Activation of sperm respiration by a low molecular weight egg factor and by 8-bromoguanosine 3‘,5‘-monophosphate

Individual female differences in chemoattractant production change the scale of sea urchin gamete interactions

Sperm selection by females is an important process influencing fertilization and, particularly in broadcast-spawning organisms, often occurs before sperm reach the egg. Waterborne sperm chemoattractants are one mechanism by which eggs selectively influence conspecific sperm behavior, but it remains an open question whether the eggs from different females produce different amounts of sperm chemoattractant, and how that might influence sperm behavior. Here, we quantify the differences in attractant production between females of the sea urchin species Lytechinus pictus and use computational models and microfluidic sperm chemotaxis assays to determine how differences in chemoattractant production between females affects their ability to attract sperm. Our study demonstrates that there is significant individual female variation in egg chemoattractant production, and that this variation changes the scope and strength of sperm attraction. These results provide evidence for the importance of individual female variability in differential sperm attraction and fertilization success.

Sperm chemotaxis promotes individual fertilization success in sea urchins

Reproductive success fundamentally shapes an organism's ecology and evolution, and gamete traits mediate fertilization, which is a critical juncture in reproduction. Individual male fertilization success is dependent on the ability of sperm from one male to outcompete the sperm of other males when searching for a conspecific egg. Sperm chemotaxis, the ability of sperm to navigate towards eggs using chemical signals, has been studied for over a century, but such studies have long assumed that this phenomenon improves individual male fitness without explicit evidence to support this claim. Here, we assess fertilization changes upon use of a chemoattractant-digesting peptidase and use a microfluidic device coupled with a fertilization assay to determine the effect of sperm chemotaxis on individual male fertilization success in the sea urchin Lytechinus pictus. We show that removing chemoattractant from the gametic environment decreases fertilization success. We further find that individual male differences in chemotaxis to a well-defined gradient of attractant correlate with individual male differences in fertilization success. These results demonstrate that sperm chemotaxis is an important contributor to individual reproductive success.

Evolution of gamete attraction molecules: evidence for purifying selection in speract and its receptor, in the pantropical sea urchin Diadema

Many free-spawning marine invertebrates, such as sea urchins, lack any courtship or assortative mating behavior. Mate recognition in such cases occur at the gametic level, and molecules present on the sperm and egg are major determinants of species-specific fertilization. These molecules must also coevolve in relation to each other in order to preserve functional integrity. When sea urchins release their gametes in seawater, diffusible molecules from the egg, termed sperm-activating peptides, activate and attract the sperm to swim toward the egg, initiating a series of interactions between the gametes. Although the compositions and diversity of such sperm-activating peptides have been characterized in a variety of sea urchins, little is known about the evolution of their genes. Here we characterize the genes encoding the sperm-activating peptide of the egg (speract) and its receptor on the sperm, and examine their evolutionary dynamics in the sea urchin genus Diadema, in the interest of determining whether they are involved in reproductive isolation between the species. We found evidence of purifying selection on several codon sites in both molecules and of selectively neutral evolution in others. The diffusible speract peptide that activates sperm is invariant across species, indicating that Diadema egg peptides do not discriminate between con- and hetero-specific sperm at this stage of the process. Speract and its receptor do not contribute to reproductive isolation in Diadema.

The role of jelly coats in sperm-egg encounters, fertilization success, and selection on egg size in broadcast spawners

Sperm limitation may be an important selective force influencing gamete traits such as egg size. The relatively inexpensive extracellular structures surrounding many marine invertebrate eggs might serve to enhance collision rates without the added cost of increasing the egg cell. However, despite decades of research, the effects of extracellular structures on fertilization have not been conclusively documented. Here, using the sea urchin Lytechinus variegatus, we remove jelly coats from eggs, and we quantify sperm collisions to eggs with jelly coats, eggs without jelly coats, and inert plastic beads. We also quantify fertilization success in both egg treatment groups. We find that sperm-egg collision rates increase as a function of sperm concentration and target size and that sperm are not chemotactically attracted to eggs nor to jelly coats in this species. In fertilization assays, the presence of the jelly coat is correlated with a significant but smaller-than-expected improvement in fertilization success. A pair of optimality models predict that, despite the large difference in the energetic value of egg contents and jelly material, the presence of the jelly coat does not diminish selection for larger egg cell size when sperm are limiting.

