Glucose is widespread in living organisms and, with protein and fat, completes the triad of the major metabolic fuels. To a much lesser extent than in plants, glucose also constitutes a building block for structural and enzymatic components of cells as well as the extracellular matrix. As a metabolic substrate, glucose is present in organisms essentially in its simple, monomeric form (α-D-glucopyranose) and as a branched polymer of α-glucose, namely glycogen. Disaccharides of glucose (lactose, maltose, and sucrose) are quantitatively less important. Glucose is present in plasma water at a concentration that — in a healthy adult who has fasted overnight — ranges between 3.6 and 5.5 mmol L−1 (65 – 99 mg dL−1 ). A family of proteins residing in the plasma membrane (and in microsomal membranes) can specifically and reversibly bind glucose molecules, and transfer them across cell membranes in both directions. Of such proteins, known as glucose transporters (or, more generally, SoLute Carriers 2A or SLC2A), there are 14 different species that have so far been identified [1,2]. They differ from one another in both tissue distribution and physiologic regulation, particularly with respect to sensitivity to insulin stimulation. It is usual for more than one species of glucose transporter to be expressed in a tissue. One type of glucose transporter (GLUT4) represents the major insulin-stimulated glucose transporter in vivo, and this transporter is abundantly expressed in the classic insulin-sensitive tissues (adipocytes, brown fat, and skeletal, cardiac, and smooth muscle). Variable dominance of the other types of glucose transporters is found in tissues in which glucose metabolism does not respond to insulin acutely (erythrocyte, liver, kidney, brain, pancreatic β cells).
A non-insulin-regulatable transporter (GLUT1) effects facilitated glucose diffusion in red blood cells (RBCs). The abundance of this transporter in RBCs has the following physiologic consequences:
- Glucose diffuses very rapidly across RBC membranes, with an estimated equilibration time of only 4 s (a total RBC mass of 25 × 109 cells with a mean diameter of 7 μm and a spherical shape occupies a surface area of approximately 4 m2 ) . The rate of glycolytic utilization of transported glucose by the erythrocyte mass is estimated to be 25 μmol min−1 or about 6 μmol × min−1 × m−2 of diffusion surface. Since this rate is ∼17,000 times less than the rate of glucose transport in these cells, glucose concentration will be the same in plasma and erythrocyte water. Plasma proteins make up ∼8% of plasma volume, while RBC proteins and ghost occupy about 38% of packed red cell volume (which, in turn, averages 40% of blood volume). Thus, 20% (i.e., 0.38 × 0.4 + 0.08 × 0.6 = 0.2) of total blood volume is inaccessible to glucose. It follows that glucose concentration should be identical in plasma and RBC water under most circumstances, and that a blood water glucose concentration of 5.0 mmol L−1 translates into a plasma glucose concentration of 4.6 mmol L−1 and a whole-blood glucose concentration of 4.0 mmol L−1 , that is, a 15% systematic difference between plasma and whole-blood glucose level under typical conditions of hematocrit, proteinemia, and erythrocyte volume.
- As both RBC and plasma convey glucose, the total amount of the sugar reaching any given organ is the product of arterial whole-blood glucose concentration times the total blood flow to that organ. Similarly, the total amount of glucose leaving a body region is the product of whole-blood glucose level in the venous effluent times the blood flow rate. Thus, under steady-state conditions of blood flow (F), arterial glycemia (A) and organ metabolism, the net balance of glucose movement across a body region is given by the product of blood flow and the arteriovenous (A-V) whole-blood glucose concentration difference [(A-V) × F, or Fick principle] (Figure 14.1) . It is evident that the use of plasma flow rate and plasma glucose concentration systematically underestimates the net organ balance of glucose (and, for that matter, of any substance that travels in plasma as well as in erythrocytes, for example lactate and some amino acids). In fact, plasma flow is less than blood flow by an amount equal to the hematocrit (∼40%), while plasma glucose is higher than whole-blood glucose by only 15% (0.6 × 1.15 = 0.69, i.e., a 31% underestimation).
