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The possibility of increasing the percentage motility and the number of Y-bearing spermatozoa by passage through graduated layers of bovine serum albumin (BSA) was first reported by Ericsson, Langevin & Nishino (1973), but their conclusions have become the subject of controversy (Ross, Robinson & Evans, 1975; Evans, Douglas & Renton, 1975). We therefore undertook a series of experiments, some involving the techniques described by Ericsson et al. (1973), in an attempt to clarify this issue.
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In the male reproductive tract, very high concentrations (mmol l−1) of free l-carnitine and acetyl-l-carnitine are found in the epididymides, seminal plasma and spermatozoa. It has been reported that the uptake of free l-carnitine by spermatozoa might be related to the epididymal maturation of the sperm membrane, since a greater uptake was found by caput than by cauda spermatozoa in vitro. However, the free l-carnitine concentrations estimated inside the gametes were never greater than those of the surrounding medium. In this study, we investigated the mechanism of transport of free l-carnitine and its ester acetyl-l-carnitine, through the plasma membrane of mature and immature epididymal boar spermatozoa. In vitro, we found a passive diffusion of both compounds to the spermatozoa, whatever the maturation stage. The spermatozoa might progress in the epididymal lumen and accumulate high amounts of free l-carnitine. The active uptake of free l-carnitine occurs only across epididymal mucosa. These results are in agreement with those reported on cells of other organs that exchange pharmacological free l-carnitine concentrations (mmol l−1) by a passive mechanism through the plasma membrane. The acetylation of high amounts of free l-carnitine inside the spermatozoa was found only in caudal spermatozoa. This result suggests that oxidative metabolism (producing acetyl CoA) might be more active in mature cells. The acetyl-l-carnitine added to the incubation medium of boar spermatozoa was hydrolysed. Enzymatic activity of the sperm membrane is low and this may partially explain the low concentrations of acetyl-l-carnitine found in the caudal epididymal plasma.
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Summary. In the epididymal fluid of boars, the concentration of carnitine (nmol/mg protein) began to increase from 20 in the distal caput, then rose progressively to 700 in the distal cauda. By contrast, the carnitine content of spermatozoa only started to increase in the proximal cauda where the concentration of carnitine in the fluid was 200–300 nmol/mg protein then gradually increased in spermatozoa from more distal sites. The increase in the acetylcarnitine content of spermatozoa paralleled that of the carnitine amount and represented 50% of the total carnitine (carnitine + acetylcarnitine). We conclude that the acetylcarnitine content of epididymal spermatozoa may be used as a marker of maturation.
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The mechanism by which a hypo-osmotic shock activates motility of carp spermatozoa was studied. The direct role of osmolality at the axoneme was investigated after demembranation of spermatozoa with Triton X-100 and reactivation in various ionic or anionic solutions containing Mg–ATP: demembranated spermatozoa remain motile in solutions of osmolality up to 550 mOsm kg−1 while non-demembranated spermatozoa are immotile when osmolality rises above 250 mOsm kg−1 with the same salt solutions as well as in non-ionic solutions. Suspension in hypo-osmotic saline solutions triggered the swelling of native carp spermatozoa. No motility or swelling occurred above 200–300 mOsm kg−1 and this osmolality is probably that of the cytosol. The swelling of carp spermatozoa is the result of an entrance of water but this was not affected by pCMBS, an inhibitor of the aquaporin CHIP28, or by various inhibitors of the co-transport of water with ions. Various pharmacological agents that affect the motility of different sperm species had no effect on carp sperm motility when used under similar conditions. However, prolonged exposure to a solution devoid of K+ or Cl− affects the activation of motility in a reversible manner, suggesting that these ions have a role in the perception or transduction of the osmotic signal. Altering the concentration of intracellular second messengers such as Ca2+ and cAMP, and the pH did not affect the motility of carp spermatozoa. However, DMSO at 1–20% (400–3200 mOsm kg−1) affects the motility of carp spermatozoa 3–4 min after mixing. These results show that the activation signal of carp sperm motility differs from that known for spermatozoa of other species of fish such as trout. Our results indicate that the activation mechanism may involve a co-transport of ions or specific 'stretch-activated channels' that are sensitive to osmotic pressure.
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Summary. Two groups of men were retrospectively selected according to their observed success in in-vitro fertilization. Seminal and post-migration sperm samples from a low fertilization rate group (≤33% cleaved embryos) have been compared to results obtained from a high fertilization rate group (≥ 66%). It was found that a low mean value of the amplitude of lateral sperm head displacement and an increased percentage of abnormal acrosomes were related to in-vitro fertilization failure. None of the individual sperm factors studied was found to determine in-vitro fertilization success with certainty; only when they were considered in combination was it possible to predict the likelihood of successful in-vitro fertilization of human oocytes.