Life‐History Consequences of Investment in Free‐Spawned Eggs and Their Accessory Coats

The optimal trade-off between offspring size and number can depend on details of the mode of reproduction or development. In marine organisms, broadcast spawning is widespread, and external coats are a common feature of spawned eggs. Egg jelly coats are thought to influence several aspects of fertilization and early development, including the size of the target for sperm, fertilization efficiency, egg suspension time, polyspermy, embryo survival, and fecundity. These costs and benefits of investment in jelly result in trade-offs that can influence optimal reproductive allocation and the evolution of egg size. I develop an optimization model that sequentially incorporates assumptions about the function of egg coats in fertilization. The model predicts large variation in coat size and limited variation in ovum size under a broad range of conditions. Heterogeneity among spawning events further limits the range of ovum sizes predicted to evolve under sperm limitation. In contrast, variation in larval mortality predicts a broad range of optimal ovum sizes that more closely reflects natural variation among broadcast-spawning invertebrates. By decoupling physical and energetic size, egg coats can enhance fertilization, maintain high fecundity, and buffer the evolution of ovum size from variation in spawning conditions.

Ligands and receptors mediating signal transduction in sea urchin spermatozoa

Sea urchins have long been a model system for the study of fertilization. Much has been learned about how sea urchin sperm locate and fertilize the egg. Sperm and eggs are spawned simultaneously into the surrounding seawater. Sperm signaling pathways lead to downstream events that ensure fertilization. Upon spawning, sperm must acquire motility and then they must swim towards or respond to the egg in some way. Finally, they must undergo a terminal exocytotic event known as the acrosome reaction that allows the sperm to bind to the vitelline layer of the egg and then to fuse with the egg plasma membrane. Motility is stimulated by exposure to seawater, while later events are orchestrated by factors from the egg. The sperm signaling pathways are exquisitely tuned to bring the sperm to the egg, bind, and fuse the two cells as quickly as possible.

Ion channels in sperm physiology

Fertilization is a matter of life or death. In animals of sexual reproduction, the appropriate communication between mature and competent male and female gametes determines the generation of a new individual. Ion channels are key elements in the dialogue between sperm, its environment, and the egg. Components from the outer layer of the egg induce ion permeability changes in sperm that regulate sperm motility, chemotaxis, and the acrosome reaction. Sperm are tiny differentiated terminal cells unable to synthesize protein and difficult to study electrophysiologically. Thus understanding how sperm ion channels participate in fertilization requires combining planar bilayer techniques, in vivo measurements of membrane potential, intracellular Ca2+ and intracellular pH using fluorescent probes, patch-clamp recordings, and molecular cloning and heterologous expression. Spermatogenic cells are larger than sperm and synthesize the ion channels that will end up in mature sperm. Correlating the presence and cellular distribution of various ion channels with their functional status at different stages of spermatogenesis is contributing to understand their participation in differentiation and in sperm physiology. The multi-faceted approach being used to unravel sperm ion channel function and regulation is yielding valuable information about the finely orchestrated events that lead to sperm activation, induction of the acrosome reaction, and in the end to the miracle of life.

Characterization of the main egg envelope proteins of the sea bass Dicentrarchus labrax L. (Teleostea, Serranidae)

Fish eggs are surrounded by a resistant acellular coat commonly called the chorion or zona radiata. This study characterizes the eggshell proteinaceous content of unfertilized eggs of the sea bass Dicentrarchus labrax, with a view to the preparation of immunogens. Solubilization of the purified eggshells was achieved in 8 M urea followed by one- and two-dimensional gel electrophoresis. Glycoproteins were detected using concanavalin-A in one and two-dimensional gels, and the principal glycoproteins had a molecular weight of 47 kDa and 170 kDa. Partial purification of a few polypeptides in the 45 kDa to 55 kDa range was achieved by gel filtration chromatography. Although whole eggshells were relatively insoluble even in 8 M urea, partial purification of these polypeptides enable them to dissolve completely in solutions at low ionic strength.

