Lactic acid output of cat gastrocnemius-plantaris during repetitive twitch contractions.

Because fatigable, white (FF) muscle fibers have been reported to have a greater glycolytic capacity and a lower oxidative capacity than fatigue-resistant, red (FR and SR) muscle fibers, it is generally supposed that FF muscle fibers produce and therefore release more lactic acid into the blood during contractions than FR and SR muscle fibers. To test this supposition, the net lactic acid output, L, and O2 uptake, VO2, were measured for the cat gastrocnemius-plantaris muscle during repetitive isometric contractions. The results obtained from this low metabolic capacity (FF, FR and SR muscles) were compared to results obtained previously in the high metabolic capacity (FR and SR muscles) of the same muscle group in the dog during similar contractions. Preliminary studies established that 1 twitch X 2 s-1 provided a similar VO2 pattern during the contractions of cat muscle as 4 twitches X s-1 produced in the dog muscle. The decline in VO2 over a 30-min period of contractions was 12 to 18%, as developed tension declined with fatigue. Thus, the contractions of the cat muscles were matched with the dog muscles in terms of the relative aerobic capacity and development of fatigue. During the 1 twitch X 2 s-1 twitches, the VO2 reached 24.4 +/- 1.41 (SE) microliter X g-1 X min-1 at 10 min of contractions. The VO2 declined to 21.3 +/- 2.3 microliter X g-1 X min-1 by 30 min. The VO2 and tension developed changed parallel to each other. The net L reached 0.21 +/- 0.06 mumol X g-1 X min-1 at 10 min and fell to 0.13 +/- 0.05 mumol X g-1 X min by 30 min.(ABSTRACT TRUNCATED AT 250 WORDS)