A study of bend formation in locally reactivated hamster sperm flagella.

Mature golden hamster sperm were demembranated with Triton X-100, and the flagellum was reactivated locally by iontophoretic application of ATP at various distances from the base. The response was a brief local straightening of a short length of the flagellum followed by the formation of a pair of bends beyond the two ends of the straight region. The two possible proximo-distal sequences of bends, either PR (principal and reverse bends) or RP, could be distinguished and their incidence studied. The formation of PR and RP bend pairs is interpreted as the result of active sliding of the axonemal doublet subsets 1-4 and 6-9 respectively. The probability of obtaining a PR response increased (1) with the initial local curvature of the resting R bend and (2) with the distance of the stimulated site from the flagellar base; it decreased with the duration of incubation after demembranation. The patterns of response in the middle and the principal piece of the flagellum were basically similar although the former was weaker and more complicated. Quantitative analysis of the ATP-induced movements indicates little or no net microtubule displacement distal to the pair of induced bends, suggesting the cancelling of microtubule displacements in the two bends. However, the expected balance in the rate of growth of the two bends was upset by the decay of one bend simultaneously with decay of the original adjacent bend. Propagation of the interbend region started before the growth of the pair of bends reached its maximum, and seemed to be triggered by a critical bend curvature. Propagation was always in the direction base to tip. Experimental findings also suggest a role in the determination of the waveform for the fibrous structures on the periphery of the axoneme which are characteristic of the mammalian sperm flagellum. The present study strengthens the experimental evidence for the mathematical model which proposes that active sliding occurs mainly in the interbend region and causes bending of segments in opposite directions. In addition our findings indicate that the activation of alternate halves of the axoneme is curvature dependent, suggesting a basis for the flagellar oscillation.