Na + /glucose transporter

Active transport of a molecule across a membrane against its concentration gradient requires an input of metabolic energy. In the case of ATP-driven active transport, the energy required for the transport of the molecule (Na+, K+, Ca2+ or H+) across the membrane is derived from the coupled hydrolysis of ATP (e.g Na+/K+-ATPase). In ion-driven active transport, the movement of the molecule to be transported across the membrane is coupled to the movement of an ion (either Na+ or H+) down its concentration gradient. If both the molecule to be transported and the ion move in the same direction across the membrane, the process is called symport (e.g. Na+/glucose transporter) if the molecule and the ion move in opposite directions it is called antiport (e.g. erythrocyte band 3 anion transporter). 

Fig. 4. Ion-driven cotransport mechanisms, (a) Symport process involving a symporter (e.g. Na+/glucose transporter) (b) antiport process involving an antiporter (e.g. erythrocyte band 3 anion transporter).

The Na+-myoinositol cotransporter is 46% identical with the glucose carrier (Kwon et al., 1992). The Na+-proline (Nakao et al., 1987) and Na+-pantothenate (Jackowski and Alix, 1990) carriers from E. coli show 28% and 25% identity, respectively (Pajor et al., 1992), to the Na+ glucose transporter from rabbit intestine. 

Glucose Unabsorbed glucose appears in the urine (=glucosuria), associated with diabetes mellitus Re-absorbed, almost 100% via Na -glucose transporters (apical) and GLUT (basolateral) ... 

Transport systems can be described in a functional sense according to the number of molecules moved and the direction of movement (Figure 41-10) or according to whether movement is toward or away from equilibrium. A uniport system moves one type of molecule bidirectionally. In cotransport systems, the transfer of one solute depends upon the stoichiometric simultaneous or sequential transfer of another solute. A symport moves these solutes in the same direction. Examples are the proton-sugar transporter in bacteria and the Na+ -sugar transporters (for glucose and certain other sugars) and Na -amino acid transporters in mammalian cells. Antiport systems move two molecules in opposite directions (eg, Na in and Ca out). 

Ion-dependent solute transport processes such as Na+-glucose and Na+-amino acid cotransporters can be identified in epithelial tissues by observing an elevation in /sc following solute addition in Na+-containing but not Na+-free... 

The effects of D-glucose observed in vivo are not well reproduced in vitro. Madara [203] reported that cytoskeletal contraction and enhanced paracellular permeability were observed only in an in situ perfusion preparation and not in an isolated tissue preparation. Although its in vivo effect was not tested, 25 mM D-glucose, an effective concentration in the jejunum [47], failed to enhance the in vitro transport of sotalol (log PC = -0.62), atenolol (log PC = 0.16), or nadolol (log PC = 0.93) across the isolated conjunctiva [213], For a similar reason and possibly due to the absence of a Na+-glucose cotransporter in the cornea, 25 mM D-glucose was ineffective in increasing the corneal transport of these three drugs. 

RK Crane, FC Dorando. (1982). The kinetics and mechanism of Na+ gradient-coupled glucose transport. In AN Martonosi, ed. Membranes and Transport. New York Plenum Press, pp 153-160. 

Na+ co-transporter (symport) allows uptake of X (e.g. amino acid or glucose)... 

Figure 5.9 The sodium ion/glucose transporter and sodium ion/ amino acid transporter. The biochemistry of the two processes is identical. To maintain electroneutral transport K ion replaces Na ion, via NaVK ATPase. The broader arrow indicates overall effect (i.e. unidirectional) transport.

Na+-glucose symporter in epithelial cells 2.A.73. HCOsT transporters HCOi -CF antiporter... 

That is, the cotransporter can pump glucose inward until its concentration within the epithelial cell is about 9,000 times that in the intestine. As glucose is pumped from the intestine into the epithelial cell at the apical surface, it is simultaneously moved from the cell into the blood by passive transport through a glucose transporter (GLUT2) in the basal surface (Fig. 11-44). The crucial role of Na+ in symport and antiport systems such as these requires the continued outward pumping of Na+ to maintain the transmembrane Na+ gradient. 

Predict the effects of the following on the initial rate of glucose transport into vesicles derived from animal cells that accumulate this sugar by means of Na+ symport. Assume that initially A P = 0, ApH = 0 (pH = 7), and the outside medium contains 0.2MNa+, whereas the vesicle interior contains an equivalent amount of K+. 

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