Effect of Osmotic Pressure on the Activation and Storage of Channel Catfish Sperm

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Artificial spawning of channel catfish Ictalurus punctatus relies on the removal of testis and suspension of sperm in an extender solution for storage and use in fertilization. Little is known about the relationships among osmotic pressure, sperm activation, motility, and storage. Our objectives were to (1) estimate motility of channel catfish sperm diluted in solutions ranging in osmotic pressure from 8 to 295 milliosmols (mosmol)/kg, (2) identify the osmotic pressure that induces threshold activation (10% motility) and the highest pressure that induces complete activation, (3) determine the role of ionic dilution in activation by use of ion-deficient solutions, and (4) evaluate the effect of osmotic pressure on the retention of motility during storage. Motility (percentage of actively swimming sperm) was estimated in diluted Hanksˈ balanced salt solution (HBSS) and sucrose solutions over a range of osmotic pressures. The HBSS osmotic pressures of threshold and complete activation were 218 ± 15 mosmol/kg and 132 ± 9 mosmol/kg, respectively. We found that osmotic pressures above 220 mosmol/kg induced minimal (<10%) activation and pressures below 130 mosmol/kg induced complete activation. Within the zone of incomplete activation (220–130 mosmol/kg) a decrease of 15 mosmol/kg in osmotic pressure increased motility about 10%. We used sucrose solutions to provide an osmotic environment with minimal ionic influence for the testing of activation. Osmotic pressures for threshold (214 ± 1 mosmol/kg) and complete (125 ± 2 mosmol/kg) activation were consistent with those obtained with diluted HBSS, suggesting that reduction in osmotic pressure plays a major role in the activation of channel catfish sperm. Sperm stored in 122 mosmol/kg HBSS lost 82% of initial motility after 2.5 h. However, sperm stored in solutions with higher osmolalities retained motility significantly longer (P = 0.0017). A minimum reduction of osmotic pressure to below 130 mosmol/kg is essential to induce complete motility of channel catfish sperm for motility estimates and artificial fertilization.