Guanylate cyclase receptor family

The plasma membrane forms of guanylate cyclase contain a highly conserved catalytic domain, which is also conserved in the soluble form of the enzyme and in mammalian adenylate cyclase. A protein kinase-like domain lies to the amino-terminal side of the catalytic domain and appears to be required for signaling via cGMP; it might also signal, itself, through phosphotransferase activity. This domain is present in the growth factor receptors, but appears not to be a component of other guanylate cyclases or adenylate cyclases. A single transmembrane domain then separates the cyclase catalytic and protein kinase-like domains from the putative ligand-binding domain. At least two plasma membrane forms of gunaylate cyclase (i.e., GC-A and GC-B) have now been identified, and their ligand specificities appear to be distinctly different. The tissue/cellular distribution of this family of receptors is now of potential importance, since specific agonists might differentially regulate physiological processes via the secondary messenger, cGMP, dependent on cellular distribution of the receptors.

Molecular basis of signalling in the spermatozoon

The spermatozoon contains cell surface receptors for various egg-associated molecules, one of which has now been identified as the enzyme guanylate cyclase. A single membrane-spanning region divides the enzyme such that about one-half is extracellular and one-half is intracellular. A new paradigm for signal/transduction is established by these observations, in that a hormone or effector molecule binding to an extracellular site activates the catalytic domain of the same protein, resulting in the increased formation of a low molecular weight second messenger. Research on signalling mechanisms in the spermatozoon clearly apply to a diversity of cellular processes as well as to fertilization.

Coordinate interactions of cyclic nucleotide and phospholipid metabolizing pathways in calcium-dependent cellular processes

It is hoped that his review enables the reader to appreciate the complexities implicit in the interactions among Ca2+, cyclic nucleotides, and phospholipid-metabolizing pathways in cell signal transduction. The interactions are varied and intricate, often involving several levels of cell amplification mechanisms. Upsetting the balance of fatty acids in membrane phospholipids can have detrimental effects on adenylate cyclase. Thus, n - 3 fatty acid enrichment of phospholipids suppresses adenylate cyclase activity. The effects of significant alterations in dietary fatty acids, such as might occur with the current vogue for n - 3 eicosapentaenoic acid and docosahexaenoic acid (fish oil) dietary enrichment regimens, will need to be assessed more fully with regard to stimulus-induced changes in cyclic nucleotide production in various tissues. Since the n - 3 fatty acids have not been demonstrated to affect guanylate cyclase activity, dietary changes in certain of these fatty acids would not be expected to contribute to changes in cGMP generation as much as in cAMP production. Moreover, the ingestion of large quantities of these n - 3 fatty acids can alter the profile of cyclooxygenase and lipoxygenase products produced in cells. According to the paradigm developed in this article, changes in the metabolism of fatty acids are amplified by alterations in cyclic nucleotide production and phospholipase activities, with the eventual physiological impact predicated on the tissue type and the specific stimulus response. There appears to be a rather clear distinction between the regulatory properties of eicosanoids regarding adenylate and guanylate cyclase activities. Whereas prostaglandins often stimulate adenylate cyclase activity, they have little effect on guanylate cyclase activity. On the other hand, the HETE compounds seem to play an important role in guanylate cyclase regulation in certain cells. Moreover, arachidonic acid affects adenylate cyclase activity without prior peroxidation, whereas endoperoxides and hydroperoxides are more effective than arachidonic acid with regard to guanylate cyclase stimulation. However, in the intact cell there is a strong implication that the dual stimulation of guanylate cyclase by Ca2+ and fatty acid evokes optimal enzyme activity. An advantage of multidimensional response mechanisms in cells includes the ability to recognize different stimuli and to respond with specific, coordinated responses modulated in their intensity and/or duration by messenger interaction. Few cell types respond to receptor stimulation in an all-or-none fashion, and the "milieu interior" depends on specific, graded responses to the autonomic nervous system and endocrine stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)

Internal GTP stimulates the speract receptor mediated voltage changes in sea urchin spermatozoa membrane vesicles