At least two non-insulin-regulatable glucose transporters (GLUT1 and GLUT3) have been identified in the blood – brain barrier and placenta [1,2], where transendothelial glucose passage occurs via facilitated diffusion. This is the structural basis for the long-held notion that brain and placental glucose utilization are not regulated by insulin, so that vital functions for the adult and fetal organism can be maintained in the face of variable metabolic conditions.
In general, diffusion of glucose from the intravascular com- partment into the interstitial fluid space is very rapid. Whether this is accomplished by facilitated diffusion mediated by specific cell transporters (as in the brain) or by simple diffusion through intercellular clefts of endothelial cells is still uncertain. A lower limit of glucose diffusion through endothelial membranes is represented by cellular glucose uptake (which does not include glucose back-diffusion into the intravascular compartment), and can be estimated by carrying out the following calculation. In healthy subjects under conditions of maximal stimulation (i.e., combined hyperinsulinemia and hyperglycemia), whole-body glucose uptake can reach 12 mmol min−1 which, for a total capillary surface of 700m2 available for diffusion (and considering a mean surface of 19,000 μ2 per capillary ), corresponds to 0.3 pmol min−1 per capillary. Thus, about 200 billion glucose molecules must pass through each capillary surface each minute to travel through the interstitial space.
Direct measurement of interstitial glucose concentrations has proven to be difficult, and has yielded conflicting results. Difficult as it may be to measure or calculate this “bathing” concentration of glucose, it is this concentration that dictates the activity of cellular glucose transport together with the state of specific activation of the glucose transporter. In addition to substrate mass action, cellular glucose uptake is influenced by changes in blood flow and hormonal stimulation by insulin. Thus, glucose metabolism is regulated by a distributed control between tissue glucose delivery (blood flow), transit through the interstitium, plasma membrane glucose transport and glucose phosphorylation (hexokinase) [6,7].
Blood flow varies considerably between insulin-sensitive tissues. Thus, basal blood flow to muscle and adipocytes in low (0.03 – 0.04 mL min−1 × gram tissue)  and blood flow can play an important role in glucose delivery, cellular uptake, and subsequent metabolism. Baron  was amongst the first to show that physiologic hyperinsulinemia, while maintaining euglycemia, stimulated muscle blood flow resulting in enhanced muscle glucose uptake. The insulin-mediated increase in muscle blood flow results from two separate effects: (i) relaxation of resistance of vessels, and (ii) recruitment of previously unperfused muscle tissue secondary to relaxation of terminal arterioles . This recruitment effect of insulin has been elegantly documented using microbubbles in combination with contrast-enhanced ultrasonography in rodents and humans . The result is expansion of the capillary surface area for nutrient and insulin delivery. Impaired insulin-mediated vasodilation is associated with metabolic resistance to the stimulatory effect of insulin on muscle glucose utilization in type 2 diabetic and obese nondiabetic individuals [8,9]. Evidence in humans has demonstrated that the insulin concentration in lymph and interstitial fluid is considerably less than in plasma [10,11]. Transport rates of large molecules, such as insulin, into the interstitial space, and thus the cell membrane, is slow. Insulin binding to its receptor in the vascular endothelium and subsequent incorporation into caveolae are involved in the transcytosis of the hormone from the intravascular to the interstitial space . Since the rate of insulin entry into skeletal muscle appears to be a critical step for insulin action, elucidation of the molecular and anatomical mechanisms involved in the transvascular process is likely to produce novel insights into the etiology of metabolic insulin resistance.
Another non-insulin-regulatable glucose transporter (GLUT2) is abundantly expressed in the plasma membrane of liver, kidney, intestinal cells, and pancreatic β cells [1,2]. In liver and kidney, net glucose release occurs in vivo. Thus, in the only organs in the body in which the presence of glucose-6-phosphatase (G6Pase) — the enzyme catalyzing the formation of free intracellular glucose from glucose-6-phosphate (G6P) — makes glucose available to the circulation, the transporter is of a type that only responds to the concentration gradient between the internal and external side of the plasma membrane. This ensures that, when insulin is around to stimulate inward glucose transport in tissues with sensitive transporters, the liver can release glucose into the bloodstream as long as sufficient G6P is derived from glycogenolysis or gluconeogenesis (or both) and sufficient G6Pase activity is there to accumulate free glucose on the inside of the plasma membrane. Under these conditions of reversed gradient, the presence of insulin-responsive glucose transport activity on hepatocyte cell membranes would enhance glucose outflow, thereby opposing the plasma glucose-lowering action of insulin. In contrast, physiologic control of the direction and rate of glucose flux through the hepatocyte membrane is not on the transport step but on intracellular processes.
