Figure 2.

Model of electrolyte transport in the principal cells of cortical collecting duct (CCD). Basolateral Na⁺/K⁺-ATPase (Na⁺/K⁺ pump) create the electrochemical gradient which provides a conductive pathway for Na⁺ entry into the cell down through an epithelial Na⁺ channel (ENaC) on the apical membrane. The higher permeability of the luminal membrane for Na⁺ creates a lumen-negative transepithelial potential difference, and provides an important driving force for the secretion of K⁺ into the lumen via apical K⁺ channels, apical inwardly rectifying channel ROMK (K_ir1.1), and the Ca²⁺-activated K⁺ channel (BK). Basolateral membrane K⁺ channels (heteromeric K_ir4.1/K_ir5.1 is the main basolateral K_ir channel) provide K⁺ recycling necessary for Na⁺/K⁺ pump activity and Na⁺ transport in principal cells. Apical aquaporin 2 (AQP2) and basolateral aquaporin 3/4 (AQP3/4) water channels carry H₂O molecules across cell membranes, and are responsible for water transport in the CD.