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It has been widely recognized that aging will cause a profound decrease in glucose tolerance end increase insulin resistance. These changes have been found to occur at a relatively early age. Narimiya [54], has documented these changes in younger rots end has shown alterations in glycogen metabolism to occur prior to nine months of age. Exercise in the form of running has been shown to attenuate these changes. In Vivo, glucose, insulin end muscle glycogen have interrelated functions. The role of muscle glycogen is to provide energy for the muscle's contractile process. Insulin is needed at rest to allow glucose to enter the muscle and be stored as glycogen. The purpose of this study is to pinpoint when changes in glycogen metabolism occur while looking at the influence of exercise end weight restriction on the process. METHODS: Male Sprague Dawley rats ages 1.5-4.0 months of age were divided into three groups control (CN), pairfed (PF), and exercise trained (ET). The ET cages were equipped with voluntary running wheels attached to an automatic revolution counter. At 1.5 months, a group of controls were sacrificed and treated as the 4 mo. animals described below. Following training the hindlimbs of CN, ET, end PF were surgically isolated and glucose uptake examined by perfusing them with a bovine blood preparation, which contained insulin and glucose. Pre and post samples of the soleus, plantaris, and red and white vastus were removed and assayed for glycogen. RESULTS: The 1.5 mo. CN had significantly greater glucose uptake then any other group for both the insulin and non-insulin infused groups. The ET had significantly greater uptake than the other 4 mo. groups at 90 and 120 min in the insulin infused group. In the 1.5 mo. CN when insulin is not present all muscles will break down glycogen. When insulin is present, glycogen is used in all but the white vastus. In the 4 mo. CN all muscles except the soleus break down glycogen. In the ET the glycogen breakdown pattern is similar regardless of whether insulin is present or not. In the PF, glycogen breakdown is depressed and decidely different from the other treatment groups. CONCLUSIONS: There is an age dependent decrease in glucose tolerance and insulin resistance at or before 4 mo. of age. Training prevented some of this loss but did not stop the decline. Weight restriction had a nominal, if any, benefit in reducing insulin resistance and raising glucose tolerance with aging. |
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