Selective modulation by cGMP of the K+ channel activated by speract

The egg peptide speract stimulates sperm guanylyl cyclase and presumably enhances fertilization, but the roles of cGMP in sperm responses are yet undetermined. Here we show that speract-induced accumulation of cGMP or cAMP is selectively enhanced by the phosphodiesterase inhibitors, 3-isobutyl-1-methylxanthine (IBMX) or papaverine, respectively. These inhibitors provided the unusual opportunity to examine the consequences of manipulating cGMP- and cAMP-dependent responses. The following observations suggest that cGMP mediates activation of K channels, the earliest known ionic event in speract signal transduction: 1) both cGMP content and K+ permeability are maximal within 15 s of speract stimulation and both decline after intracellular pH (pHi) increases in response to hyperpolarization; 2) IBMX prolongs elevation of cGMP and sustains K+ permeability after pHi increases; 3) both cGMP accumulation and K+ permeability also are enhanced when the pHi increase is prevented by an elevated concentration of external K+ (Ko); 4) elevating pHi with NH4Cl bypasses the blockade imposed by high Ko and decreases K+ permeability. Because IBMX antagonizes this action of NH4Cl, these results further suggest that elevation of pHi initiates an inactivation of guanylyl cyclase that leads to K channel closure. However, K+ permeability is restored upon subsequent elevation of intracellular [Ca2+] (Cai), indicating either that sperm K channels possess an alternate regulatory mode, or that a distinct Ca(2+)-activated K permeability also participates in speract signal transduction. Regardless of the mechanism that mediates Cai action, sperm K channels are identified as downstream targets of cGMP and are implicated in a feedback loop that both terminates guanylyl cyclase activity and leads to their own inactivation.