As noted previously, the Na+,K+-ATPase and K+-selective channels play an important role in establishing cellular ion gradients for Na+ and K+ and in generating the Vm. In all epithelial cells except the choroid plexus and retinal pigment epithelium, the Na+,K+-ATPase channel is located in the basolateral membrane of the cell. Numerous K+selective channels are in epithelial cells and may be located in either membrane domain. Through the establishment of these chemical and voltage gradients, the transport of other ions and solutes can be driven (e.g., Na+-glucose symporter, Na+-H+ antiporter, 1Na+,1K+,2Cl− symporter, 1Na+-3HCO3 − symporter). The direction of transepithelial transport (reabsorption or secretion) depends simply on which membrane domain the transporters are located. Because of the dependence on the Na+,K+-ATPase, epithelial transport requires the expenditure of energy. Other ATP-dependent transporters, such as the H+-ATPase, H+,K+-ATPase, and a host of ABC transporters—such as P-glycoprotein (PGP) and multidrug resistance-associated protein 2 (MRP2), which transport xenobiotics (drugs), and cystic fibrosis transmembrane conductance regulator (CFTR), which transports Cl−—are involved in epithelial transport.
hThe choroid plexus is located in the ventricles of the brain and secretes the cerebrospinal fluid. The Na+,K+-ATPase channel is located in the apical membrane of these cells.
CHAPTER 2 Homeostasis: Volume and Composition of Body Fluid Compartments •Fig. 2.11 Insymmetricalcells(A; e.g.,redbloodcells),membranetransportproteinsaredistributedovertheentiresurfaceofthecell.Epithelialcells(B), incontrast,areasymmetricalandtargetvariousmembranetransportproteinstoeithertheapicalorthebasolateralmembrane.Whenthetransportersareconfinedtoamembranedomain,vectorialtransportcanoccur.Inthecelldepicted,Na+ istransportedfromtheapicalsurfacetothebasolateralsurface.ATP,adenosinetriphosphate.
Solutes and water can be transported across an epithelium by traversing both the apical and basolateral membranes (transcellular transport) or by moving between the cells across the tight junction (paracellular transport). Solute transport via the transcellular route is a two-step process, in which the solute molecule is transported across both the apical and basolateral membrane. Uptake into the cell, or transport out of the cell, may be either a passive or an active process. Typically, one of the steps is passive, and the other is active. For the example shown in
Fig. 2.11B , the uptake of Na+ into the cell across the apical membrane through the Na+-selective channel is passive and driven by the electrochemical gradient for Na+ . The exit of Na+ from the cell across the basolateral membrane is primary active transport via the Na+,K+-ATPase channel. Because a transepithelial gradient for Na+ can be generated by this process (i.e., the [Na+] in the apical compartment can be reduced below that of the basolateral compartment, the overall process of transepithelial Na+ transport is said to be active). Any solute that is actively transported across an epithelium must be transported via the transcellular pathway.