The male obese Wistar diabetic fatty rat is a new model of extreme insulin resistance.

The male obese Wistar Diabetic Fatty (WDF) rat is a genetic model of obesity and non-insulin dependent diabetes (NIDDM). The obese Zucker rat shares the same gene for obesity on a different genetic background but is not diabetic. This study evaluated the degree of insulin resistance in both obese strains by examining the binding and post binding effects of muscle insulin receptors in obese rats exhibiting hyperinsulinemia and/or hyperglycemia. Insulin receptor binding and affinity and tyrosine kinase activity were measured in skeletal muscle from male WDF fa/fa (obese) and Fa/? (lean) and Zucker fa/fa (obese) and Fa/Fa (homozygous lean) rats. Rats were fed a high sucrose (68% of total Kcal) or Purina stock diet for 14 weeks. At 27 weeks of age, adipose depots were removed for adipose cellularity analysis and the biceps femoris muscle was removed for measurement of insulin binding and insulin-stimulated receptor kinase activity. Plasma glucose (13.9 vs. 8.4 mM) and insulin levels (14,754 vs. 7440 pmol/L) were significantly higher in WDF obese than in Zucker obese rats. Insulin receptor number and affinity and TK activity were unaffected by diet. Insulin receptor number was significantly reduced in obese WDF rats ( 2.778 +/- 0.617 pmol/mg protein), compared to obese Zucker rats (4.441 +/- 0.913 pmol/mg potein). Both obese strains exhibited down regulation of the insulin receptor compared to their lean controls. Maximal tyrosine kinase (TK) activity was significantly reduced in obese WDF rats (505 +/- 82 fmol/min/mg protein) compared to obese Zucker rats (1907 +/- 610 fmol/min/mg protein). Only obese WDF rats displayed a decrease in TK activity per receptor. These observations establish the obese WDF rat as an excellent model for exploring mechanisms of extreme insulin resistance, particularly post-receptor tyrosine kinase-associated defects, in non-insulin dependent diabetes.

Adrenergic stimulated skeletal muscle glycogenolysis in perfused hindlimbs of young and old male Fischer 344 rats.

Epinephrine (Epi)- and forskolin (FSK)-stimulated glycogenolysis of skeletal muscle was evaluated in perfused hindlimb isolated from male Fischer 344 (F344) rats, ages 6, 12, and 26 mo. Muscle glycogen stores were reduced by sciatic nerve stimulation and replenished by infusing 10 mM glucose, 500 microU insulin, and 5 microCi [14C]glucose via a left carotid artery cannula. Then the hindlimb was perfused with a modified Krebs-Henseleit buffer (pH 7.4). At minute 20 of the perfusion, Epi [0.0 (perfusate), 0.25, 0.50, or 0.75 microM] or 40 microM FSK were infused for 10 min. Radioactivity (14C) in the effluent perfusate was collected every 60 s during a 20-min preinfusion, a 10-min Epi infusion, and a 20-min postinfusion period and was used to determine the rate of muscle glycogen utilization. Total 14C release increased with Epi and 40 microM FSK. However, the pattern of release did not differ significantly with age. In general, the fraction of the perfusate released as 14CO2 increased in the presence of FSK and Epi but did not significantly differ with age. [14C]lactate released in response to Epi increased in the 6-mo-old group, remained unchanged in the 12-mo-old group, and decreased in the 26-mo-old group compared with 0.0 Epi (perfusate) values. It appears that stimulation of skeletal muscle glycogenolysis via adrenergic receptor or postreceptor/adenosine 3',5'-cyclic monophosphate-mediated mechanisms is unaffected by age. However, the utilization of carbohydrate by isolated hindlimb muscle is altered in the aging rat, resulting in a more oxidative metabolism.

Effect of age and high sucrose diet on 2-deoxyglucose uptake in perfused hindlimb.

The purpose of this investigation was to evaluate insulin-stimulated skeletal muscle glucose uptake in male Fischer 344 rats, age 6, 12, and 27 mo, fed either a sucrose (S: 66.6% sucrose, 17.6% protein, 6.4% fat) or sucrose-free (SF: 66.6% starch, 17.6% protein, 6.4% fat) diet for 3 mo. Skeletal muscle glucose uptake (Rg) in perfused hindlimbs was estimated from the uptake and subsequent phosphorylation of radiolabeled 2-deoxyglucose (2DG) in the gastrocnemius (GN), extensor digitorum longus (EDL), and soleus (SOL) muscles. Rat hindlimbs were perfused at a rate of 10 ml.min-1 with a modified Krebs Henseleit buffer containing bovine red blood cells (hematocrit: 40%) and 5.85 mmole.L-1 glucose along with 358 pmoles.L-1 followed by 3580 pmoles.L-1 insulin. There was no effect of diet on plasma glucose levels measured at weeks 1, 7, and 11 of the dietary period. A significant effect of age on estimated glucose uptake in the GN was demonstrated due primarily to greater uptake in the 27-mo compared to the 6-mo-old animals. This significant effect of age was not evident in the EDL or SOL, nor were there significant effects of diet in any muscle. These data suggest that insulin-stimulated glucose uptake in perfused hindlimbs is not attenuated with senescence or with the feeding of a sucrose diet for 3 months.

High sucrose diet and exercise: effects on insulin-receptor function of 12- and 24-mo-old Sprague-Dawley rats.

The purpose of this study was to evaluate the effect of aging (12 vs. 24 mo) on skeletal muscle insulin receptor function of male Sprague-Dawley rats fed either a 33% sucrose (wt/wt) or sucrose-free diet. The effect of exercise in combination with the sucrose diet was also evaluated by exercising half of the sucrose-fed group on a motorized treadmill. Insulin-receptor function was assessed in vitro by measuring the binding capacity of [125I]-insulin to partially purified receptors of the biceps femoris and vastus lateralis. Tyrosine kinase activity was measured as an index of postreceptor function. Insulin-receptor number was significantly decreased in 24-mo-old sucrose-fed rats compared to 12-mo-old rats fed the sucrose or sucrose-free diets. The affinity of insulin for the receptor did not significantly differ among groups. Maximal tyrosine kinase activity in vastus lateralis was significantly decreased in 12-mo-old sucrose-fed rats compared with sedentary 24-mo-old rats fed the sucrose-free diet or 24-mo-old rats fed the sucrose diet in combination with exercise. Exercise prevented the decrease in receptor function in both 12- and 24-mo-old sucrose-fed rats as measured by insulin binding and tyrosine kinase activity. These data suggest that diet and/or exercise rather than aging per se has a greater influence on insulin-receptor function.