Implications of Hyperglycemia for Post-exercise Resynthesis of Glycogen in Trout Skeletal Muscle

Rates of whole-body glucose turnover and muscle-specific glucose utilization were determined in rainbow trout (Oncorhynchus mykiss) at rest and at intervals during recovery from burst swimming. Plasma glucose level was high in the experimental animals (range 6­38 mmol l-1), but hyperglycemia was not related specifically to exercise. Estimated glucose turnover, 19.3±2.6 (rest) and 15.8±3.9 µmol min-1 kg-1 (recovery), was also highly variable, but was linearly associated with plasma glucose concentration (turnover=0.97[glucose]+0.57, r=0.93) in both resting and recovering fish. While utilization of glucose in the whole animal was clearly responsive to plasma glucose availability, estimated total skeletal muscle disposal of glucose accounted for less than 15 % of glucose turnover, indicating that glucose was utilized largely by tissues other than locomotory muscle. Rates of glucose utilization in white muscle (range 0.5­4 nmol min-1 g-1) provide direct evidence that glucose, regardless of plasma concentration, accounted for less than 10 % of glycogen repletion during exercise recovery. In red muscle, glucose uptake was influenced by plasma glucose level below 10­12 mmol l-1 (utilization range 1­15 nmol min-1 g-1), but was independent of glucose concentration above about 12 mmol l-1 (utilization plateaued at 15­20 nmol min-1 g-1). Trout red muscle is similar to mammalian white muscle in the sense that glucose is estimated to account incompletely for glycogen restoration (25­60 %), suggesting dependence on both glycogenesis and glyconeogenesis during recovery. It is concluded that hyperglycemia may be important to the pattern of substrate incorporation into red muscle glycogen and to the rate of repletion observed, but glucose availability has, as predicted from earlier indirect studies, little relevance to white muscle glycogen restoration. The regulatory mechanisms that govern apparently very high glucose turnover rates during extreme hyperglycemia, concomitant with low disposal rates in skeletal muscle, require further investigation.