Mechanical factors affecting the <i>ATP</i> ase activity of glycerol-extracted insect fibrillar flight muscle

Abstract

Abstract The ATPsplit by glycerol-extracted fibrillar flight muscle of the tropical water bug Lethocerus cordofanus was measured in the presence of an inhibitor of mitochondrial ATPase in a calciumbuffered solution at pH 7.1 and under controlled mechanical extension and oscillation. In the absence of calcium (pCa &amp;gt; 8.5) the ATPase activity of the fibres was low (ca. 10 nmole ATP split (mg protein)-1 min-1) and was not significantly changed by oscillating or stretching the muscle by a few percent of its rest length. When the calcium concentration was raised to 10-7 to 10-6 M in an unstretched muscle the ATPase increased, and on stretching the muscle by up to 5 % of its rest length the calcium-activated ATPase rose again, to 30 to 90 nmole (mg protein )-1 min-1. Further extension of the muscle did not increase the ATPase. The effect of extension on ATPase was found whether or not calcium was present during the extension. Oscillation of the stretched muscle by up to 3 % of its rest length at a frequency of 3 to 5 c/s produced a further increase of the calcium-activated ATPase, apparently proportional to the work done by the muscle (mechanochemical coefficient = 1.0 to 2.4 kcal/mole). Oscillation at higher or lower frequencies produce less increase of ATPase. Evidence is presented to show that these changes were effects of the mechanical events on the myofibrillar enzymic activity in a constant chemical environment, and were not caused by variation in the diffusion of ATP within the fibres.