Alteration of Intracellular [ Ca " ' ] in Sea Urchin Sperm by the Egg Peptide Speract

The egg peptide speract increases intracellular pH (pHi) and cyclic nucleotides in sperm of the sea urchin Strongylocentrotus purpuratus by a mechanism dependent on seawater Na+ but not Ca2+ (Hansbrough, J. R., and Garbers, D. L. (1981) J. Biol. Chem. 256, 2235-2241; Repaske, D. R., and Garbers, D. L. (1983) J. Biol. Chern. 258, 6025-6029). Using the Ca2+ indicators quin2 and indo1, we show that speract stimulates a transient rise in intracellular [Ca"'] ([Ca"'];) when millimolar Ca2+ is present in seawater. The rise is increased and extended by the phophodiesterase inhibitor, l-methyl-3-isobutylxanthine (MIX), which also enhances "Na+ uptake with or without Ca2+. Without MIX, speract initiates a rise in [Ca2+]i that peaks within -5 s and decreases with a trh of -9 s. Activation of Na+:H+ exchange without speract by either Na+ addition to sperm in Na+-free seawater (NaFASW) or by monensin also increases [Ca2+Ii, but neither change is transient. Inhibition of Na+:H+ exchange by increased seawater [K+] prevents the rise in [Ca2+Ii nitiated by either speract or Na+ addition to sperm in NaFASW. Increasing pHi by adding 10 mM NH,+ or by addition of Li+ to sperm in NaFASW does not increase [Ca"+li. The data suggest that speract binding leads to rapid activation of Na+:H+ exchange; and, as a consequence, [Ca"'] entry increases transiently through either Na+:Ca2+ exchange or else through a verapamil-insensitive Ca2+ channel. MIX prevents the inactivation of this entry mechanism.