Carbon dioxide reversibly inhibits sperm motility and fertilizing ability in steelhead (Oncorhynchus mykiss)

The effects of different carbon dioxide (CO2) levels on the short-term stor age of semen samples from hatchery-produced steelhead (Oncorhynchus mykiss) were evaluated. Sperm motility and fertilizing ability were significantly reduced following 4 h incubation under a relatively modest (greater than or equal to0.9 kPa = 1%) amount of CO2. The dose-dependent reductions, howeve r, were not the result of cell death as sperm viability was unaltered even at the highest (5.2 kPa = 5.6%) CO2 exposures. Reductions in sperm motility and fertilizing ability were reversible. Although previous work has indica ted a direct relationship between salmonid sperm motility and sperm ATP con tent, the inhibitory effects of CO2 on sperm motility were not the result o f reduced sperm ATP levels. Decreasing the pH of the seminal fluid (to belo w similar to7.5) significantly reduced sperm motility. However, this effect was only observed after prolonged (4 h) exposure; short-term (1 min) expos ure to this lowered pH did not alter sperm motility. Moreover, acetazolamid e, a carbonic anhydrase inhibitor, attenuated the inhibitory effects of CO2 on sperm motility. These results suggest that CO2 inhibits steelhead sperm motility and therefore fertility in a dose-dependent manner, by reversibly lowering intracellular pH.