Microtubule sliding, bend initiation, and bend propagation parameters of Ciona sperm flagella altered by viscous load

The effect of altered viscous resistance on flagellar bending has been reexamined, utilizing ATP-reactivated sperm flagella from Ciona and newer methods that resolve metachronous and synchronous components of microtubule sliding and allow the examination of bend initiation as well as bend propagation. Large changes in amplitude and wavelength of bend propagation occur with little change in bend initiation parameters, other than frequency, indicating that bend initiation and bend propagation are regulated by quite different mechanisms. At increased viscosity, reduced amplitude of propagating bends, measured as metachronous shear amplitude, is associated with both reduced amplitude during bend initiation and amplitude adjustment after bends begin to propagate. This combination of effects was seen previously when reduced amplitudes were induced by increased salt concentration, and it was suggested to be caused by an imbalance between active moments and viscous resistances. However, in contrast to the results at increased salt concentrations, which involved significant reduction in bend curvature and little reduction in wavelength, increased viscosity causes very little change in curvature and causes a major reduction in wavelength. This difference can be explained by a model of flagellar bending in which inner arm dyneins have primary responsibility for maintaining bend curvature and outer arm dyneins have primary responsibility for performing work against viscous resistances. Both sets of dyneins would be inhibited by increased salt concentration, but increased viscous resistance would be irrelevant to the operation of inner arm dyneins.