Sodium Absorption By The Small And Large Intestines

Four mechanisms have been identified that are responsible for the movement of Na+ from the luminal solution across the apical membranes of different Na+-absorbing epithelial cells. As illustrated in Fig. 3, the first and simplest (a) is restricted diffusion of Na+ through water-filled, highly selective channels that are often inhibited by the diuretic agent amiloride; the second (b) is Na+ entry coupled to the entry of organic solutes such as sugars and amino acids (i.e., cotran-sport); the third (c) is Na+ entry coupled to the entry of Cl" most often mediated by a so-called "tritransporter" that brings about the entry of 1 Na+ ion and 1 K+ ion coupled to the entry of 2 Cl" ions (another example of

FIGURE 3 A composite model illustrating the four different mechanisms responsible for the movement of Na+ from the luminal solution across the apical membranes of small and large intestinal cells. S, organic solute.

cotransport); and the fourth (d) is Na+ entry coupled to the countertransport of H+. In every instance, Na+ entry is a "downhill" or "passive" process. In the first (a), it is driven entirely by its electrochemical potential difference across the apical membrane, and, in the second (b), Na+ entry is energized by the combined electrochemical potential differences for Na+ and those of the cotransported solutes. Mechanisms (c) and (d) are electrically neutral so that Na+ entry by these routes is driven by the combined chemical potential differences of Na+ and those of the cotransported or countertran-sported solutes and is not directly affected by the electrical potential difference across the apical membrane.

The Na+ that enters these absorptive cells by any of these four different mechanisms is then extruded from the cell across the basolateral membrane by the Na + -K+ pump. This pump is responsible for maintaining the low intracellular Na+ concentration and, because Na+ extrusion is coupled to K+ uptake, the high intracellular K+ concentration characteristic of all these cells. Most, if not all, of the K+ pumped into the cell in exchange for Na+ recycles across the basolateral membrane through K+ leaks or channels.

The distribution of the four Na+ entry mechanisms illustrated in Fig. 3 differs along the intestinal tract. Na+ entry across the apical membranes of small intestinal absorptive (villus) cells is mediated predominantly by the carrier mechanisms b, c, and d.

On the other hand, Na+ entry across the apical membranes of colonic absorptive cells, particularly in the distal colon, is the result for the most part of restricted diffusion through highly selective channels (mechanism a). Further, the number of active channels in the apical membranes of colonic absorptive cells appears to be regulated by mineralocorticoids such as aldosterone. Any stimulus that brings about an increase in plasma aldosterone levels (e.g., sodium deprivation, hypovolemia) results in an increase in the number of active channels, an increase in the rate of Na + entry into these cells and, in turn, an increase in the rate of Na + absorption.

Thus, in many respects, Na + absorption by small intestine cells resembles that of renal proximal tubule cells, whereas Na + absorption by large intestine cells resembles that of the distal nephron (see Chapters 29 and 30).

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