Glucose infusion partially attenuates glucose production and increases uptake during intense exercise

Abstract

Glucose infusion can prevent the increase in glucose production (Ra) and increase glucose uptake (Rd) during exercise of moderate intensity. We postulated that 1) because in postabsorptive intense exercise (>80% maximal O2 uptake) the eightfold increase in Ra may be mediated by catecholamines rather than by glucagon and insulin, exogenous glucose infusion would not prevent the Ra increment, and 2) such infusion would cause greater Rd. Fit young men were exercised at >85% maximal O2 uptake for 14 min in the postabsorptive state [controls (Con), n = 12] or at minute 210 of a 285-min glucose infusion. In seven subjects, the infusion was constant (CI; 4 mg . kg-1 . min-1), and in seven subjects it was varied (VI) to mimic the exercise Ra response in Con. Although glucose suppressed Ra to zero (with glycemia approximately 6 mM and insulin approximately 150 pM), an endogenous Ra response to exercise occurred, to peak increments two-thirds those in Con, in both CI and VI. Glucagon was unchanged, and very small increases in the glucagon-to-insulin ratio occurred in all three groups. Catecholamine responses were similar in all three groups, and correlation coefficients of Ra with plasma norepinephrine and epinephrine were significant in all. In all CI and VI, Rd at rest was 2x Con, increased earlier in exercise, and was higher for the 1 h of recovery with glucose infusion. Thus the Ra response was only partly attenuated, and the catecholamines are likely to be the regulators. This suggests that an acute endogenous Ra rise is possible even in the postprandial state. Furthermore, the fact that more circulating glucose is used by muscle during exercise and early recovery suggests that muscle glycogen is spared.

Combined Infusion of Epinephrine and Norepinephrine During Moderate Exercise Reproduces the Glucoregulatory Response of Intense Exercise

“…Glucose specific activity (SA) was maintained by increasing the tracer infusion incrementally to a maximum of sixfold the rates at rest during the infusion period in CCI and then returning it to the pre-exercise rate in early recovery. The goal was to introduce labeled glucose into the circulation at a rate proportional to endogenous R a , thereby attenuating changes in [ 3 H]glucose SA to Ͻ25% during the rapid changes in glucose kinetics, as in previous experiments (5,6,9,10,22,23). This step would ensure the validity of glucose turnover calculations, even if there were changes in pool fraction during this time.…”

“…Samples for glucose turnover, insulin and glucagon, catecholamines, and lactate and pyruvate measurements were collected and processed as previously described (9) and analyzed as previously detailed (6). R a and glucose uptake (R d ) were calculated from the variable isotope infusions with the one-compartment model using a glucose distribution space of 25% of body weight and a pool fraction of 0.65.…”

“…Plasma norepinephrine (NE) and epinephrine (Epi) concentrations both increase 15-fold, and we have demonstrated highly significant correlations of both with R a during IE (5). We recently found that the patterns of responses of plasma catecholamines and R a persist in glucose-infused subjects (9) and in the postprandial state (10). These are situations in which endogenous R a suppression from hyperinsulinemia must be overcome, the latter being the situation under which most IE is performed.…”

Intense Exercise Has Unique Effects on Both Insulin Release and Its Roles in Glucoregulation

“…In contrast, in intense exercise there is a marked 14-to 18-fold increase of both EPI and NE ( Fig. 1C and D) (e.g., 19,[21][22][23]25,26) to levels seen in pheochromocytoma. This induces a seven-to eightfold increase in GP (Fig.…”

Fuel metabolism during exercise in euglycaemia and hyperglycaemia in patients with type 1 diabetes mellitus—a prospective single-blinded randomised crossover trial

“…However, it is uncertain whether increased carbohydrate availability also induces a saving of intramyocellular glycogen during exercise [9,[14][15][16][17][18][19][20]. Interestingly, in healthy individuals, hyperglycaemia is generally accompanied by physiological hyperinsulinaemia, thereby hampering direct application of these findings to diabetic patients.…”