The chemical energetics of muscle contraction. II. The chemistry, efficiency and power of maximally working sartorius muscles

Abstract

Sartorius muscles from Rana pipiens were stretched and then stimulated electrically at 0°C whilst being allowed to shorten at constant predetermined velocities on a Levin-Wyman ergometer. The muscles developed tensions appropriate to their instantaneous lengths and velocities. A comparison with the unstimulated paired control muscle allowed measurements to be made of the changes in phosphate compounds during these maximally-working, constant-velocity contractions. In contractions lasting less than 1.5 s, no significant dif­ferences were found in the usage of adenosine triphosphate or production of inorganic phosphate, for the performance of a constant amount of work, in normal aerobic muscles, anaerobic muscles pretreated with iodoacetate to inhibit lactate production, or muscles pre­treated with 2, 4-dinitrofluorobenzene so that adenosine triphosphate was the only energy source. In slow contractions lasting longer than 1.5 s allowance had to be made for myokinase and other enzymic reactions. The amount of external work done by the muscles, as a result of the hydrolysis of each mole of adenosine triphosphate, was found to be very dependent on velocity, being low at low and high speeds of shortening with a maximum below 1 muscle length/second. The free energy available per mole of adenosine triphosphate was calculated and the thermo­dynamic efficiency of the muscles was found to be high. On the basis of 10 kcal/mol adenosine triphosphate the overall efficiency was over 66 ± 6% at 2 cm/s in experiments with muscles pretreated with 2, 4-dinitrofluorobenzene. The amount of adenosine triphosphate used for processes other than mechanical work (mainly calcium pumping) was estimated to be about one quarter of the total. After allowance had been made for this the efficiency was found to be 98 ± 15% at a constant shortening velocity of 2 cm/s. Conversely, the minimum free energy available for doing external work from ATP hydrolysis under these conditions must be 9.8 ± 1.5 kcal/mol.