Glucose, active transport

H D-glucose Actively transported via the Na+/D-glucose co-transporter Steady-state permeability values for D-glucose indicate that active transport processes are operational (typically at concentration >100 mM). At low concentrations (e.g. 1 mM), absorption is assumed to be ABL controlled and hence at these concentration can serve as marker for resistance of the ABL... 

The history of observations of efflux associated with PTS carriers is nearly as old as PTS itself. Gachelin [82] reported that A -ethylmaleimide inactivation of a-methyl-glucoside transport and phosphorylation in E. coli was accompanied by the appearance of a facilitated diffusion movement of both a-methylglucoside and glucose in both directions, uptake and efflux. His results could not discriminate, however, between one carrier operating in two different modes, active transport for the native carrier and facilitated diffusion for the alkylated carrier, or two distinct carriers. Haguenauer and Kepes [83] went on to show that alkylation of the carrier was not even necessary to achieve efflux NaF treatment which inhibits P-enolpyruvate synthesis was sufficient but this study did not address the question of one carrier or two. 

Glucose and galactose enter the absorptive cells by way of secondary active transport. Cotransport carrier molecules associated with the disaccharidases in the brush border transport the monosaccharide and a Na+ ion from the lumen of the small intestine into the absorptive cell. This process is referred to as "secondary" because the cotransport carriers operate passively and do not require energy. However, they do require a concentration gradient for the transport of Na+ ions into the cell. This gradient is established by the active transport of Na+ ions out of the absorptive cell at the basolateral surface. Fructose enters the absorptive cells by way of facilitated diffusion. All monosaccharide molecules exit the absorptive cells by way of facilitated diffusion and enter the blood capillaries. 

Several active transport systems that are normally found in the small intestinal enterocytes have been characterized in the Caco-2 cell model [13]. These include transport systems for glucose [32, 33], amino acids [34-37], dipeptides [38-40], vitamins [41], and bile acids [42, 43]. 

Any mechanism suggested for carrying out the active transport of glucose in the kidney must provide at least this much work. 

The extracellular glucose concentration in the brain of the rat is about 0.5 mmol/L whereas the intracellular concentration is estimated to be 2 mmol/L. Since there is no active transport into the brain, some form of com-partmentation of glucose must exist. 

Some cells couple the pure transport forms discussed on p. 218—i.e., passive transport (1) and active transport (2)—and use this mechanism to take up metabolites. In secondary active transport (3), which is used for example by epithelial cells in the small intestine and kidney to take up glucose and amino acids, there is a symport (S) located on the luminal side of the membrane, which takes up the metabolite M together with an Na" ion. An ATP-dependent Na transporter (Na /lC ATPase see p. 350) on the other side keeps the intracellular Na+ concentration low and thus indirectly drives the uptake of M. Finally, a uniport (U) releases M into the blood. 

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