three-quarters of the IMCD. Thin descending limbs and thin ascending limbs from all regions of the inner medulla had low but readily measurable values. Based on these results, we conclude that all inner medullary renal tubule segments have measurable Na+-K+-ATPase activities that could potentially drive solute active transport.