GLUT4 is the insulin-regulated transporter present in muscle and adipocytes [1,2]. In the fasting state, less than 5 – 10% of GLUT4 is present in the plasma membrane. The other 90% resides in GLUT4 storage vesicles (GSV) and endosomes within the cell. Following stimulation by insulin, the GSV translocate to the plasma membrane via exocytosis and mediate glucose transport into myocytes and adipocytes. Following binding of insulin to its receptor and activation of the insulin signaling pathway, a series of small (20 – 35 kDa) GTPases are activated and they interact with multiple motor proteins, membrane tethers, and fusion-regulating proteins to direct flow of the GSV to the plasma membrane [1,2]. GSV exocytosis can be divided into three separate processes: translocation to the cell periphery, targeting of the vesicles to the plasma membrane, and ultimately fusion with the cell membrane. In myocytes the phosphoinositol-3-kinase (PI3K) signaling pathway is an absolute requirement for GLUT4 exocytosis. In individuals with type 2 diabetes mellitus (T2DM) defects in both the PI3K signaling pathway and in the GLUT4 exocytotic pathway contribute to insulin resistance in muscle and adipocytes . Following exposure to insulin, GLUT4-containing vesicles can be demonstrated in the cell periphery and plasma membrane within 5 minutes. Disruption of the microtubular/actin system within myocytes/adipocytes completely inhibits insulin-stimulated GLUT4 translocation. Fusion of GSV with the plasma membrane requires interaction with SNARE (soluble N-ethylmolemide-sensitive factor attachment receptor regulatory) proteins including VAMP2, STX4, SNAP23, and others. In adipocytes a PI3K-independent signaling pathway mediated via the adaptor protein APS (adaptor protein with pleckstrin homology and Src homology domains) also plays an important role in GLUT4 exocytosis; in myocytes a role for the insulin-stimulated APS pathways has yet to be defined .
In summary, the differential distribution and acute insulin sensitivity of the various classes of glucose transporters provide the backbone for the functional characteristics of glucose diffusion and exchange in the body. On the whole, free glucose is present in blood water, interstitial fluid, and the intracellular water compartment of insulin-independent tissues (liver, brain, kidney, intestine, placenta) in total amounts which, in the overnight-fasted healthy adult, average 80 mmol (14g or 1.2 mmol kg−1 of body weight), of which one fifth is in the blood volume. Free glucose is found at concentrations that (a) are uniform in the intravascular water compartment; (b) decline across the interstitial space toward the cell; (c) fall precipitously within cells that consume glucose avidly (e.g. brain) or in which glucose transport is relatively slow in the basal state (e.g. muscle, adipose tissue); (d) are increased in the cytoplasm of cells that produce free glucose (mostly liver); and (e) gradually and continuously decrease in the vascular bed as arterial blood turns into capillary blood and then runs back toward the right heart as venous blood. The regional characteristics of tissue composition, blood flow rate, capillary density (i.e., the average distance between the capillary axis and the cell surface), and cellular glucose uptake concur to determine the A-V glucose gradient in any region of the body. Glycogen is present in most cells in cytoplasmic granules that encase the enzymes that regulate its metabolism. In normal humans, the largest part of glycogen stores is in liver and skeletal muscle. In the former, 3 – 4 g of glycogen are packed in each 100 g of parenchyma; in striated muscle, the concentration is much lower (0.7 – 1.0% weight by weight). As a consequence, a normal human liver (1.5 kg) contains some 60 g of glycogen, whereas muscle (28 kg) depots keep 250 g. Thus, approximately 25 times more glycosyl units are stored in intracellular depots as glycogen than are dissolved in body water as free glucose. Glycogen metabolism is controlled by irreversible cascades of enzymatic reactions ultimately acting upon the proximal enzymes that catalyze glycogen synthesis (glycogen synthase) and degradation (glycogen phosphorylase) (see subsequent discussion).