Effects of sarcomere length and temperature on the rate of ATP utilisationby rabbit psoas muscle fibres

1. The steady state rate of ATP utilisation by single permeabilised fibres from rabbit psoas muscle immersed in silicons oil was measured using a link ed enzyme assay that coupled ADP production to the oxidation of NADH. 2. At sarcomere length 2.5 mum, at 10 degreesC, the rate of ATP utilisation in relaxing conditions M as 6 +/- 1 muM s(-1) (mean +/- S.E.M., n = 8 fibr es); during isometric contraction it was 310 +/- 10 muM s(-1) (mean +/- S.E .M., n = 11). Assuming a myosin active site concentration of 150 muM, these values correspond to rates of ATP utilisation per active site of about 0.0 4 and 2.1 s(-1), respectively. 3. The rate of ATP utilisation in relaxing conditions was independent of sa rcomere length in the range 2.5-4.0 mum. The rate of ATP utilisation during isometric contraction had a dependence on resting sarcomere length similar to that of isometric force in the range 2.5-4.0 mum, but was less strongly dependent on sarcomere length than was isometric force in the range 1.5-2. 5 mum. 4. The rate of ATP utilisation in relaxing conditions had a Q(10) of 2.5 in the temperature range 7-25 degreesC, but this increased to 9.7 in the rang e 25-35 degreesC, suggesting that some active force may have been generated in relaxing solution at temperatures above 25 degreesC. 5. The rate of ATP utilisation during isometric contraction had a Q(10) of 3.6 throughout the temperature range 7-25 degreesC; this was similar to the Q(10) for isometric force at low temperature (3.5 at 7-10 degreesC) but mu ch larger than that for isometric force at higher temperature (1.3 at 20-25 degreesC). 6. Application of the NADH-linked assay to single muscle fibres in oil impr oves the effective sensitivity and time resolution of the method, and allow s continuous measurements of the rate of ADP production during active contr action.