Effect of pentoxifylline on the intrinsic swimming forces of human sperm assessed by optical tweezers.

It is still controversial whether in vitro exposure of sperm to pentoxifylline increases sperm motility and force, which is defined as the product of velocity by beat frequency of the tail. Laser optical tweezers have been successfully used in the past to evaluate sperm force in basal conditions. The aim of this prospective study was to determine whether exposure of human sperm to pentoxifylline has any effect on sperm intrinsic forces. Twelve healthy subjects undergoing routine semen analysis were enrolled in the study. Ten exhibited normal semen parameters, 2 exhibited asthenozoospermia. Each semen specimen was washed and, after swim-up, resuspended in human tubal fluid (HTF) and divided into 2 aliquots. One aliquot was incubated with pentoxifylline for 30 minutes (final concentration = 3.6 mM); the second aliquot, without pentoxifylline, served as a control. After 30 minutes the pentoxifylline-treated aliquot was divided into 2 portions, 1 of which was washed to remove the pentoxifylline, the other was left in prolonged coincubation with the chemical. The main outcome was the measurement of sperm intrinsic force in milliwatts (mW), which was assessed by means of a noninvasive infrared laser optical trap created by a continuous wave, 1064-nm Nd:YAG laser beam directed in an inverted microscope. Exposure of sperm to pentoxifylline consistently increased sperm relative escape force from the laser optical trap. The increase ranged from 33% to 154% over baseline force compared with controls. The average absolute increase in sperm force rose from 37 mW to 79 mW (P < .05). Specimens with sperm having an initial low relative escape force gained the highest relative increase. The effect of pentoxifylline on sperm force, already apparent after 5 minutes, reached a peak at 30 minutes and persisted for up to 3 hours in sperm that were left in coincubation and in those on which the pentoxifylline had been washed off. In conclusion, pentoxifylline significantly increases sperm intrinsic relative force in normozoospermic and asthenozoospermic samples. This experiment confirms that optical tweezers can provide an accurate determination of sperm force in in vitro conditions. Clinical data must now establish whether a documented increase in sperm force is an important parameter for assessing sperm fertilizing capacity.