Hco3

Short-chain fatty acids

Apical

Basolateral membrane Apical and basolateral membrane Apical

Apical

Apical and basolateral channels

Active; antiport

Secondary; antiport

Antiport; passive; electrochemically neutral

Diffusion; passive (secretion) and some active transport proteins at the apical surface (absorption and secretion), including CFTR

Antiport; active transport (basolateral membrane) Active secretion at the apical membrane;

linked to Cl transport function Absorptive active apical K-ATPase pumps in distal colon Alkaline phosphatase linked Passive transport mechanisms Na-HCO3 cotransporter postulated CFTR-synchronized apical channel and

Cl-HCO3 exchanger postulated Postulated link to NA-H ion transport

Principal ion involved in water absorption

Principal ion involved in water secretion

Basal rate of secretion influenced by several mediators (endocrine, paracrine, neural, luminal, etc.)

Principal anion of the colon allowing water absorption. This osmotic gradient facilitates water absorption via both transcellular and paracellular pathways.

Transcellular water transport mechanisms such as aquaporins, or water channels, have been described. The paracellular pathway of water transport has been studied extensively, a process often described as 'solvent drag' (Figure 4A).

The leakiness of paracellular pathways, which varies by location in the lower alimentary tract (more prominent in the jejunum, with subsequent decrease distally), and the magnitude of the osmotic gradient (also affected by dietary Na content) are important factors affecting solvent drag. The nature of the intercellular junctions in a particular region of the colon determines the permeability or leakiness of that particular epithelial area. Several intercellular structures have been described, including the zona accludens (tight junction), desmosomes (connections between cells), and the zona adherens, which functions in cell adhesion and contributes to maintaining cellular polarity across the membrane. Zona occul-dens are more apical in location and form junctional complexes between cells. It has been postulated that these junctional complexes may be more dynamic than previously believed, responding to signaling mechanisms and subject to regulation, thereby influencing their function and resultant permeability characteristics (Figure 4B).

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