A voltage-sensitive Na+/H+ exchanger in the flagellar membrane is responsible for regulating the intracellular pH of the sea urchin spermatozoa. A previous study has shown that the egg peptide speract can modulate this Na+/H+ exchanger through its hyperpolarizing effect on the membrane potential. The effect of GTP on this speract receptor mediated process is investigated in this study. Plasma membrane vesicles with an outwardly directed K+ gradient were prepared from the isolated flagella by osmotic lysis. Vesicular membrane potential was monitored by a cationic probe, diS-C3-(5), and an anionic probe, diS-BA-C2-(3). Results show that the presence of internal GTP greatly stimulated the speract induced membrane hyperpolarization in this vesicle system. The analog GTP gamma S was not only active but could, by itself, induce partial hyperpolarization which was further enhanced by speract addition. Internal GDP was partially active in supporting the speract effect, whereas GDP beta S, cGMP, GMP, and ATP were all inactive. The ionic selectivity of the speract effect was investigated by increasing the external concentration of various cations. K+ and Rb+ abolished the hyperpolarization while Cs+ had no effect. These results indicate that internal GTP is involved in the coupling between the speract receptor and the membrane hyperpolarization, which is most likely due to the activation of K+ selective channels.

A fast transient hyperpolarization occurs during the sea urchin sperm acrosome reaction induced by egg jelly

The egg jelly-induced sea urchin sperm depolarization, assayed with the membrane potential-sensitive dye diS-C3-(5), is preceded by a fast quenching, which was initially taken as an artifact. Here we show that part of this quenching results from a K+-dependent transient hyperpolarization (approximately 4 s). Seawater containing 25-35 mM KCl inhibits this hyperpolarization, the depolarization and the acrosome reaction induced by egg jelly, or by the acrosome reaction inducing factor purified therefrom. These results suggest that egg jelly induces a transient hyperpolarization mediated by an increase in K+ permeability, which may be involved in triggering the acrosome reaction.

Peptides associated with eggs: mechanisms of interaction with spermatozoa

Speract (Gly-Phe-Asp-Leu-Asn-Gly-Gly-Gly-Val-Gly), a peptide obtained from the culture medium of Strongylocentrotus purpuratus eggs, stimulates the respiration and motility of S. purpuratus spermatozoa under appropriate conditions. Resact (Cys-Val-Thr-Gly-Ala-Pro-Gly-Cys-Val-Gly-Gly-Gly-Arg-LeuNH2), a peptide obtained from Arbacia punctulata eggs also stimulates the metabolism and motility of A. punctulata spermatozoa, however, it fails to stimulate S. purpuratus spermatozoa. Early biochemical responses of the spermatozoa to the egg peptides include a net H+ efflux and elevations of cyclic AMP and cyclic GMP concentrations. In addition, in A. punctulata spermatozoa, a major plasma membrane protein is modified in response to resact such that its apparent molecular weight shifts from 160,000 to 150,000. If cells are incubated with 32P, the 160,000 molecular weight form of the protein becomes radiolabeled; subsequent addition of resact causes a rapid loss of 32P from the protein. The plasma membrane protein appears to be the enzyme, guanylate cyclase; coincident with the shift in apparent molecular weight, enzyme activity decreases by as much as 90%. Since speract fails to cause these responses in A. punctulata, it can be concluded that the events are receptor-mediated.

Extracellular localization of cyclic GMP in the house cricket male accessory reproductive gland and its fate in mating

The male accessory reproductive gland (ARG) of the house cricket, Acheta domesticus (L.), contains an exceedingly high concentration of cyclic GMP, about 1,000 pmol/mg protein. Immunofluorescent localization and radioimmunoassay measurements show that cyclic GMP is concentrated in a small number of tubules. It accumulates in the tubule lumina where it is protected from degradation by phosphodiesterases. Cyclic GMP is secreted by the ARG and is incorporated into spermatophores. Over 80% of spermatophore cyclic GMP is found in the handle-capillary tube, a thin conduit through which sperm pass during transfer to the female. The concentration of cyclic GMP in the insemination fluid is about 20 microM but does not appear to be specifically associated with the sperm. Cyclic GMP enters the female spermatheca during insemination but disappears rapidly. Physiological effects of cyclic GMP on sperm were not observed nor was an effect of cyclic GMP observed on egg laying by mated females. Cyclic AMP was localized on sperm flagella in the spermatophore and in the spermatheca. These studies indicate that cyclic nucleotides have important roles in insect reproduction and that the house cricket is a good model for elucidating these functions.

Effects of extracellular egg factors on sperm guanylate cyclase

Extracellular factors from the sea urchin egg induce a change in the electrophoretic mobility of an abundant sperm membrane phosphoprotein. The modified protein was identified as guanylate cyclase. The mobility shift of the cyclase was shown to be associated with a decrease in its enzymatic activity.

