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Studies on the flagellar movement of carp spermatozoa induced by dilution in distilled water allowed us to describe a sequence of early, rapid morphological and kinetic changes which begin at the very tip of the flagellum. They cause the progressive folding of the axoneme which ends stuck to the head within 90–120 seconds after the initiation of motility. However, the axonemal machinery remains functional as the folding can be reversed after transfer back into a high osmolality medium and partially folded flagella were able to propagate efficient waves along the non‐folded proximal portion of the axoneme. The data also revealed that the membrane area of the terminal piece exhibits strong sensitivity to hypotonicity. These results suggest that in the normal freshwater medium, the brief swimming period allowing fertilization of oocytes is limited by the osmotic stress induced coiling of the carp sperm tail and not by ATP stores. © 1996 Wiley‐Liss, Inc.

Carp spermatozoa are immotile in seminal plasma or in saline solution of high osmolality (> 400 mosmol kg-1). These ‘quiescent’ spermatozoa initiate a progressive forward motility when transferred in freshwater or in saline solution with low osmolality (< 160 mosmol kg-1). In this study we investigated ‘in vitro’ the relationship between sperm ATP content (measured by bioluminescence) and sperm motility (analysed by videomicroscopy). Sperm ATP content remained high in the immobilizing medium (200 mM KCl, Tris 30 mM, pH 8.0) where no flagellar movement occurs. Dilution of these spermatozoa in the activating medium (45 mM NaCl, 5 mM KCl, Tris 30 mM, pH 8.0) triggered forward motility which varied with temperature. At 20 degrees C, sperm ATP content decreased rapidly during the progressive forward motility phase from 12 to 4 nmol/10(8) spermatozoa, concomitantly with decreases in velocity (130 to 10 microns s-1) and the beat frequency (50 to 7 Hz). An inhibitor of mitochondrial respiration (KCN 10 mM) produced a drop in sperm ATP content irrespective of the incubation medium (activating or immobilizing). A second phase of sperm motility in the activating medium was induced following a previous transfer of spermatozoa into a medium of high osmolality for a few minutes prior to the second phase. Within 10 minutes, spermatozoa recover 90% of the initial ATP level as well as forward motility. These results suggest that motility of carp spermatozoa depends on sperm ATP synthesized by mitochondrial respiration mainly stored before activation. In low osmolality conditions, the mitochondrial oxidative phosphorylation is unable to compensate for the ATP hydrolysis required to sustain motility.(ABSTRACT TRUNCATED AT 250 WORDS)