Structure and function of the cytoskeleton in heliozoa: 2. Measurement of the force of rapid axopodial contraction in Echinosphaerium

The large heliozoan Echinosphaerium extends a number of needle-like axopodia by which it captures food organisms. Every axopodium contains a bundle of several hundreds of axonemal microtubules as a cytoskeletal element. When the tip of a poly-L-lysine-coated glass micro-needle came into contact with the distal part of an axopodium, a rapid axopodial contraction (2.6 mm/s) occurred with a concomitant bending of the needle toward the cell body. In this report, we measured the force of the axopodial contraction by utilizing the relation between force and bending displacement of the micro-needle, and examined a possibility that the axopodial contraction is ascribed to the axopodial tension (surface tension and/or cytoplasmic elasticity) that is developed as a result of microtubule degradation. The force of the axopodial contraction was estimated in the order of 10⁻⁹ N. Treatment with 10 mM colchicine induced disassembly of the axopodial microtubules and a resulting slow retraction of the axopodia (0.1 μm/s) occurred. The force of the slow retraction was also measured by the same procedure to estimate the axopodial tension, and was in the order of 10⁻¹¹ N. It was thus demonstrated that the motive force for axopodial contraction cannot be explained as an axopodial tension generated as a result of disassembly of the microtubules.