Sodium-dependent pH regulation in active sea urchin sperm

Extracellular sodium ion is required for activation of motility and respiration in sea urchin sperm when semen is diluted in seawater. We have investigated the role of sodium ion in maintenance of sperm activity. Active sperm lose activity on transfer to sodium-free artificial seawater and can be reactivated with external Na+. Reactivation occurs in the range of Na+ concentration required for initial activation; ammonium can substitute for sodium in reactivation. Sperm withdrawn from sodium and sperm prior to activation share a characteristic morphology with straight or gently bent flagella. Activation of sperm by amines in the absence of Na+ is unstable. It is followed by a steady respiratory decline which is temporarily reversed by addition of more amine and stably reversed by addition of Na+. Measurements of intracellular pH indicate that the internal pH rises during amine activation. Internal reacidification occurs during the period of respiratory decline, and Na+ again elevates internal pH. Treatment with cyanide abolishes the reacidification, indicating that it depends on respiration. We conclude that the sodium requirement persists in active sperm; respiration-dependent production of H+ must be balanced by sodium-dependent H+ removal to maintain activity.

Mediation of insulin release by cGMP and cAMP in a starved animal model

Islets isolated from fed rats released insulin in response to glucose, 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP) and 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP), but not to 8-bromoinosine 3',5'-cyclic monophosphate. Starving rats for 48 h significantly diminished insulin release from islets in response to these agents, and lowered endogenous levels of cGMP and cAMP. The analogs of cGMP and cAMP potentiated the glucose response in a dose-dependent manner in islets from starved rats, whereas in fed rat islets the cyclic nucleotide analogs did not potentiate glucose-stimulated insulin release. Sodium nitroprusside, which enhances endogenous cGMP levels in islets, also enhanced the glucose response in islets from starved rats. Mannoheptulose inhibited glucose and 8-Br-cGMP-stimulated insulin release, but not 8-Br-cAMP-stimulated release. These results suggest that the impaired glucose response of islets from starved animals is in part due to diminished levels of cyclic nucleotides, and that the role(s) of cGMP in insulin secretion may include enhancement of glucose metabolism.

The stimulation of bovine caudal epididymal sperm forward motility by bovine cumulus-egg complexes in vitro

We have determined the effects of cumulus oophorus-egg complexes, and of separated eggs and cumulus oophorus cells on bovine sperm forward motility. The forward motility index of spermatozoa was elevated by the incubation of spermatozoa in the presence of cumulus oophorus-egg complexes. Subsequently, it was shown that these complexes could release substance(s) into the medium which stimulated sperm motility. FSH appeared to cause a greater release of the stimulatory substance(s) from the cumulus oophorus-egg complexes. Separation of cumulus oophorus cells from eggs demonstrated that the stimulatory factor(s) were being released from the cumulus oophorus cells and not the eggs. These results demonstrate that bovine cumulus oophorus cells associated with the egg can influence spermatozoan motility.

Methylxanthines stimulate calcium transport and inhibit cyclic nucleotide phosphodiesterases in abalone sperm

Experiments were designed to determine the mechanism by which methylxanthines elevate abalone sperm cAMP concentrations and induce the acrosome reaction (AR). Theophylline or, more effectively, 1-methyl-3-isobutylxanthine (MIX) inhibit the cyclic nucleotide phosphodiesterase activities of abalone sperm homogenates. 45Ca2+ uptake by sperm is also stimulated by theophylline, and more effectively by MIX, and this stimulatory effect is blocked by KCN. Verapamil, a compound known to antagonize Ca2+ conductance, has no effect on the Ca2+ or MIX-induced cAMP elevation at concentrations up to 200 microM. However, verapamil reduces the sperm cAMP elevation caused by the addition of Ca2+ plus MIX. This inhibition is not complete, even at 200 microM verapamil. The AR induced by Ca2+ plus MIX is completely inhibited by 200 microM verapamil. The data suggest that these methylxanthines elevate abalone sperm cyclic nucleotide concentrations by inhibiting cyclic nucleotide phosphodiesterase activities. Furthermore, since sperm cAMP metabolism is modulated by Ca2+ flux, methylxanthines also appear to elevate abalone sperm cAMP concentrations by their effects on Ca2+ transport. The Ca2+-induced cAMP elevation occurs through a verapamil-insensitive mechanism, whereas the potentiation by MIX of the Ca2+ effect to elevate cAMP occurs through both verapamil-insensitive and -sensitive mechanisms. The methylxanthine-induced AR is mediated by a primary effect on Ca2+ transport and occurs through a verapamil-sensitive mechanism. Cyclic AMP may play a role in the methylxanthine-induced AR, but does not appear to act as the primary mediator of this exocytotic event.

