EFFECT OF OSMOTIC-PRESSURE ON THE ACTIVATION AND STORAGE OF CHANNEL CATFISH SPERM

Artificial spawning of channel catfish Ictalurus punctatus relies on t he removal of testis and suspension of sperm in an extender solution f or storage and use in fertilization. Little is known about the relatio nships among osmotic pressure, sperm activation, motility, and storage . Our objectives were to (1) estimate motility of channel catfish sper m diluted in solutions ranging in osmotic pressure from 8 to 295 milli osmols (mosmol)/kg, (2) identify the osmotic pressure that induces thr eshold activation (10% motility) and the highest pressure that induces complete activation, (3) determine the role of ionic dilution in acti vation by use of ion-deficient solutions, and (4) evaluate the effect of osmotic pressure on the retention of motility during storage. Motil ity (percentage of actively swimming sperm) was estimated in diluted H anks' balanced salt solution (HBSS) and sucrose solutions over a range of osmotic pressures. The HBSS osmotic pressures of threshold and com plete activation were 218 +/- 15 mosmol/kg and 132 +/- 9 mosmol/kg, re spectively. We found that osmotic pressures above 220 mosmol/kg induce d 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 m otility about 10%. We used sucrose solutions to provide an osmotic env ironment with minimal ionic influence for the testing of activation. O smotic 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 ma jor role in the activation of channel catfish sperm. Sperm stored in 1 22 mosmol/kg HBSS lost 82% of initial motility after 2.5 h. However, s perm stored in solutions with higher osmolalities retained motility si gnificantly longer (P = 0.0017). A minimum reduction of osmotic pressu re to below 130 mosmol/kg is essential to induce complete motility of channel catfish sperm for motility estimates and artificial fertilizat ion.