Water Absorption

It is well established that water absorption by epithelial tissues is dependent on and proportional to total solute absorption. The nature of this proportionality is illustrated in Fig. 6. Thus, in leaky epithelia such as gall bladder and small intestine, the absorbed fluid is isotonic with that in the lumen and plasma; in short, approximately 1 L of water accompanies the absorption of 300 mOsm of solute. In other tighter epithelia, less than 1 L of water accompanies the absorption of 300 mOsm so that the absorbed fluid or absorbate is hypertonic with respect to plasma, and the fluid in the lumen becomes increasingly hypotonic.

FIGURE 6 Relation between water (or volume) and solute absorption observed in GI epithelia.

36. Intestinal Electrolyte and Water Transport

FIGURE 7 (A) Cellular model of fluid absorption by leakey epithelia. (B) Cellular model of fluid absorption by tight epithelia.

In tight epithelia, the situation is generally quite different, as illustrated in Fig. 7B. Whereas the baso-lateral membranes are highly permeable to water, the junctions and apical membrane are, generally, far less so. Thus, although solute absorption still generates an osmotic driving force, water has difficulty keeping up with the rate of solute absorption. The result is the absorption of a hypertonic solution from the lumen that may render the latter hypotonic. In the distal colon, however, this is offset by the secretions of K+ and HCO" and the accumulation of osmotically active breakdown products of bacterial digestion. Consequently, the fecal water may be hypertonic to plasma.

In summary, it is universally accepted that water absorption is a passive process that is always secondary to and completely dependent on solute absorption. Although several models have been suggested to explain this observation, the simplest and most widely held is that described earlier in which the mechanism is simple osmosis into subepithelial regions that are rendered hypertonic by solute absorption. The osmolarity of the absorbate is determined by the ability of water absorption to keep pace with solute absorption, and this is largely determined by the permeabilities of the apical membranes and junctional pathways to water.

Intestinal Secretion

The small and large intestines also have the ability to secrete water and electrolytes. This secretory capacity appears to reside mainly in cells located in their crypts, whereas absorption by these organs is primarily carried out by cells located in the upper two-thirds of the villi.

The mechanism of secretion is illustrated in Fig. 8. The basolateral membranes of these secretory cells possess a NaKCl2 cotransport ("tritransport") mechanism that mediates the neutral influx of Na + , K + , and Cl" and is capable of driving Cl" into the cell against an electrochemical potential difference; thus, the intra-cellular concentration of Cl" is much greater than that predicted by the Nernst equation (see Chapter 2) for a passive distribution. The energy needed to achieve this is derived from the coupling of the uphill movement of Cl" to the downhill movement of Na+ (i.e., the Na + gradient).

The apical membranes of these secretory cells possess two types of Cl" channels. One type is activated by elevations in cell Ca2+. The other type is called the cystic fibrosis transmembrane conductance regulator and is identical to Cl" channels found in the apical membranes of secretory cells in the airway epithelia (i.e., bronchioles) and pancreatic acinar cells. These channels are activated by phosphorylation mediated by (cyclic adenosine monophosphate) cAMP-dependent protein kinase A. Defects in these channels may result in impaired secretory ability by these cells, which is responsible for the pulmonary, pancreatic, and intestinal

Mucosal Solution

Serosal Solution

Serosal Solution

Mucosal Solution

Ouabain

FIGURE 8 Cellular model of secondary active Cl" secretion accompanied by passive Na+ secretion by secretory cells in the crypts of the small and large intestines. S, secretagogue. (Note: Although only one Cl" is depicted, as discussed in the text, there are at least two families of apical membrane Cl" channels: one activated by Ca2+ and the other by cAMP.)

Ouabain

FIGURE 8 Cellular model of secondary active Cl" secretion accompanied by passive Na+ secretion by secretory cells in the crypts of the small and large intestines. S, secretagogue. (Note: Although only one Cl" is depicted, as discussed in the text, there are at least two families of apical membrane Cl" channels: one activated by Ca2+ and the other by cAMP.)

abnormalities characteristic of the inherited congenital disease cystic fibrosis.

Under resting or unstimulated conditions, the Cl— channels are inactive, and these secretory cells are poised to secrete Cl—. Stimuli that bring about an increase in cell cAMP and/or Ca2+ open these channels and permit Cl— to diffuse from the cell into the lumen. This negative current, directed from the serosal fluid (plasma) into the lumen, generates a crypt-negative (lumen-negative) electrical potential difference that provides the driving force for the movement of Na+ through paracellular pathways. The end result is the secretion of NaCl into the crypts. This establishes an osmotic pressure difference that draws water into the crypts. The fluid that emerges from the crypts into the lumen is essentially isotonic saline.

Intestinal secretion is an important physiologic event as well as a potentially life-threatening pathologic event. Secretion is stimulated by many of the GI hormones and neurotransmitters activated after a meal. These secretagogues interact with receptors on the basolateral membranes of the secretory cells and initiate the cascade of events leading to an elevation of cell cAMP and/or Ca2+. For example, acetylcholine, which is released by the enteric nervous system during a meal, brings about an increase in cell Ca + , whereas the GI hormones secretin and VIP result in an elevation of cell cAMP. The resulting secretion undoubtedly assists in the

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