The purposes of this study were to: (1) compare the patterns of responses for plasma ammonia concentration ([NH3]) during moderate- and heavy intensity cycle ergometry; and (2) examine the relationship between the V˙O2 slow component (V˙O2SC) and plasma [NH3]. Thirteen healthy, untrained college males who performed less than a total of two hours of aerobic and/or resistance exercise per week visited the laboratory on six separate occasions. The initial visit included a V˙O2max test, whereas subsequent visits involved constant power output exercise (7-minutes in duration) at two different intensities [moderate 60%GET = 60% of the gas exchange threshold; and heavy, Δ50% = 50% of the difference between GET and V˙O2max)]. Plasma was collected from an antecubital vein prior to the exercise, during the last three minutes of the six minute warm-up, and during each minute of the seven minute constant power output workbout. The time course of changes in plasma [NH3] and V˙O2 during the two constant power output exercise intensities (60%GET and Δ50%) were assessed separately using 2 (intensity: 60%GET and Δ50%) × 7 (time: each minute of 7-minute workbout) repeated-measures ANOVAs. When appropriate, Scheffé\u27s post-hoc analyses were used to determine significantly different mean values. In addition, a Pearson\u27s correlation coefficient was calculated to assess the relationship between plasma [NH3] and the amplitude of the V˙O2SC for the heavy intensity exercise. For moderate intensity exercise (60%GET), there were no significant differences in the mean normalized plasma [NH3] during the 7-minute workbout. In addition, the mean normalized V˙O2 values were not significantly different from each other during the final 5-minutes of the exercise workbout. For heavy intensity exercise (Δ50%), there was a significant increase in the mean normalized plasma [NH3] during the 7-minute workbout. Similarly, the mean normalized V˙O 2 response increased significantly during the entire 7-minute workbout. Pearson\u27s correlation coefficient indicated a significant (r = 0.76, p \u3c 0.05) relationship between [NH3]Δ7-3 (the difference in normalized [NH3] between the 7 th- and 3rd-minute ([NH3]Δ7-3 ) of the Δ50% constant power output exercise) and the V˙O2SC amplitude. The results indicated that during moderate intensity exercise, the V˙O2 response reached a steady-state with no change in plasma [NH3] from baseline values. For the heavy intensity exercise, however, there was a V˙O 2SC and plasma [NH3] increased continuously until the end of exercise. These findings suggest a potential relationship between exercise-induced increase in plasma [NH3] and the V˙O 2SC during heavy intensity exercise
