Exercise-stimulated glucose transport in skeletal muscle is nitric oxide dependent

It has been suggested that there are separate insulin-stimulated and contraction-stimulated glucose transport pathways in skeletal muscle. This study examined the effects of nitric oxide on glucose transport in rat skeletal muscle by use of an isolated sarcolemmal membrane preparation and the nitric oxide synthase inhibitor N omega-nitro-L-arginine methyl ester (L-NAME), administered in the drinking water (1 mg/ml). Female Sprague-Dawley rats were divided into five groups: control, acute exercise, acute exercise+L-NAME, insulin stimulated, and insulin stimulated+L-NAME. Exercise (45 min of exhaustive treadmill running) increased glucose transport (37 +/- 2 to 76 +/- 5 pmol.mg-1.15 s-1) and this increase was completely inhibited by L-NAME (40 +/- 4 pmol.mg-1.15 s-1). A maximum dose of insulin increased glucose transport (87 +/- 10 pmol.mg-1.15 s-1), and adding L-NAME had no effect (87 +/- 11 pmol.mg-1.15 s-1). In addition, exercise, but not exercise+L-NAME, increased sarcolemma GLUT-4 content. This study confirms that there are separate pathways for contraction- and insulin-stimulated glucose transport. More importantly, although exercise and insulin both significantly increased glucose transport, L-NAME had no effect on insulin-stimulated glucose transport but blocked the exercise-stimulated transport. We conclude that nitric oxide is involved in the signal transduction mechanism to increase glucose transport during exercise.

Heart CD36 expression is increased in murine models of diabetes and in mice fed a high fat diet

High levels of CD36 expression are found in triglyceride storing and secreting cells such as differentiated adipocytes and mammary secretory epithelial cells and in some capillary endothelial cells. We have found high levels of CD36 in the capillary endothelium of murine adipose tissue and in cardiac and skeletal muscles. Muscle cells themselves were CD36 negative. No CD36 was found in brain endothelium. Cardiac and skeletal muscle tissues are highly oxidative and catabolize long-chain fatty acids as a source of energy while brain tissue does not use long-chain fatty acids for energy production. Since capillary endothelial cell CD36 expression appeared to correlate with parenchymal cell fatty acid utilization and since CD26 has been identified recently as a long-chain fatty acid-binding protein, we examined heart tissue CD36 expression in murine models of insulin-dependent (nonobese diabetic, NOD) and non-insulin-dependent diabetes mellitus (KKAY). Diabetic NOD and KKAY mice had serum triglyceride levels 2.6- and 4.2-fold higher, respectively, than normal mice and exhibited 7- and 3.5-fold higher levels of heart microsomal CD36, respectively, than control mice. Mice fed a 40% fat diet expressed heart tissue CD36 at a level 3.5-fold higher than those fed a 9% fat diet. These data suggest that endothelial cell CD36 expression is related to parenchymal cell lipid metabolism.

Effects of maturation and aging on the skeletal muscle glucose transport system

Insulin resistance in old, compared with young, humans and animals has been well documented. The resistance is due primarily to defects in skeletal muscle. In the present study, skeletal muscle sarcolemmal membranes were purified from five age groups of female Fischer rats ranging from 2 to 24 mo. Basal specific D-glucose transport was not significantly different among any of the groups. Maximum insulin-stimulated transport was progressively decreased from 96.4 +/- 5.0 pmol.mg-1.15 s-1 in the 2-mo-old animals to 70.8 +/- 8.9 pmol.mg-1.15 s-1 in the 24-mo-old animals. Most of the decrease occurred during maturation, and in fact there was no significant difference in maximum transport among the 8-, 16-, and 24-mo-old rats. The decrease in insulin-stimulated transport in the 24-mo-old animals was due to a reduction in the number of glucose transporters translocated into the sarcolemma membrane (9.8 +/- 0.6 vs. 7.8 +/- 0.6 pmol/mg protein). The intracellular or microsomal pool of glucose transporters was not significantly different between the 2- and 24-mo-old animals (8.8 +/- 0.6 vs. 8.5 +/- 0.9/mg protein). Western blotting revealed no differences in the cellular GLUT-4 contents between the 2- and 24-mo-old rats. The number of insulin receptors (2.3 +/- 0.4 vs. 2.1 +/- 0.5 pmol/mg protein) was not significantly different. Tyrosine kinase activity of the insulin receptor was, however, significantly reduced in the 24-mo-old compared with the 2-mo-old animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of NIDDM on the glucose transport system in human skeletal muscle

The purpose of this study was to investigate cellular changes in the glucose transport system in skeletal muscle of lean non-insulin-dependent diabetes mellitus (NIDDM) compared to lean nondiabetic control patients. NIDDM patients had significantly elevated fasting levels (means +/- SE) of serum glucose (10.1 +/- 1.3 vs. 5.4 +/- 0.4 mM, P less than 0.001) and serum insulin (110.8 +/- 31.1 vs. 35.9 +/- 3.6 pM, P less than 0.0025). Basal glucose transport (35.1 +/- 5.5 vs. 30.8 +/- 8.0 pM/mg protein) and cytochalasin-beta binding (3.5 +/- 1.2 vs 3.8 +/- 1.0 pM/mg protein) in isolated sarcolemmal vesicles were not significantly different between NIDDM and control groups. Insulin binding was reduced in NIDDM (0.82 +/- 0.03 vs. 1.63 +/- 0.18 pM/mg protein) as was the Kd (0.93 +/- 0.03 vs. 1.38 + 0.12 nM). Tyrosine kinase activity, as assessed from incorporation of [32P]ATP into Glu 4:Tyr 1, was significantly (P less than 0.005) reduced in NIDDM at insulin concentrations from 1-100 nM. Maximum kinase activity was depressed (1.88 +/- 0.04 vs. 2.97 +/- 0.07 fM 32P/fM insulin binding at 100 nM insulin). The number of glucose transporters in the low-density microsomes was not significantly different between NIDDM and control groups (7.01 +/- 1.40 vs. 7.65 +/- 0.90 pM cytochalasin-beta bound/mg protein). These results suggest that decreased insulin binding and diminished receptor tyrosine kinase activity play a substantial role in the development of skeletal muscle insulin resistance associated with NIDDM.