Zinc plays an essential role in mitochondrial function, motility and plasma membrane organisation of boar spermatozoa

Zinc is the most abundant trace element in boar seminal plasma. The current in vitro study investigated to which extent mitochondrial function, motility, and plasma membrane organization depend on the presence of extra- and intracellular Zn²⁺ in boar spermatozoa. Spermatozoa were incubated in chemically-defined media in the presence of increasing concentrations of ZnCl₂ (0.01-2 mM) or TPEN, an intracellular zinc chelator (0.01-1000 μM). Extracellular Zn²⁺-levels (≥1 mM) prevented the downregulation of free intracellular Zn²⁺ levels over time (P < 0.05) observed in the absence of zinc supplementation. Concomitantly, extracellular Zn²⁺ led to a dose-dependent reduction in mitochondrial transmembrane potential in viable spermatozoa (P < 0.05). At the highest Zn²⁺ concentrations, sperm motility was virtually absent (P < 0.05). Neither Ca²⁺ nor Mg²⁺ had a similar effect (P > 0.05). Notably, extracellular Zn²⁺ induced a persistent, dose-dependent increase in cellular hydroxyl radical and peroxide levels in viable spermatozoa (P < 0.05), while simultaneously decreasing cellular and mitochondrial superoxide levels. Chelation of free intracellular zinc with TPEN decreased the mitochondrial transmembrane potential in viable sperm by up to 60 % (P < 0.05). Negative effects on sperm motility were apparent with high TPEN concentrations, but varied in intensity between experimental runs. TPEN consistently reduced cellular superoxide levels and, after prolonged incubation, increased cellular hydroxyl radical and peroxide levels in viable spermatozoa. Zinc showed a significant synergistic effect with caffeine to increase the percentage of viable spermatozoa with high membrane fluidity. Altogether, the results underscore the importance of zinc as an essential factor for mitochondrial function and balance of reactive oxygen species in boar spermatozoa.