Hypotonic challenge reduces human sperm motility through coiling and folding of the tail

Human ejaculates collected for in vitro procedures show variably rapid increases in osmolality, depending on enzymatic degradation of compounds. Changes in osmolality can affect cell functions due to the energy consuming processes needed to control cell volume. The aim was to examine the effects of a hypotonic challenge for spermatozoa exposed to increased osmolality. Spermatozoa were selected by density gradient centrifugation and washed in media with different osmolalities. Osmolality was measured by freezing-point depression and sperm velocities by CASA. Swimming pattern observations and assessments of tail morphology of fixed spermatozoa were done with phase contrast microscopy. Increased osmolality did not change the curvilinear velocity (VCL), while decreased osmolality reduced or abolished VCL nonreversibly. For spermatozoa first exposed to 400 mOsm/kg, reversal of osmolality to 290 mOsm/kg reduced the VCL and the average path velocity (VAP) permanently. Hypotonic challenges increased sperm tail coiling and folding in a dose-response pattern. Spermatozoa once adjusted to high osmolality in the liquefied ejaculate are likely to suffer if exposed to a medium with a lower osmolality. For improved success of Assisted Reproductive Technologies (ART), it appears to be important to minimise the duration of sperm exposure to the ejaculate, by early dilution or sperm preparation.