The force-velocity relationship for microtubule sliding in demembranated sperm flagella of the sea urchin.

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Abstract
We studied the relationship between the force and velocity of microtubule sliding in demembranated sperm flagella of the sea urchin, Hemicentrotus pulcherrimus, under auxotonic conditions following a quick release of the tension between sliding microtubules. The shape of the force-velocity curve was independent of the concentration of Mg-ATP over the range of 3.7 to 350 .mu.M and appeared either linear or was the reverse to the hyperbolic curve seen for muscle. The power, caluculated as the product of velocity and force, passed through a peak at c.0.7 Fmax (the maximal isometric force). Thus, the maximal power is attained at a larger relative load than in muscle. The sliding velocity at 0.1 Fmax showed a hyperbolic dependence on Mg-ATP concentration, with a Km of 210 .mu.M and a Vmax of 19 .mu.m .cntdot. sec-1. The maximal force did not significantly change over the Mg-ATP concentration range of 3.7 to 350 .mu.M. These results are discussed in terms of a crossbridge model similar to the one originally proposed by Huxley. It is suggested that the dynein crossbridge cycle is characterized by a relatively rapid rate of attachment and a relatively slow rate of detachment.