Since many of studies have been conducted to elucidate the relationship between energy metabolic pathways (glycolysis and respiration) and flagellar motility in mammalian sperm, contribution of glycolysis to sperm motility has not been fully elucidated yet. In the present study, we performed detailed analysis of mouse sperm flagellar motility for further understanding of the contribution of glycolysis to mammalian sperm motility. Mouse sperm maintained vigorous motility by substrates either for glycolysis or for respiration. By contrast, inhibition of glycolysis by alpha-chlorohydrine (ACH) caused significant decrease in bend angle of flagellar bending wave, sliding velocity of outer doublet microtubules and ATP content even in the presence of respiratory substrates (pyruvate or beta-hydroxybutyrate; BHB). The decrease of flagellar bend angle and sliding velocity are prominent in the distal part of the flagellum, indicating that glycolysis inhibition caused the decrease in ATP concentration especially at the distal part of the flagellum. These results suggest that glycolysis potentially act as a spatial ATP buffering system, transferring energy (ATP) synthesized by respiration at mitochondria located in the basal part of the flagellum to the distal part of the flagellum. In order to validate glycolytic enzymes can transfer high energy phosphoryls, we calculated intraflagellar concentration profiles of adenine nucleotides along the flagellum by computer simulation analysis. The result demonstrated that the involvement of glycolysis for maintaining the ATP concentration at the tip of the flagellum. It is likely that glycolysis plays a key role in energy homeostasis in mouse sperm not only through ATP production but also through energy transfer.

Salmonid fish sperm motility is known to be suppressed in millimolar concentrations of extracellular K⁺, and dilution of K⁺ upon spawning triggers cAMP-dependent signaling for motility initiation. In a previous study, however, we demonstrated that suspending sperm in a 10% glycerol solution and subsequent dilution into a low-osmotic solution induced motility independently of extracellular K⁺ and cAMP. In the present study, we further investigated the glycerol-induced motility mechanism. We found that treatment with solutions consisting of organic or inorganic ions, as well as glycerol, induced sperm motility in an osmolarity-dependent manner. Elimination of intracellular Ca²⁺ by BAPTA-AM significantly inhibited glycerol-treated sperm motility, whereas removal of extracellular Ca²⁺ by EGTA did not. Monitoring intracellular Ca²⁺, using fluo-4, revealed that intracellular Ca²⁺ increased when sperm were suspended in hypertonic solutions, and a subsequent dilution into a hypotonic solution led to a decrease in intracellular Ca²⁺ concomitant with motility initiation. In addition, upon dilution of sperm into a hypertonic glycerol solution prior to demembranation, the motility of demembranated sperm was reactivated in the absence of cAMP. The motility recovery suggests that completion of axonemal maturation occurred during exposure to a hypertonic environment. As a result, it is likely that glycerol treatment of sperm undergoing hypertonic shock causes mobilization of intracellular Ca²⁺ from the intracellular Ca²⁺ store and also causes maturation of axonemal proteins for motility initiation. The subsequent dilution into a hypotonic solution induces a decrease in intracellular Ca²⁺ and flagellar movement. This novel mechanism of sperm motility initiation seems to act in a salvaging manner for the well-known K⁺-dependent pathway.

During capacitation, motility of mammalian spermatozoon is changed from a state of “activation” to “hyperactivation.” Recently, it has been suggested that some hormones present in the oviduct are involved in the regulation of this hyperactivation in vitro. Progesterone, melatonin, and serotonin enhance hyperactivation through specific membrane receptors, and 17β-estradiol suppresses this enhancement by progesterone and melatonin via a membrane estrogen receptor. Moreover, γ-aminobutyric acid suppresses progesterone-enhanced hyperactivation through the γ-aminobutyric acid receptor. These hormones dose-dependently affect hyperactivation. Although the complete signaling pathway is not clear, progesterone activates phospholipase C and protein kinases and enhances tyrosine phosphorylation. Moreover, tyrosine phosphorylation is suppressed by 17β-estradiol. This regulation of spermatozoal hyperactivation by steroids is also disrupted by diethylstilbestrol. The in vitro experiments reviewed here suggest that mammalian spermatozoa are able to respond to effects of oviductal hormones. We therefore assume that the enhancement of spermatozoal hyperactivation is also regulated by oviductal hormones in vivo.

It has been recently shown that mammalian spermatozoa were hyperactivated by steroids, amines and amino acids. In the present study, we investigated whether hyperactivation of hamster sperm is regulated by progesterone (P) and γ-aminobutyric acid (GABA). Although sperm hyperactivation was enhanced by P, GABA significantly suppressed P-enhanced hyperactivation in a dose-dependent manner. Suppression of P-enhanced hyperactivation by GABA was significantly inhibited by an antagonist of the GABAA receptor (bicuculline). Moreover, P bound to the sperm head, and this binding was decreased by GABA. Because the concentrations of GABA and P change in association with the estrous cycle, these results suggest that GABA and P competitively regulate the enhancement of hyperactivation through the GABAA receptor.