A sea urchin egg jelly peptide induces a cGMP-mediated decrease in sperm intracellular Ca(2+) before its increase

Speract, a sperm-activating peptide (SAP) from sea urchin eggs, increases the intracellular concentration of Ca²⁺ (Ca²⁺]i) and modulates sperm motility. We measured the initial sperm response to speract using its caged analog and observed, for the first time, a small but significant decrease in sperm Ca²⁺]i before the increase. Both directions of the Ca²⁺]i change were completely blocked in high K⁺ seawater. Using membrane-permeant caged cyclic nucleotides (cNMP), only cGMP induced the decrease in Ca²⁺]i although both cGMP and cAMP increased the Ca²⁺]i. The decrease in the Ca²⁺]i induced by cGMP was more notable following a second photolytic event, once Ca²⁺]i had been elevated by an initial flash. This pattern of Ca²⁺]i change was confirmed in individual sperm. These results together with pharmacological evidence suggest that the initial Ca²⁺]i decrease is due to a Na⁺/Ca²⁺ exchanger activity, stimulated by hyperpolarization mediated by K⁺ efflux through cGMP-regulated K⁺ channels.