It seems likely that depletion of body
carbohydrates may account for the rise in the sympathetic activity during prolonged exercise, since
glucose given during or before exercise reduces the increase in plasma
catecholamines. The aim of the present study was to find out whether the increase in plasma
noradrenaline (NA) in response to exercise can be reduced by 1. increasing of the amount of
carbohydrate available for metabolism without producing
hyperinsulinemia and 2. by inhibition of afferent activity from hepatic
glucoreceptors. The study was performed on dogs which exercised whilst receiving either the intravenous
fructose infusion (2.2 mmol/min) or a slow
glucose infusion (0.25 mmol/min) which was given either via the portal or a peripheral vein.
Fructose infusion reduced the muscle
glycogen depletion during exercise and reduced the increase in plasma NA and
glycerol concentrations without altering the
blood glucose or
insulin levels. The exercise-induced increases in plasma NA and
glycerol concentrations were significantly smaller with intraportal than with peripheral
glucose infusion but there were no differences between these two cases in the concentration of
glucose in the systemic circulation. These findings indicate that the reduction of the plasma NA response to physical effort under conditions of increased
carbohydrate availability cannot be attributed to the inhibitory effect of
insulin on sympathetic activity and provide evidence for the participation of hepatic
glucoreceptors in the control of the sympathetic activity during exercise.