1. The influence of 30 mM inorganic phosphate (P(i)) and pH (6.2-7.4) on the rate of ATP utilization was determined in mechanically skinned bundles of myofibrils from the iliofibularis muscle of Xenopus laevis at approximately 5°C. 2. BDM (2,3-butanedione monoxime; 10 mM) depressed isometric force production and actomyosin (AM) ATPase activity equally. Therefore sarcoplasmic reticular (SR) ATPase activity could be determined by extrapolation of the total ATPase activity to zero force. 3. The SR ATPase activity without added P(i) at pH 7.1 was 42 ± 2% of the total ATPase activity. Addition of 30 mM P(i) reduced SR ATPase activity slightly, by 9 ± 5%, and depressed force by 62 ± 2% and AM ATPase activity by 21 ± 6%. 4. At pH 6.2, force, SR ATPase activity and AM ATPase activity were reduced by 21 ± 5, 61 ± 5 and 10 ± 4% of their respective values at pH 7.1. 5. The SR ATPase activity at 30 mM P(i) and pH 6.2 was reduced markedly to 20 ± 6% of the value under control conditions, suggesting that the maximum rate of Ca2+ uptake during muscle fatigue was strongly depressed. This reduction was larger than expected on the basis of the effects of P(i) and pH alone.
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