Changes in plasma proenkephalin peptide F and catecholamine levels during graded exercise in men.

Proenkephalin peptide F immunoreactivity, epinephrine, and norepinephrine were measured in the plasma of endurance-trained and untrained male subjects riding on a bicycle ergometer at 28%, 54%, 83%, and 100% of maximum oxygen consumption (VO2). At rest the trained group had peptide F levels almost twice the level of the untrained group, whereas all other variables measured were the same. The maximum epinephrine and norepinephrine levels were found at 100% exercise intensity, with a precipitous drop in the levels at 5 min of recovery. In contrast, the peptide F immunoreactivity reached a maximum at 5 min of recovery and was still substantially above the initial level after 15 min of rest. In addition, the trained subjects showed another peak of peptide F immunoreactivity at 54% VO2max. Possible explanations for the different patterns of catecholamine and peptide F levels are presented.