The pH within the jelly coat of sea urchin eggs

Because low molecular weight factors isolated from sea urchin egg jelly increase sperm motility and respiration, but only at a pH less than 7.4, H. Ohtake (J. Exp. Zool. 198, 303-312, 1976) suggested that the pH within the jelly experienced by a sperm swimming to the egg might be 6.5-7.0. With a pH microelectrode, the pH in the jelly coat of single eggs of Strongylocentrotus purpuratus and Lytechinus pictus was measured and found to be not significantly different from the pH of seawater, 8.0; it thus does not seem likely that these low molecular weight factors are important in maintaining sperm motility or respiration during fertilization.

Regulation of abalone sperm cyclic AMP concentrations and the acrosome reaction by calcium and methylxanthines

When Ca2+ is added to abalone sperm (Haliotis rufescens) in Ca2+-free artificial seawater (CaFASW) to a final concentration of 9.6 mM a 4-fold elevation in sperm cAMP occurs within 15-30 sec. The methylxanthines, theophylline and 1-methyl-3-isobutylxanthine (MIX), also elevate sperm cAMP concentrations. In CaFASW, either compound causes up to a 3-fold increase in cAMP within 15-30 sec. MIX (150 microM), added to sperm in the presence of 9.6 mM Ca2+, elevates sperm cAMP 100-fold within 15-30 sec and also triggers the acrosome reaction (AR) in the same period. Under identical conditions theophylline (1.67 mM) is much less potent at elevating cAMP and inducing the AR. The effects of methylxanthines on cAMP of sperm incubated in the presence of Ca2+ appear to represent a potentiation by these compounds of the action of Ca2+. Neither compound induces the AR in the absence of Ca2+. All of the observed effects on sperm cAMP and the AR are dependent on Ca2+ and methylxanthine concentrations. Added cyclic nucleotides or their derivatives do not induce the AR in either the absence or presence of Ca2+. Experiments with isolated sperm heads and flagella indicate that the dramatic stimulatory response of sperm cAMP to Ca2+ plus MIX is present in the head region (acrosome, nucleus, midpiece) of the cell. The data suggest that the dramatic elevation of cAMP by MIX in the presence of Ca2+ may occur directly by an inhibition of phosphodiesterase activity and indirectly by an increase in cellular Ca2+. A strong temporal correlation between the cAMP elevation and the abalone AR exists, although cAMP elevation by itself does not act as the primary mediator of this exocytotic event.

Interactions between sperm and sea urchin egg jelly

The addition of egg jelly to sea urchin sperm induces multiple changes in morphology and behavior. When jelly is added to sperm diluted in seawater, the acrosome reaction is triggered, the mitochondrion rounds up, the internal pH is transiently alkalinized and then reacidified, and respiration becomes uncoupled and rapidly decreases. Sperm also become unable to fertilize eggs within a few minutes after jelly addition. In order to explore in more detail the effect of egg jelly on sperm, we have studied the response to jelly in the presence of inhibitors of the acrosome reaction. When jelly is added to sperm under conditions which are inhibitory for the acrosome reaction, an alkalinization takes place without the subsequent reacidification, the mitochondria remain coupled, and respiration and intracellular ATP levels remain high. Sperm viability is prolonged by some of these conditions, but not others. The addition of jelly to sperm in the absence of calcium elicits an internal alkalinization but no other rapid change in sperm physiology. The capacity of egg jelly to alter sperm physiology even when the overall acrosome reaction is inhibited indicates that some of the physiological changes either are early events in the triggering pathway that happen before the inhibitory step or are unrelated to the acrosomal reaction itself. The reacidification of the internal pH, the uncoupling and decrease of the respiration, and the decrease of the ATP levels might be linked together by the large influx of calcium that occurs after the acrosome reaction.