Separate regulation of Na+ and anion transport by IMCD: Location, aldosterone, hypertonicity, TGF-beta 1, and cAMP

We have investigated some of the factors known or suspected to influence ion transport by the rat inner medullary collecting duct and have analyzed their actions on active Na+ absorption and active anion secretion by primary cultures. Cells from the terminal 1-2 mm (tip) of the papilla had a lower basal rate of Na+ absorption (2.0 mu A/cm(2)) than cells from the more proximal portions (6.5 mu A/cm(2)). Aldosterone increased Na+ transport approximately sevenfold in the tip cells and approximately threefold in the proximal cells. The magnitude of anion secretion in response to adenosine 3',5'-cyclic monophosphate (cAMP) agonists was similar in the two regions and was unaffected by aldosterone. The morphology of monolayers from both regions was also similar. In monolayers cultured from the entire inner medulla, hypertonic (100 mosM) urea, NaCl, or sucrose reduced Na+ transport but had no significant effect on anion secretion. Transforming growth factor-beta 1, known to blunt the effect of steroids on Na+ transport, had no effect on anion secretion. Finally, cAMP had no effect on Na+ transport, a result that contrasts with its effect on Na+ transport by other epithelial cells demonstrating steroid-responsive, electrogenic Na+ transport. These results demonstrate some potential differences in the magnitude of Na+ transport by position along the inner medulla. They further demonstrate separate regulation of Na+ and anion transport.