GLUCOSE-ADMINISTRATION BEFORE EXERCISE MODULATES CATECHOLAMINERGIC RESPONSES IN GLYCOGEN-DEPLETED SUBJECTS

In glycogen-depleted subjects (GD) a nonlinear increase in epinephrine (Epi) and norepinepkrine(NE) parallels blood lactate (Lai during grad ed exercise. The effect of glucose (Glc) supplementation and route of administration on these relationships was studied in 26 GD athletes wh o were randomly assigned to receive 1.3 g/kg Glc by slow intravenous i nfusion (IV; n = 9), oral administration (PO; n = 9), or artificially sweetened placebo in 1 liter of water (Asp; n = 8) in the 2 h precedin g a graded maximal exercise. Performance and La were similar among the three groups in normal glycogen (NG) or GD conditions. However, sligh tly improved performances were observed in GD compared with NG and wer e associated with a shift to the right in La curves. Blood Glc concent rations were higher in IV and PO before exercise, but they rapidly dec reased to lowest levels in IV, gradually decreased over time in PO, an d remained stable in Asp or MG. Insulin concentrations were highest in IV and lowest in Asp and NG at onset of exercise, rapidly decreasing in IV and PO although remaining at higher levels than in Asp or NG. In contrast, higher serum levels of free fatty acids were measured durin g exercise in Asp with no significant differences in glucagon or glyce rol among the three groups. Free and sulfated NE increases were smalle r in IV than in PO and Asp on exhaustion. In contrast, free and conjug ated Epi were most increased in IV, with smallest increases in Asp. Do pamine levels were most increased in IV at exhaustion. We conclude tha t the changes of Epi and NE concentrations, associated with the activa tion of glucoregulatory mechanisms, including hyperinsulinemia, displa y different magnitude and time courses during exercise in GD subjects who received oral vs. intravenous load of Glc before exercise. We spec ulate that the magnitude of insulin surge after acutely increased Glc before exercise in GD subjects may exert dissociative effects on adren al-dependent glycogenolysis and on sympathetic responses.