Induction of acrosomal reaction and calcium uptake in ram spermatozoa by ionophores

Deglycosylation effect of the mammalian sperm maturation antigen (SMA2) on serological reaction and acrosome reaction

Spermatozoal membrane proteins are considered to possess several immunological unique characteristics as the cell is formed behind the blood-testes barriers. Major goat sperm maturation antigen (SMA2) contains one hexosamine along with mannose, galactose and glucose. In the present study, effects of deglycosylation of SMA2 antigen on immuno-reactivity and the serological activity was investigated. SMA2 glycoantigen showed positive immunoreactivity after treatment with sodium borohydride (NaBH(4)) and moreover this generated a 44 kDa protein band which was negative for periodic acid Schiff reagent. Trifluoromethanesulfonic acid (TFMS) caused aggregation and restricted the free mobility of the treated antigen on SDS-PAGE and the protein band generated by TFMS treatment also showed positive immuno-reactivity. The results supported the views that the protein portion retains its immuno-reactivity even after oxidation of the vicinal hydroxyl group of saccharide component of SMA2 antigen. These data suggest that immunodominent epitopes exist on the core protein by which the SMA2 antigen retains its immuno-reactivity even after disruption of the saccharide portion. Additional experiments demonstrate that protein epitopes have a role in capacitation and the acrosome reaction (AR) in presence of antibody which is raised against this protein part of SMA2 using the negative staining of FITC-PSA (fluorescein isothiocyanate-labeled Pisum sativum agglutinin) probe. Altogether these findings indicate that the protein portion of SMA2 might fulfill the serological activity of the antigen as well as the protein epitope affects the acrosome reaction. In view of this property, we propose that the protein portion of SMA2 antigen might be considered as a potential antigenic target for an immune response.

Extracellular adenosine triphosphate stimulates acrosomal exocytosis in bovine spermatozoa via P2 purinoceptor

The presence of ATP in the genital tract fluid of mammals provokes questions regarding its function in the fertilization process. We investigated the effect of extracellular ATP (ATPe) on the activation of bovine spermatozoa. A signal transduction mechanism for ATP involving the receptor-mediated release of second messengers is described. Treatment of spermatozoa with ATP, uridine triphosphate (UTP), or 2-methylthio-ATP resulted in a concentration-dependent increase of acrosomal exocytosis, whereas treatment with either AMP or adenosine induced little exocytosis. This suggested that the receptor involved is of the P2 and not the P1 type. Several lines of evidence also suggest that the ATP purinoceptor is of the P2y and not the P2x type. First, the acrosome reaction was induced by the P2y-agonists ATP, UTP, or 2-methylthio-ATP, but no effects were shown by the P2x-agonists alpha,beta-methylene-ATP or beta,gamma-methylene-ATP. Second, ATP-induced acrosomal exocytosis was inhibited by the P2y antagonists, but not by the P2x antagonists. Third, enhanced Ca2+ uptake into the cells was observed with ATP and 2-methylthio-ATP, but not with beta,gamma-methylene-ATP. Additionally, ATP induced elevation of intracellular Ca2+ and cAMP, and the effect on cAMP was predominantly enhanced by including Ca2+ and the Ca2+-ionophore A23187 in the incubation medium. Extracellular ATP also activates protein kinase Calpha (PKCalpha), and the acrosome reaction, stimulated by ATPe, is inhibited by a PKC-specific inhibitor. In summary, we suggest that ATPe activates the P2 purinoceptor that elevates [Ca2+]i, which leads to PKCalpha activation and culminates in acrosomal exocytosis.

