Transcellular glucose transport

Figure 18-3 Site 1 The Na transport systems responsible for the reabsorption of Na and associated solutes in the proximal tubule. A. Transcellular reabsorption of Na THCOj, which is controlled by carbonic anhydrase (CA). Acetazolamide and other CA inhibitors block Na reabsorption by this route. B. Transcellular reabsorption of Na coupled to glucose, amino acids, and phosphate C Paracellular transport of Na /Cl. No commercially available agents inhibit Na reabsorption by routes B or C, Na"/K -ATPase is indicated by filled circles on the antiluminal membrane.

The net result of this two-stage process is movement of Na Ions, glucose, and amino acids from the intestinal lumen across the intestinal epithelium into the extracellular medium that surrounds the basolateral surface of intestinal epithelial cells. Tight junctions between the epithelial cells prevent these molecules from diffusing back into the intestinal lumen, and eventually they move into the blood. The increased osmotic pressure created by transcellular transport of salt, glucose, and amino acids across the intestinal epithelium draws water from the intestinal lumen into the extracellular medium that surrounds the basolateral surface. In a sense, salts, glucose, and amino acids carry the water along with them. 

In the intestinal epithelial cell, the coordinated operation of Na" -linked symporters in the apical membrane with Na /K ATPases and uniporters in the basolateral membrane mediates transcellular transport of amino acids and glucose from the intestinal lumen to the blood (see Figure 7-27). 

Movement of glucose from one side to the other side of the intestinal epithelium is a major example of transcellular transport. How does the Na /K ATPase power the process Why are tight junctions essential for the process Rehydra-tlon supplements such as sport drinks Include a sugar and a salt. Why are both Important to rehydratlon ... 

Reconsider Example 10.9 with active transport of Na ions from the intracellular to the extracellular space. Assume that 25% of the energy of hydrolysis of ATP can be used for the transport and that 38 mol of ATP are formed per mole of glucose (G) combusted. In the steady state, the extracellular concentration of Na" " is observed to be 145 mM and the intracellular concentration is observed to be 12 mM and the electrical potential of the intracellular fluid is determined to be -90.5 mV in relation to the extracellular space. The transcellular flux of Na+ ions is 1 mmol/min and the temperature of the tissue is 37 °C. Estimate the oxygen consumption, heat output and energy expenditure of the process when A//r,AXP = -20 kJ/mol for hydrolysis of ATP and A//r,G= —2867 kJ/mol for glucose combustion. Assume 44/ 5 = 4/5. 

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