Fluorometric assessments of mitochondrial function and viability in cryopreserved bovine spermatozoa

Mitochondrial function and sperm viability were quantified in samples of cryopreserved bovine spermatozoa from 12 bulls using fluorometric techniques. The active mitochondria of the spermatozoa were fluorescently stained using three different fluorophores: rhodamine 123 (R123), 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolyl-carbocyanine iodide (JC-1) or MitoTracker Green FM (MITO). The stained spermatozoa, and companion aliquots that had been stained with SYBR-14 (a living-cell nucleic acid stain) and propidium iodide to assess viability, were quantified using flow cytometry. The resulting fluorescent measurements of mitochondrial function were compared with microscopic assessments of progressive sperm motility immediately after thawing, with motility after 3-h incubation at 37 degrees C, and with the fluorescent assessment of sperm viability. Staining with either R123 or MITO resulted in a single green population. In contrast, the JC-1 staining of mitochondria produced both green and red-orange populations of spermatozoa and sometimes a progressive gradient between the two populations. The ability of IC-l to discriminate between mitochondria exhibiting high membrane potential from those having low to medium membrane potential provided a more rigorous estimate of metabolic function than the other two fluorescent stains. Overall, the three fluorometric measurements of mitochondrial function were highly correlated with each other, with the SYBR-14 assessment of viability, and with the microscopic estimates of motility.