Participation of a K+ channel modulated directly by cGMP in the speract-induced signaling cascade of Strongylocentrotus purpuratus sea urchin sperm

Speract, a decapeptide from Strongylocentrotus purpuratus sea urchin eggs, transiently stimulates a membrane guanylyl cyclase and activates a K+-selec tive channel that hyperpolarizes sperm. However, previous studies of sperm and of sperm membrane vesicles reached conflicting conclusions about the me chanisms that open these channels. We find that speract hyperpolarizes and increases the cGMP content of flagellar vesicles. We confirm previous findi ngs that intravesicular GTP gamma S and GTP enhance this hyperpolarization, but not GDP beta S. The G protein activators AlF4- and mastoparan also are ineffective. Thus, it is unlikely that a G protein participates in the spe ract response. In contrast, hyperpolarization responses to speract are incr eased by 3-isobutyl-1-methylxanthine, which preferentially inhibits cGMP-se lective phosphodiesterases of sperm, and the 8Br-cGMP derivative hyperpolar izes vesicles in the absence of speract. The responses to speract and to 8B r-cGMP have similar ionic selectivities (K+ > Rb+ > > Li+ > Na+) and sensit ivities to the channel blockers 4-aminopiridine and 3,4-dichlorobenzamil, i ndicating that they likely result from opening of the same K+ channel. Inhi bitors that preferentially inhibit cAMP-selective phosphodiesterases do not alter responses to speract, and permeant cAMP analogs do not hyperpolarize vesicles. In addition, inhibitors of protein kinases and phosphatases fail to alter vesicle hyperpolarization by speract. The increase in vesicular c GMP content produced by speract therefore may directly mediate opening of t he channel that hyperpolarizes sperm membrane vesicles. Similar mechanisms presumably operate in intact sperm. (C) 2000 Academic Press.