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

Purpose: beta-Adrenergic blockade increases blood ammonia concentration dur ing exercise. The purpose of this study was to assess the role of decreased carbohydrate availability in this process. Methods: Wistar rats (N = 47) w ere injected intravenously with a selective beta(2)-adrenoceptor blocker (I CI 118,551), placebo, or beta(2)-blocker + glucose 1 h before a treadmill e xercise test. Blood samples were taken to measure the concentration of ammo nia, glucose, lactic acid, free fatty acids (FFA), glycerol, branched-chain amino acids (BCAA), and muscle samples for determination of glycogen conte nt. Results: beta(2)-adrenergic blockade shortened running time to exhausti on (23 +/- 4.3 min compared to 44 +/- 5.2 min with placebo), increased bloo d ammonia levels (146.7 +/- 16.21 mu mol.L-1 compared to 47.5 +/- 0.92 mu m ol.L-1 with placebo) and prevented exercise-induced glycogen breakdown in s oleus and gastrocnemius muscles. Pre-exercise supplementation of glucose du ring beta(2)-blockade restored exercise-induced glycogen breakdown and redu ced blood ammonia concentration during exercise (66.5 +/- 5.65 mmol.L-1) bu t did not improve exercise capacity (26 +/- 3.2 min) when compared with bet a(2)-blockade alone. Conclusion: The results suggest that the enhanced rise in blood ammonia concentration during exercise after beta-blockade is caus ed by impaired carbohydrate availability.