Ionic dependence of cell volume regulation by the thin ascending limb of Henle's loop

Thin ascending limb cells from Henle's loop were studied with optical and video techniques to evaluate cell volume regulation in response to anisoosmotic media and its ionic dependence. Cell volume regulation was observed when these cells were exposed to hypoosmotic solutions. Under hyperosmotic conditions only an osmometric response was found, with no volume regulatory increase (VRI). The removal of Cl- or HCO3- abolished the volume regulatory decrease (VRD) normally observed during exposure to hypoosmotic solutions. Re-addition of these ions did not elicit the VRD response. The removal of K+ from hypoosmotic solutions abolished VRD but its re-introduction restored the volume regulatory response. In the absence of Na+, a partial inhibition of VRD was found; re-addition of Na+ completely restored the regulatory response. These indicate that cells from the thin ascending limb of Henle's loop regulate their volume under hypoosmotic conditions, and that this process is dependent upon Cl-, HCO3-, Na+ and K+, with different patterns of response being observed upon addition or deletion of these ions.