Lactate as the sole energy substrate induces spontaneous acrosome reaction in viable stallion spermatozoa

Abstract

AbstractBackgroundEquine spermatozoa appear to differ from spermatozoa of other species in using oxidative phosphorylation preferentially over glycolysis. However, there is little information regarding effects of different energy sources on measured parameters in equine spermatozoa.ObjectiveTo determine the effect of three individual energy substrates, glucose, pyruvate, and lactate, on motion characteristics, membrane integrity, and acrosomal status of stallion spermatozoa.Materials and methodsFreshly ejaculated stallion spermatozoa were incubated with combinations of glucose (5 mm), pyruvate (10 mm), and lactate (10 mm) for 0.5 to 4 h. Response to calcium ionophore A23187 (5 μm) was used to evaluate capacitation status. Motility was evaluated using computer‐assisted sperm analysis, and plasma membrane and acrosomal integrity were evaluated by flow cytometry.ResultsIncubation with lactate alone for 2 h increased acrosomal sensitivity to A23187. Notably, incubation with lactate alone for 4 h induced a significant spontaneous increase in acrosome‐reacted, membrane‐intact (viable) spermatozoa, to approximately 50% of the live population, whereas no increase was seen with incubation in glucose or pyruvate alone. This acrosomal effect was observed in spermatozoa incubated at physiological pH as well as under alkaline conditions (medium pH approximately 8.5). Sperm motility declined concomitantly with the increase in acrosome‐reacted spermatozoa. Sperm motility was significantly higher in pyruvate‐only medium than in glucose or lactate. The addition of pyruvate to lactate‐containing medium increased sperm motility but reduced the proportion of live acrosome‐reacted spermatozoa in a dose‐dependent fashion.DiscussionThis is the first study to demonstrate that incubation with a specific energy substrate, lactate, is associated with spontaneous acrosome reaction in spermatozoa. The proportion of live, acrosome‐reacted spermatozoa obtained is among the highest reported for equine spermatozoa.ConclusionThese findings highlight the delicate control of key sperm functions, and may serve as a basis to increase our understanding of stallion sperm physiology.