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The bat Corynorhinus mexicanus provides an interesting experimental model for the study of epididymal sperm maturation because after spermatogenesis and the regression of the testes, this bat stores sperm in the epididymal cauda for several months. Earlier research conducted by our group suggested that sperm maturation in this species must be completed in the caudal region of the epididymis. One of the major signal transduction events during sperm maturation is the tyrosine phosphorylation of sperm proteins. The aim of the present study was to comparatively evaluate tyrosine phosphorylation in spermatozoa obtained from the caput, corpus and cauda of the epididymis during the sperm storage period. The maturation status of the sperm was determined by the percentage of capacitation and tyrosine phosphorylation in sperm obtained from the epididymis. The highest proportion of tyrosine phosphorylation was registered after the sperm had reached the cauda epididymis during the middle of the storage period. In conclusion, in Corynorhinus mexicanus and most likely in other chiropteran species with an asynchronous male reproductive pattern, epididymal sperm maturation ends in the caudal region of the epididymis and is related to the time that the sperm remains in the epididymis before mating activity.

The results of this work support the existence of sperm storage in Corynorhinus mexicanus, a vespertilionid bat endemic to Mexico, and evidence is presented that modulation of peroxidative toxicity plays a role in the mechanism of prolonged sperm storage in Chiroptera. Spermatozoa were obtained from C. mexicanus by retrograde perfusion of the cauda epididymis, and from genital washings from previously inseminated females captured during reproductive activity. Ejaculated pig spermatozoa were simultaneously studied as controls. Lipid peroxidation was determined in both the presence and absence of genital secretions obtained from previously inseminated female C. mexicanus by measuring malondialdehyde generated during aerobic incubation of spermatozoa suspensions. The number of spermatozoa recovered from the cauda epididymis decreased steadily from November through January. Corpora lutea were observed in January. None of the female bats captured between October and January were pregnant, but some females captured in mid-February were already pregnant. Spermatozoa of C. mexicanus showed substantial lipid peroxidation activity (0.64 ± 0.11 nmol malondialdehyde was produced by 108 spermatozoa per 22 h), about half of that observed in ejaculated pig spermatozoa. Incubation of pig and C. mexicanus spermatozoa in the presence of bat female genital tract washings induced highly significant concentration-dependent inhibition of lipid peroxidation.

We studied the activities of reactive oxygen species (ROS) scavenging enzymes during epididymal spermatozoon maturation and storage in Corynorhinus mexicanus (G.M. Allen, 1916), a vespertilionid bat that stores spermatozoa in the epididymides for several months after regression of the testes. Depending on the phase of the epididymal reproductive cycle, two different patterns of antioxidant enzyme activities were observed in C. mexicanus. Catalase activity is clearly present in both caput and cauda epididymides throughout the entire annual reproductive cycle, being particularly high during the post-testicular phase of epididymal function. Superoxide dismutase (SOD) activity, present during the testicular phase of epididymal transport and maturation of spermatozoa, is almost completely absent or inhibited in both epididymal segments during the post-testicular epididymal storage period. GPx activity is low during the testicular phase of epididymal spermatozoon maturation and is high in both epididymal segments during the storage phase of epididymal function. From our results, we postulate that (i) the pattern of epididymal antioxidant enzyme activities in C. mexicanus is significantly different from the pattern that is proposed to be unique for mammals; (ii) epididymal function in these species of bats can be clearly divided into two phases, a testicular-dependent phase that is related to the spermatozoon maturation function of the epididymides and a testicular-independent phase that is related to the long-term spermatozoon storage function observed in these mammals; (iii) the study of the regulation of the redox potential of the microenvironment, associated with mammalian spermatozoa as they transit through the epididymides, must be particularly focused on the anatomical region where ROS generation scavenging and spermatozoon maturation storage processes take place.

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