Roles for Potassium and Calcium Channels in the Initiation of Sperm Motility in Rainbow Trout.

Abstract

It is well known that the motility of spermatozoa in rainbow trout is suppressed by K+. We showed here that although trout sperm are completely immotile in medium containing 5 mM K+, motility was initiated by the subsequent addition of several mM Ca2+, suggesting that both K+ and Ca2+ are related to the process of the initiation of sperm motility. It was further found that Kt channel blockers tetraethylammonium, nonyltriethylammonium, Ba2+ and Cs+ , as well as the Ca2+ channel blocker verapamil, inhibited the initiation of sperm motility at doses at which these reagents inhibit chnnel-related functions in other cells. However, Na+ channel blocker, tetrodotoxin and anion channel blocker 4, 4-diisothiocyatatostilbene-2, 2'-disulfonic acid inhibited the motility only at extremely high doses. These results suggest that transport of K+ and Ca2+ through ion channels at the plasma membrane of spermatozoa is the first event that triggers the initiation of sperm motility in rainbow trout.The mechanism underlying the initiation of sperm motility is attracting much attention and one of the most successful investigations concerning this phenomenon were performed in studies using fish sperm, especially the sperm of salmonid fishes (15). The sperm motility of rainbow trout (22,28) as well as other salmonid fishes (19,22) is suppressed completely in the medium containing K f , which explains why the motility of trout sperm is suppressed in the sperm duct by seminal K+ surrounding the spermatozoa. Motility is initiated under experimental conditions when the semen is diluted into K+-free solution (19, 22, 28), suggesting that spermatozoa become motile when they are released into fresh water at natural spawning. In addition, it was also established that the decrease of K+ immediately caused the significant and transient increase of intracellular cAMP (17) which triggers the initiation of sperm motility (18) , through the CAMP-dependent phosphorylation of axonemal protein (16) in rainbow trout. However, how the decrease of K+ outside of the cell induces the synthesis of intracellular cAMP and causes the initiation of axoneme movement is poorly understood.Baynes et al. has suggested that inhibition of sperm motility by K+ is antagonized by the presence of divalent cations such as Ca2+ and Mg2+ ( 3 ) . We confirmed the above result and furthermore demonstrated that intracellular cAMP increased and spermatozoa became motile when several mM of Ca2+ was added to the sperm suspension in which spermatozoa were immotile in the presence of K+ (15). These imply that transport of Kf and Ca2+ may occur through plasma membrane, inducing the synthesis of cAMP in the cell, resulting in the initation of trout sperm motility.In order to clarify the contribution of ion channels to the process of initiation of sperm 117

Membrane hyperpolarization removes inactivation of Ca 2+ channels, leading to Ca 2+ influx and subsequent initiation of sperm motility in the common carp

“…An increase in intracellular calcium is a prerequisite for the initiation of sperm motility in rainbow trout (28)(29)(30)(31). It was also shown by Okuno and Morisawa (32) that demembranated trout sperm showed vigorous motility in the reactivating solution containing Ca 2ϩ in a concentration below 10 Ϫ8 M, suggesting that the initiation of fish sperm motility requires a transient influx of Ca 2ϩ , although its origin, from intracellular storage or of extracellular origin, is controversial (29).…”

“…Membrane hyperpolarization (29) through the direct effect of transmembrane K ϩ efflux (30,31) is the first trigger for the initiation of sperm motility in salmonid fishes. Membrane hyperpolarization also triggers the activation of ascidian sperm motility through hyperpolarization-induced direct activation of adenylyl cyclase to synthesize cAMP in Ciona (35).…”

Tale of Fish Sperm and Factors Affecting Sperm Motility: A Review

“…Although [K + ] o and cAMP were known to influence motility, their relationship is still unknown. Potassium channel blockers like, Tetraethylammonium (TEA + ), nonyltriethylammonium1, Ba 2+ , and Cs + , inhibited sperm motility initiation [98]. The above results obtained on induction of sperm motility suggest the hypothesis: ''the inhibition of motility in salmonids is mainly due to K + ion''.…”

Ionic Environment Necessary for Maintenance of Potential Motility in the Common Carp Spermatozoa during <i>In Vitro</i> Storage

“…21,22) Trout spermatozoa do not move in the presence of EGTA22) or in the presence of Ca 21-channel blockers. 22,23) Moreover, upon the initiation of movement of sper matozoa, an increase occurs in the intracellular concentra tion of Ca2+ ions in these cells.24,25) These data strongly suggest that external Ca2+ ions might be involved in the in itiation of flagellar movement in salmonid spermatozoa.…”