Question: is the thick ascending limb permeable to water

Answer: No, The medullary ascending limb is impermeable to water. Sodium (Na), potassium (K) and chloride (Cl) ions are reabsorbed by active transport. K is passively transported along its concentration gradient through a K leak channel in the apical aspect of the cells, back into the lumen of the ascending limb. This K ``leak'' generates a positive electrochemical potential difference in the lumen. This drives more paracellular reabsorption of Na, as well as other cations such as magnesium (Mg) and importantly calcium Ca due to charge repulsion.