Hidden effects of cryopreservation on quality of human spermatozoa

The effects of cryopreservation on two characteristics of human spermatozoa were investigated: the early phases of disturbed plasma membrane function and the activity of enzymes in intact spermatozoa. The membrane function was detected by means of the calcium-dependent binding of fluorescein isothiocyanate (FITC)-conjugated Annexin V to sperm plasma membranes. Annexin V monitors the translocation of phosphatidylserine from the inner to the outer leaflet of the plasma membrane, which is one of the earliest features of membrane disintegration. For the second aim synthetic fluorogenic substrates for peptidases, proteinases, esterases, elastases and collagenases were applied. These substrates, CellProbe trade mark reagents consist of different peptide sequences, specific for the enzymes, and a fluorescein- or rhodamine 110-dye moiety. They enter the cells without previous membrane permeabilisation and exhibit fluorescence after cleavage depending on enzyme activity. The number of positive cells and the intensity of the fluorescence were determined by flow cytometric analysis comparing fresh spermatozoa with cryopreserved ones. Thirty-five semen samples collected from 35 donors were cryopreserved using the freezing medium TEST yolk buffer. All specimens showed normal spermiogram parameters. Twenty-five of the samples were used for detection of Annexin V-FITC binding and 10 semen samples for investigations of the intracellular enzymes. The Annexin V-assay applied two fluorescent dyes (Annexin V, AN and propidium iodide, PI) which led to three groups of spermatozoa (a) viable spermatozoa (AN V-negative and PI-negative), (b) dead spermatozoa (AN V-positive and PI-positive) and (c) cells with impaired but integer plasma membrane (AN V-positive and PI-negative). The percentage of vital Annexin V-negative spermatozoa (x +/- SEM) decreased significantly (p < 0.001) from fresh spermatozoa (51.6 +/- 3.1) to cryopreserved spermatozoa (26.6 +/- 2.2%) and was associated with their motility (57.9 +/- 1.9% motile fresh spermatozoa vs. 22.6 +/- 3.9% motile sperm after cryopreservation). Of the spermatozoa 28.2% were Annexin V-positive before and 44.4% after cryostorage even though they did not bind to PI. Thus, vital spermatozoa showed a disturbed membrane function indicating viability before as well as after cryostorage. Moreover, after cryopreservation the spermatozoal fluorescence increased applying substrates for butyryl esterase (p < 0.05), prolyl-aminopeptidase (p < 0.001) and val-lys-(VK)-cathepsin (p < 0.001). In contrast, the activities of fluorescein diacetate (FDA)- and FDA/sodium fluoride (NAF)-esterase (p < 0.05), ala-ala-pro-val-(AAPV)-elastase (p < 0.001), gly-pro-leu-gly-pro-(GPLGP)-collagenase (p < 0.05) and gly-gly-leu-(GGL)-subtilisin (p < 0.001) decreased after cryopreservation. The substrates for arg-gly-glut-ser-(RGES)-elastase, gly-phenyl-gly-ala-(GFGA)-collagenase and threo-pro-(TP)-cathepsin were not cleaved before as well as after cryostorage. In addition to the known effects of sperm cryopreservation our results showed two further alterations of human ejaculated spermatozoa: (a) disturbed plasma membrane function, which is not detectable by supravital staining and (b) a changed pattern of intracellular enzyme activities.