Dependence of length of muscle fibers upon ATP concentration

1. 1. The steady-state length of glycerol-extracted psoas fibers is a function of the concentration of ATP and not of the amount of ATP split. 2. 2. Reciprocal Michaelis-Menten constants for the glycerol-treated psoas muscle-ATP system have been obtained by measurements of length ( K ̄ L ) and of rates of dephosphorylation ( K ̄ c ). 3. 3. K ̄ c and V max are essentially independent of diameter in the range 200–600 μ, but both increase if the dimensions are reduced to the order of 2 μ. 4. 4. K ̄ c and V max remain approximately constant during storage in 50% glycerol for more than 1 year. 5. 5. At 0.3 M KCl, K ̄ L is greater than K ̄ c for fibers of 200–600 μ in diameter, but for fibrils of about 2 μ or less K ̄ L is about equal to K ̄ c . 6. 6. K ̄ L increases as [KCl] increases and also increases if small amounts of Mg ++ are added to fibers in 0.3 M KCl. Possible interpretations of the effects of KCl and of Mg ++ or Ca ++ are discussed. 7. 7. There is a linear relation between K ̄ L of psoas fibers and the rate of shortening of myosin B threads.