FLUID AND ION-TRANSPORT IN CORNEAL ENDOTHELIUM - INSENSITIVITY TO MODULATORS OF NA-K+-2CL(-) COTRANSPORT()

The role of N+-K+-2Cl(-) cotransport in ion and fluid transport of the corneal endothelium was examined by measuring changes in corneal hydr ation and uptake of Rb-86 by the endothelial cell layer. Isolated, int act rabbit corneas maintain normal hydration when they are superfused at the endothelial surface with bicarbonate (HCO3-)-Ringer solutions a s a result of equilibrium between active ion and fluid transport out o f the stromal tissue and leak of fluid into stromal tissue from the aq ueous humor. Furosemide and bumetanide did not alter this equilibrium when they were added to the superfusion medium. Uptake of Rb-86 by the endothelium of the incubated cornea was increased 25% by bumetanide, but uptake in the presence of ouabain (70% less than that of controls) was not changed by bumetanide. In Na+-free medium, uptake of Rb-86 wa s reduced by 58%, but it was unchanged in Cl--free medium. Calyculin A , a protein phosphatase inhibitor and activator of Na+-K+-Cl- cotransp ort, was without effect on Rb-86 uptake. Hypertonicity (345 mosmol/kg) increased uptake slightly, whereas hypotonicity (226 mosmol/kg) cause d a 33% decrease. Neither of these changes was significantly different when bumetanide was present in the media. It is concluded that Na+-K-2Cl(-) cotransporter activity is not exhibited by the in situ corneal endothelium and does not play a role in the ion and fluid transport o f this cell layer. Its presence in cultured endothelial cells may refl ect the reported importance of this protein in growth, proliferation, and differentiation.