The physiological regulation of glucose flux into muscle in vivo

Abstract

Skeletal muscle glucose uptake increases dramatically in response to physical exercise. Moreover, skeletal muscle comprises the vast majority of insulin-sensitive tissue and is a site of dysregulation in the insulin-resistant state. The biochemical and histological composition of the muscle is well defined in a variety of species. However, the functional consequences of muscle biochemical and histological adaptations to physiological and pathophysiological conditions are not well understood. The physiological regulation of muscle glucose uptake is complex. Sites involved in the regulation of muscle glucose uptake are defined by a three-step process consisting of: (1) delivery of glucose to muscle, (2) transport of glucose into the muscle by GLUT4 and (3) phosphorylation of glucose within the muscle by a hexokinase (HK). Muscle blood flow, capillary recruitment and extracellular matrix characteristics determine glucose movement from the blood to the interstitium. Plasma membrane GLUT4 content determines glucose transport into the cell. Muscle HK activity, cellular HK compartmentalization and the concentration of the HK inhibitor glucose 6-phosphate determine the capacity to phosphorylate glucose. Phosphorylation of glucose is irreversible in muscle; therefore, with this reaction, glucose is trapped and the uptake process is complete. Emphasis has been placed on the role of the glucose transport step for glucose influx into muscle with the past assertion that membrane transport is rate limiting. More recent research definitively shows that the distributed control paradigm more accurately defines the regulation of muscle glucose uptake as each of the three steps that define this process are important sites of flux control.

Figures

Fig. 1.

Distributed control of muscle glucose uptake. Modified from Wasserman and Halseth (Wasserman and Halseth, 1998) and Wasserman et al. (Wasserman et al., 1967).

Fig. 2.

Ohm's Law was applied to determine sites of resistance to muscle glucose uptake. Ga, Ge and Gi are the glucose concentrations in the arterial blood, outer sarcolemmal surface and inner sarcolemmal surface, respectively. RExtracell, RTransport and RPhos are the resistances to glucose influx in the extracellular space, across the membrane and at the phosphorylation step, respectively. Ig is the glucose ‘current’ as estimated using 2[3H]DG. Using the countertransport method, glucose gradients were calculated as described in the text. Transgenic mice were used to alter sites of resistance.

Fig. 3.

The flux of glucose from the blood to the membrane to muscle. Steps 1, 2 and 3 represent glucose delivery, membrane transport and phosphorylation steps, respectively. Hexagons labeled ‘G’ are glucose molecules; those with an associated ‘P’ are glucose 6-phosphate. Green ovals are glucose transporters. The figure illustrates the fasted, sedentary state where few transporters are in the plasma membrane. Glucose 6-phosphate inhibition of glucose phosphorylation is illustrated by a negative feedback loop. The countertransport method estimates glucose gradients across each step using radioactive glucose analogues. Transgenic mice were used to alter sites of resistance at each step.

Fig. 4.

Resistance to glucose phosphorylation and the impact of a 50% reduction in GLUT4 on the index of skeletal muscle glucose uptake (Rg) during exercise in mice. The absence of a single GLUT4 allele (GLUT4+/–) in mice does not affect Rg during exercise when resistance to phosphorylation is high. However, it leads to a marked reduction in Rg when the resistance to glucose phosphorylation is reduced by HK II overexpression. ‡P<0.05 compared with Rg in wild-type (WT) mice. *P<0.05 compared with Rg in mice overexpressing HK II (HKTg). Data are from Fueger et al. (Fueger et al., 2004c).

Fig. 5.

Fractional 2-deoxyglucose (2[3H]DG) uptake in the gastrocnemius of mice expressing 0-, 0.5-, 1.0- and ∼3.5-fold wild-type GLUT4 levels during exercise or in the sedentary state. GLUT4 only affected fractional 2[3H]DG uptake in the sedentary state when it was overexpressed. However, the absence of GLUT4 caused a marked attenuation of fractional 2[3H]DG uptake during exercise regardless of whether HK II was overexpressed. HK II overexpression had no effect on the response during saline infusion but increased the exercise. Data points are means ± s.e.m. of 8–11 in vivo mouse experiments. Modified from Wasserman (Wasserman, 2009).

Fig. 6.

Resistance to glucose phosphorylation and the impact of a 50% reduction in GLUT4 on the index of skeletal muscle glucose uptake (Rg) during the steady-state period of a 4.0 mU kg–1 min–1 insulin clamp. The absence of a single GLUT4 allele (GLUT4+/–) in mice does not affect Rg during physiological insulin stimulation when resistance to phosphorylation is high. However, it leads to a marked reduction in Rg when the resistance to glucose phosphorylation is reduced by HK II overexpression. ‡P<0.05 compared with Rg in wild-type (WT) mice. *P<0.05 compared with Rg in mice overexpressing HK II (HKTg). Data are from Fueger et al. (Fueger et al., 2004b).

Fig. 7.

Fractional 2-deoxyglucose (2[3H]DG) uptake in the gastrocnemius of mice expressing 0-, 0.5-, 1.0- and ∼3.5-fold wild-type GLUT4 levels during the steady-state period of a 4.0 mU kg–1 min–1 insulin clamp or during an equal duration saline infusion. HK II content was either normal (WT) or the protein was overexpressed. GLUT4 only affected fractional 2[3H]DG uptake during saline infusion when it was overexpressed. However, the absence of GLUT4 caused a marked attenuation of fractional 2[3H]DG uptake during insulin stimulation regardless of whether HK II was overexpressed. HK II overexpression had no effect on the response during saline infusion but increased the maximal response to insulin. Data points are means ± s.e.m. of 8–11 in vivo mouse experiments. Modified from Wasserman (Wasserman, 2009).

Fig. 8.

Glucose metabolic index measured using 2-deoxyglucose (2[3H]DG) was measured for the gastrocnemius and superficial vastus lateralis (SVL) during the last 30 min of a 120-min saline infusion or hyperinsulinemic-euglycemic clamp (4.0 mU kg–1 min–1) experiment on conscious, unrestrained mice fasted for 5 h. Wild-type (WT) or HK II overexpressing (HKTg) mice were fed either a standard diet or a high-fat diet up to the age of 4 months of age and fasted for 5 h. Data are means ± s.e.m. for 7–14 mice per group. *P<0.05 vs saline condition; †P<0.05 vs WT standard diet; ‡P<0.05 vs HKTg standard diet. Data are from Fueger et al. (Fueger et al., 2004b). Figure reproduced from Wasserman (Wasserman, 2009).

Fig. 9.

Hyperinsulinemic-euglycemic clamps on conscious, unrestrained C57Bl/6J mice fasted for 5 h and chronically treated with either vehicle or sildenafil plus arginine subcutaneously for 3 months. The the index of skeletal muscle glucose uptake (Rg) measured using 2[3H]DG is shown for the soleus, gastrocnemius and superficial vastus lateralis (SVL). Data are the means ± s.e.m. for 7–8 mice per group. *P<0.05 vs vehicle. Data are from Ayala et al. (Ayala et al., 2007). Figure reproduced from Wasserman (Wasserman, 2009).