The lactate/pyruvate shuttle in spermatozoa: operation in vitro

Area of Science:

• Biochemistry
• Mitochondrial Function
• Sperm Biology

Background:

• Spermatozoa utilize unique sperm-type mitochondria (STM) for energy metabolism.
• Reducing equivalents transfer is crucial for sperm motility and function.
• The lactate/pyruvate shuttle is a key metabolic pathway in various cell types.

Purpose of the Study:

• To investigate the functionality of the lactate/pyruvate shuttle in sperm-type mitochondria (STM) from different species.
• To identify species-specific differences in STM shuttle operation.
• To elucidate the role of lactate transport in shuttle efficiency.

Main Methods:

• Reconstituted in vitro systems using STM from mouse, rat, and rabbit.
• Enzyme assays involving lactate dehydrogenase isozyme C4.
• Measurement of pyruvate and lactate consumption by STM.
• Inhibition studies using mersalyl to assess lactate transport.

Main Results:

• The shuttle system was active in rat and rabbit STM but non-functional in mouse STM.
• Poor lactate influx into mouse STM was identified as the likely cause of shuttle inactivity.
• Pyruvate/lactate consumption ratios varied significantly: 21.6 (mouse), 1.28 (rat), and 1.6 (rabbit).
• Mersalyl, a lactate transport inhibitor, blocked shuttle operation.

Conclusions:

• The sperm mitochondrial shuttle is species-dependent, primarily due to variations in mitochondrial lactate transport.
• Efficient shuttle function in rat and rabbit STM facilitates oxidation of cytosolic NADH produced during sperm glycolysis.
• This pathway is vital for aerobic energy production in spermatozoa of species with effective mitochondrial lactate carriers.