Effect of glucose infusion on endurance performance after beta-adrenoceptor blocker administration

To investigate the effect of glucose (Glc) infusion on endurance performance after beta-adrenoceptor blockade, eight healthy male volunteers performed four endurance cycle ergometer tests at 67% of maximal work load after 80 mg of propranolol (Pr) or placebo (Pl) were administered orally in combination with a continuous infusion of Glc (0.5 g/min) or saline (Sal). The order of the tests was randomized. Endurance times were 53 +/- 6 (SE), 64 +/- 7, 26 +/- 5, and 31 +/- 6 min after Pl+Sal, Pl+Glc, Pr+Sal, and Pr+Glc, respectively (P < 0.001). Glc infusion increased endurance after Pl (P < 0.05) but not after Pr. Glc infusion resulted in significantly higher plasma Glc concentrations during exercise compared with Sal infusion (P < 0.001). Glc infusion had no effect on cardiorespiratory or other metabolic variables. Plasma ammonia concentration was increased during the Pr tests (P < 0.001) but reached similar values at exhaustion in all tests and was not affected by Glc infusion. The maintenance of plasma Glc concentration during endurance exercise at or above preexercise levels did not improve the reduction of endurance performance after Pr, indicating that the availability of blood Glc is not a limiting factor in this process.