Reversible loss of calcium control of tension in scallop striated muscle associated with the removal of regulatory light chains

Abstract

THE control by calcium of the ATPase activities of myosin, actomyosin and myofibrils from molluscan muscles requires the presence of a particular ‘regulatory’ light chain of myosin^1,2. In these muscles troponin is not found in stoichiometric amounts and does not function as a major regulatory component^2,3. In scallop muscles one of the two regulatory light chains per myosin is readily and reversibly removed by reducing the divalent cation concentration with EDTA^2,4, with the result that the preparations become ‘desensitised’ to calcium, that is, the ATPase activity is maximal whether or not calcium is present. ‘Resensitising’ can be achieved by the recombination of regulatory light chains with myosin or myofibrils. We have extended these studies to a scallop muscle preparation (chemically ‘skinned’ fibre bundles) in which the contractile machinery is intact, and we show here that the regulatory light chains can be removed and recombined, with a concomitant loss and recovery of calcium control over tension production.