Requirements for in vitro shortening and lengthening of isolated thick filaments of Limulus striated muscle

Abstract

DE VILLAFRANCA1 first reported that the A bands of Limulus skeletal muscle shortened during ATP-stimulated contraction of glycerinated myofibrils. We have found that in Limulus striated muscle, unlike vertebrate striated muscle, the length of the A band varies linearly with sarcomere length2. This is partly due to variation in thick filament length. We have isolated either long or short thick filaments from this muscle, depending on the conditions to which the muscle was exposed before the isolation procedure3. We have further determined that the paramyosin-containing thick filaments4,5 of the Limulus telson muscle decrease 30–40% in length2 and increase in diameter by a similar amount during sarcomere shortening below rest length (7.0 µm)6. The mechanism for the control of this shortening process is unknown but the series of experiments reported here implicate Ca2+ and ATP in thick filament shortening.