<prompt> A luminal cell membrane Na channel is the main pathway for Na reabsorption in:Proximal tubule cellsThick ascending limb cellsDistal convoluted tubule cellsCollecting duct principal cells  <response>: Ans D. Collecting duct principal cellsNa reabsorption by collecting duct principal cells occurs via a Na channel called ENaC epithelial sodium channel. Na reabsorption in proximal tubule cells is coupled to transport of solutes via cotransport e.g., Na-glucose and antiport e.g., NaH exchanger mechanisms. Na reabsorption in the thick ascending limb involves a Na-K-2Cl cotransporter and, in the distal convoluted tubule, a Na-Cl cotransporter. Collecting duct intercalated cells are primarily concerned with acid-base, not Na, transport. 