Carbohydrate availability affects ammonemia during exercise after beta 2-adrenergic blockade.

Abstract

Beta-adrenergic blockade increases blood ammonia concentration during exercise. The purpose of this study was to assess the role of decreased carbohydrate availability in this process. Wistar rats (N = 47) were injected intravenously with a selective beta 2-adrenoceptor blocker (ICI 118,551), placebo, or beta 2-blocker + glucose 1 h before a treadmill exercise test. Blood samples were taken to measure the concentration of ammonia, glucose, lactic acid, free fatty acids (FFA), glycerol, branched-chain amino acids (BCAA), and muscle samples for determination of glycogen content. Beta 2-adrenergic blockade shortened running time to exhaustion (23 +/- 4.3 min compared to 44 +/- 5.2 min with placebo), increased blood ammonia levels (146.7 +/- 16.21 micromol x L(-1) compared to 47.5 +/- 0.92 micromol x L(-1) with placebo) and prevented exercise-induced glycogen breakdown in soleus and gastrocnemius muscles. Pre-exercise supplementation of glucose during beta 2-blockade restored exercise-induced glycogen breakdown and reduced blood ammonia concentration during exercise (66.5 +/- 5.65 mmol x L(-1)) but did not improve exercise capacity (26 +/- 3.2 min) when compared with beta2-blockade alone. The results suggest that the enhanced rise in blood ammonia concentration during exercise after beta-blockade is caused by impaired carbohydrate availability.