Hormonal and metabolic responses to maintained hyperglycemia during prolonged exercise

We studied the effects of maintained hyperglycemia (12 mmol/l) on endurance exercise to determine the hormonal and metabolic responses, the maximal ra te of glucose infusion (i.e., utilization), and the effects on muscle glyco gen stores. Eight men undertook two trials during which they exercised on a cycle ergometer at an intensity of similar to 70% peak O-2 uptake for 120 min. In the first trial (trial A), subjects had their blood glucose concent ration clamped at 12 mmol/l 30 min before exercise and throughout exercise. The same rate and volume of infusion of saline as had occurred for trial A were used in a placebo trial (trial B). Maintained hyperglycemia resulted in significantly lowered plasma concentrations of nonesterified fatty acid, glycerol, 3-hydroxybutyrate, epinephrine, norepinephrine, and growth hormo ne (P < 0.001) during exercise, whereas concentrations of plasma insulin we re significantly elevated (P < 0.001). Calculations of the rates of total c arbohydrate oxidation showed that trial A resulted in significantly higher values when compared with trial B (P < 0.01) and that the maximal rates of glucose infusion varied between 1.33 and 2.78 g/min at 100-120 min. Muscle glycogen concentrations were significantly depleted (P < 0.01) after both t rials (trial A, 170.3 mu mol/g dry wt decrease; trial B, 206 mu mol/g dry w t decrease), although this apparent difference may be accounted for by stor age of 22.6 g glucose during the 30-min prime infusion. The results from th is study confirm that maintained hyperglycemia attenuates the hormonal resp onse and promotes carbohydrate oxidation and utilization and that muscle gl ycogen may not be spared.