Inhibition of microtubule sliding by Ni2+ and Cd2+: Evidence for a differential response of certain microtubule pairs within the bovine sperm axoneme

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Abstract
Bovine sperm, extracted with 0.1% Triton X‐100, frozen at −20°C for 48–120 hours, and thawed, disintegrated by microtubule sliding when 1 mM MgATP was added. Microtubules and outer dense fibers (ODFs) were usually extruded in groups or “bundles.” A total of 44.5% of the cells extruded two distinct bundles, one from each side of the connecting piece, exhibiting opposite curvatures. Only one bundle was observed in 46.2% of the cells, and 9.2% showed no signs of sliding. Transmission electron microscopy (T.E.M.) showed one group consisting of the 4,5‐6,7 elements, with the 9,1,2 elements on the other side of the axoneme making up the other bundle. T.E.M. revealed that when only one side of the axoneme had extruded elements, they were always from the 4,5‐6,7 group. The remainder of the axoneme (8,9,1,2,3 and the central pair) was left relatively intact, suggesting a difference in the sliding response of the nine pairs of axonemal microtubules. These results indicate a predisposition for sliding between elements 7 and 8 over that between doublets 2 and 3, perhaps due to a disparity in activation thresholds. Also, both Ni2+ and Cd2+ appear to selectively block activation of 2–3 interdoublet sliding.Incubation with 0.25 mM Ni2+ prior to adding MgATP modified the percentages of sliding patterns: 8.6% demonstrated two‐sided extrusion, 58.2% showed one‐sided, and 33.2% had no extruded bundles. Again, when half the axoneme was missing, it was always the 4,5‐6,7 group. Ten micromolar Cd2+ altered the sliding pattern similarly to Ni2+, with 28% two‐sided extrusion, 55.9% one‐sided extrusion and 16.1% with no extruded bundles.Either pretreatment regimen impeded extrusion of the 9,1,2 group in a high percentage of cells, compared to untreated cells. This specific inhibition of the 9,1,2 side by Ni2+ or Cd2+ is especially significant since Ni2+ also inhibits spontaneous wave initiation in bull sperm (Lindemann et al.: Journal of Cell Biology 87:420–426, 1980), and both Ni2+ and Cd2+ reportedly block the flagellar Ca2+‐response in rat sperm (Lindemann and Goltz: Cell Motility and the Cytoskeleton 10:420–431, 1988; Lindemann et al.: Cell Motility and the Cytoskeleton 20:316–324, 1991).