ABSTRACT
The respiration rates and cGMP concentrations of Hemicentrotus pulcherrimus spermatozoa were determined in sea water containing various cation concentrations in the presence of sperm-activating peptide I (SAP-I) or monensin. Sperm respiration rates were inhibited in Na+-free, high K+ concentration ([K+]) or acidic (pH 6.6) sea water. SAP-I did not activate the inhibited respiration in high [K+] sea water but monensin did in high [K+] sea water. Both SAP-I and monensin stimulated the inhibited respiration rate in acidic sea water although both failed to activate the respiration in Na+-free sea water. SAP-I increased sperm cGMP level in normal, high [K+], divalent cation(s)-free sea water but not in Na+-free sea water. Monensin did not increase the cGMP level of spermatozoa in normal, high [K+] or Na+-free sea water. These results suggest that the metabolic activation of sea urchin spermatozoa by SAP-I is largely dependent on external Na+ and K+ concentrations.
