MUSCLE-CONTRACTION AS A MARKOV PROCESS I - ENERGETICS OF THE PROCESS

Force generation during muscle contraction can be understood in terms of cyclical length changes in segments of actin thin filaments moving through the three-dimensional lattice of myosin thick filaments. Recen t anomalies discovered in connection with analysis of myosin step size s in io vitro motility assays and with skinned fibres can be rationali zed by assuming that ATP hydrolysis on actin accompanies these length changes. The paradoxically rapid regeneration oi tension in quick rele ase experiments. as well as classical energetic relationships, such as Hill's force-velocity curve, the Fenn effect, and the unexplained ent halpy of shortening, can be given mutually self-consistent explanation s with this model. When muscle is viewed as a Markov process, the vect orial process of chemomechanical transduction can be understood in ter ms of lattice dependent transitions, wherein the phosphate release ste ps of the myosin and actin ATPases depend only on occurrence of allost eric changes in neighbouring molecules. Tropomyosin has a central role in coordinating the steady progression of these cooperative transitio ns along actin filaments and in gearing up the system in response to h igher imposed loads.