| Popis: |
In order to assess glucose dynamics and the pathways (direct and gluconeogenic) by which muscle and liver glycogen is synthesized during and following prolonged submaximal exercise, twelve-hour-fasted catheterized rats were infused intravenously with ($\sp3$H-6) -glucose (or ($\sp3$H-3) -glucose) and either (i) NaH$\sp $CO$\sb3,$ (ii) ($\sp $C-U) -lactate or (iii) ($\sp $C-1) -glucose (three gluconeogenic tracers) throughout two-hour basal, four-hour swim (or rest) and three-hour postexercise recovery periods. During recovery, rats were infused with either saline, lactic acid or glucose. Arterial blood samples were taken throughout the protocols. Liver and three muscles (soleus, white and red gastrocnemii) were excised and assessed for total and radiolabelled glycogen content. The results indicate that: (1) While plasma glucose concentrations are maintained during the prolonged swim owing to matched doubling of both glucose production (R$\sb{\rm a}$) and clearance (MCR), glucose increases during the recovery period as the fall in R$\sb{\rm a}$ lags behind that of MCR and the rate of gluconeogenesis remains elevated. (2) Despite increases in direct glycogenesis during prolonged swimming, glycogen is significantly depleted in white and red gastrocnemii. Glyconeogenesis contributes at least 20-25% to glycogen repletion during fasted recovery. The source of this gluconeogenic substrate appears to be lactate which is formed locally in the muscles (as traced by recycled $\sp $C-glucose). Although not required to elicit muscle gluconeogenic activity, high lactate levels enhance glyconeogenesis, especially in the postexercise period. (3) There is a high turnover of soleus glycogen at rest. The soleus does not respond to prolonged swimming but rather to the availability of substrate (glucose) to synthesize its glycogen almost entirely via the direct route. (4) Glyconeogenesis is the primary pathway of hepatic glycogen synthesis. While glucose infusion is the most potent stimulus of glycogenesis, the combined effects of lactate and exercise appear synergistic in the formation of liver glycogen. |
