The effects of naloxone and timolol on plasma catecholamine levels during short-term dynamic exercise

In order to study the role of opioid- and betareceptors on exercise-induced catecholamine responses, the effects of acute intravenous administration of 1 and 4 mg naloxone and of the non-selective betablocker timolol 2 mg of on circulating concentrations of adrenalin, noradrenaline and dopamine during exercise to exhaustion were examined in eight normal, healthy young men, using a double-blind, randomized, placebo-controlled design. During maximal exercise, adrenalin levels increased from 71 +/- 17 to 821 +/- 235 pg/ml (p less than 0.05), noradrenaline from 355 +/- 58 to 4235 +/- 1031 pg/ml (p less than 0.05), and dopamine from 72 +/- 20 to 178 +/- 44 pg/ml (p less than 0.05). Naloxone did not influence basal or exercise-induced noradrenaline responses. Timolol clearly augmented peak adrenalin concentration at maximal exercise capacity (1543 +/- 510 pg/ml, p less than 0.05). Basal noradrenaline level was increased (546 +/- 86 pg/ml, p less than 0.05), while exercise-induced noradrenaline level was reduced (2954 +/- 594 pg/ml, p less than 0.05) in proportion to the reduction in maximal exercise capacity during timolol treatment. Neither naloxone nor timolol affected dopamine levels. No additive effect was seen with the combination of naloxone and timolol. It is concluded that the opioid peptides are probably not involved in noradrenaline and dopamine responses, whereas betablockers change the catecholamine response to short-term maximal exercise.