Sidedness and Coupling of Transport Processes in Small Intestinal and Renal Epithelia
Epithelial cells separate phases of different ionic composition and by transepithelial transport maintain or establish the asymmetric distribution of ions. The epithelia of the small intestine and the proximal tubule are additionally specialized in the (re)absorption of a variety of solutes, such as sugars, amino acids, and phosphate. Such vectorial transports can only be performed if a sidedness of the epithelial cell in respect to the transport properties of the luminal and contraluminal membrane exists. Maintenance of asymmetric distribution of solutes involves active transport processes that are energized by cellular metabolism. The coupling of transepithelial transport to energy can be direct or indirect. In primary active transport the movement of a solute across the cell membrane is directly coupled to an energy-transducing chemical reaction, such as the coupling of sodium transport via the Na+-K+-ATPase to ATP hydrolysis. In secondary active transport the movement of the solute across the membrane is coupled to a flux of a primary actively transported substrate.
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