VO2 max is unaffected by altering the temporal pattern of stimulation frequency in rat hindlimb in situ

It might be anticipated that fatiguing contractions would impair the aerobic metabolic response in skeletal muscle if significant fatigue developed before full activation of aerobic metabolism. On the basis of this premise, we examined two groups of rats to test the hypothesis that a gradual increase in stimulation frequency would yield a higher maximal O-2 uptake ((V) over dot O-2 max) than beginning immediately with an intense stimulation frequency because of a slower progression of fatigue under the former conditions. In one group of animals, the distal hindlimb muscles were electrically stimulated at a frequency of 60 tetani/min for 4 min (F-60; n = 6 rats); in the other group, the muscles were incrementally stimulated for 1 min at each of 7.5, 15, 30, and 60 tetani/min and for 2 min at 90 tetani/ min (F-Inc; n = 5 rats). Despite large differences in rate of fatigue [time to 60% of initial force was 47 +/- 3 (SE) vs. 188 +/- 1 s in F-60 and F-Inc, respectively] and the time at which (V) over dot O-2 max occurred (120 +/- 15 vs. 264 +/- 6 s), (V) over dot O-2 max was not different (419 +/- 24 vs. 381 +/- 44 mumol . min(-1) . 100 g(-1)). Furthermore, time x tension integral at (V) over dot O-2 max (3.82 +/- 0.41 vs. 4.07 +/- 0.31 N . s) and peak lactate efflux ( 910 +/- 45 vs. 800 +/- 98 mumol . min(-1) . 100 g(-1)) were not different between groups. Thus our results show that the more rapid progression of fatigue in F60 did not compromise the aerobic metabolic response in electrically stimulated rat hindlimb muscles. However, in both groups, O-2 uptake and lactate efflux declined after (V) over dot O-2 max was attained in similar proportion to a further fall in force, suggesting that ongoing fatigue with intense contractions reduced ATP demand below that requiring maximal aerobic and glycolytic metabolic responses once (V) over dot O-2 max was reached.