The stiffness of the flagella of impaled bull sperm

The elastic rigidity (stiffness) of impaled motionless bull sperm flagella has been determined by a manipulatory technique which permitted direct analytical treatment of the experimental system. The effects of external ATP and ADP were measured. It was found that ATP acts as a plasticizing agent, while ADP does not. The stiffness measured for flagella in a medium without ATP was 15 times greater than the value measured with 10 mM ATP present. The rigor-like stiffness measured with no ATP present is reversible with ATP and seems to be correlated to a transition in the state of the contractile system.