Electrolyte Transport Ion Channels

Fluids and electrolytes are absorbed via either the transcellular or the paracellular pathway. Active and passive transport systems exist via both of these pathways.

There is a clear polarity to the distribution of protein transporters, channels, and pumps distinguishing the apical from the basolateral membrane. Active transport utilizes a transcellular, energy-driven protein pump or channel to facilitate passage of an electrolyte from an area of low concentration to one of high concentration (electrochemical gradient). A prime example of this is the NA-K ATPase pump, the principal pump present along the baso-lateral membrane. The net effect of the three Na ions expelled for every two K ions accepted into the cell is a lowered intracellular Na content and resultant net negative charge. The negative charge formed by this active transport creates an electrochemical gradient facilitative to the passive flow for other ions across the cell membrane—a process known as secondary active transport (Figure 2).

Ion transporters may be subclassified into sym-porters, in which ions move in the same direction, or antiporters, in which ions move in opposite directions across the cell membrane (Figure 3). Cotransport of ions occurs with other molecules, such as Na and glucose. The intracellular concentration of glucose is regulated both by uptake at the apical surface and by exit through the basolat-eral membrane, allowing for conditions favorable to uptake from the lumen. The Na-glucose transporter system allows for therapeutic interventions, such as the use of oral rehydration solution in cases of severe diarrhea related to cholera or other processes. Similar cotransporters are linked to the transport of bile salts and amino acids (Table 2).

Whereas sodium is the primary cation involved in ion transport, short-chain fatty acids constitute the primary anion in the colon and the primary metabolic fuel for colonocytes. Their transport is postulated to be linked to Na-H transporters and pH, specific bicarbonate-linked transporters, and the concentration gradient across cell membranes. Chloride transport occurs via both active and passive processes, and it is the major intestinal anion involved in intestinal secretion of fluids.

Colonic smooth channels also possess ion channels and are involved in active and secondary ion

Apical

Basolateral

H2O^

H2CO3

Apical

H2CO3

Glucose

Glucose

Figure 2 Electrolyte transport at the colonocyte level. (From Despopoulos A and Silbernagl S, Color Atlas of Physiology. New York: Thieme; 2000. Reprinted with permission.)

(A)
Intestinal absorption of water and electrolytes

H2O Intestinal Na+)""w'' lumen

Cell iSjjj^Na-K-

H2O

"T

Blood

Na+ H2O

1. Na+ is concentrated between cells

2. H2O follows the osmotic gradient. Pressure rises

3. H2O and Na+ move towards base of cell and into blood

Figure 3 Electrolyte transport across the cell membrane and the different types of tranporters. (Reproduced with permission from Guyton (1991) Guyton's Textbook of Medical Physiology, 8th edn. Philadelphia: WB Saunders.)

Figure 3 Electrolyte transport across the cell membrane and the different types of tranporters. (Reproduced with permission from Guyton (1991) Guyton's Textbook of Medical Physiology, 8th edn. Philadelphia: WB Saunders.)

transport processes involving calcium. The electrochemical gradient formed by the activity of these ion channels facilitates the function of smooth muscle action potential generation upon depolarization. With the generation of smooth muscle action potentials attaining threshold voltage, contractility of the smooth muscle is possible. The efflux of calcium into these active transport channels activates the process of contraction. Interaction with the enteric nervous system stimulates the release of calcium ions in intracellular stores. The function of ion channels can be modified by calcium channel-blocking drugs. This contractile activity, when it occurs in a coordinated fashion and is modulated by neurotransmission, effects peristalsis and colonic motility.

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