Intracellular Ca(2+)-Mg(2+)-ATPase regulates calcium influx and acrosomal exocytosis in bull and ram spermatozoa

Calcium influx is required for the mammalian sperm acrosome reaction (AR), an exocytotic event occurring in the sperm head prior to fertilization. We show here that thapsigargin, a highly specific inhibitor of the microsomal Ca(2+)-Mg(2+)-ATPase (Ca(2+) pump), can initiate acrosomal exocytosis in capacitated bovine and ram spermatozoa. Initiation of acrosomal exocytosis by thapsigargin requires an influx of Ca(2+), since incubation of cells in the absence of added Ca(2+) or in the presence of the calcium channel blocker, La(3+), completely inhibited thapsigargin-induced acrosomal exocytosis. ATP-Dependent calcium accumulation into nonmitochondrial stores was detected in permeabilized sperm in the presence of ATP and mitochondrial uncoupler. This activity was inhibited by thapsigargin. Thapsigargin elevated the intracellular Ca(2+) concentration ([Ca(2+)](i)), and this increase was inhibited when extracellular Ca(2+) was chelated by EGTA, indicating that this rise in Ca(2+) is derived from the external medium. This rise of [Ca(2+)](i) took place first in the head and later in the midpiece of the spermatozoon. However, immunostaining using a polyclonal antibody directed against the purified inositol 1,4,5-tris-phosphate receptor (IP(3)-R) identified specific staining in the acrosome region, in the postacrosome, and along the tail, but not in the midpiece region. No staining in the acrosome region was observed in sperm without acrosome, indicating that the acrosome cap was stained in intact sperm. The presence of IP(3)-R in the anterior acrosomal region as well as the induction, by thapsigargin, of intracellular Ca(2+) elevation in the acrosomal region and acrosomal exocytosis, implicates the acrosome as a potential cellular Ca(2+) store. We suggest here that the cytosolic Ca(2+) is actively transported into the acrosome by an ATP-dependent, thapsigargin-sensitive Ca(2+) pump and that the accumulated Ca(2+) is released from the acrosome via an IP(3)-gated calcium channel. The ability of thapsigargin to increase [Ca(2+)](i) could be due to depletion of Ca(2+) in the acrosome, resulting in the opening of a capacitative calcium entry channel in the plasma membrane. The effect of thapsigargin on elevated [Ca(2+)](i) in capacitated cells was 2-fold higher than that in noncapacitated sperm, suggesting that the intracellular Ca pump is active during capacitation and that this pump may have a role in regulating [Ca(2+)](i) during capacitation and the AR.

A novel method for evaluating the acrosomal status of mammalian spermatozoa

A novel method was developed to evaluate the acrosomal status of mammalian spermatozoa. The method is based on the ability of the lectin Pisum sativum agglutinin (PSA) to bind specifically to glycoproteins of the acrosomal matrix released during the acrosome reaction. The amount of released acrosomal content is proportional to the fraction of spermatozoa that underwent acrosome reaction. The released glycoproteins present in the supernatant separated from the cells were detected via an ELISA-like assay. The authors suggest that one of these glycoproteins might be the acrosin as identified by anti-acrosin antibodies, using Western blot analysis. The new method (demonstrated here with ram and bull spermatozoa) correlates well with the results obtained by conventional methods. Its advantages are simplicity, objectiveness, rapidity, and low cost. In addition, many samples can be processed in parallel. The method can be used in experimental as well as clinical applications.

Assessment of the acrosomal status of membrane-intact ram spermatozoa after freezing and thawing, by simultaneous lectin/Hoechst 33258 staining

We have evaluated the effect of freezing and thawing on the acrosomal status of ram spermatozoa, especially those that withstood cryopreservation as assessed by membrane integrity. To this end, we performed simultaneous lectin/Hoechst 33258 staining, and compared the ability of three fluoresceinated lectins. Ram spermatozoa were treated with fluorescein isothiocyanate-labelled Pisum sativum lectin (PSA), fluorescein isothiocyanate-labelled Arachis hypogea lectin (PNA) and fluorescein isothiocyanate-labelled Triticum vulgaris lectin (WGA) and simultaneously with Hoechst 33258 for determination of membrane integrity and acrosomal status. In all cases, three forms were readily distinguished by their distribution pattern. For both PSA and PNA, the most abundant form found in fresh semen consisted of fluorescence on the acrosomal area. This form corresponds to acrosome-intact spermatozoa, as assessed by Differential Interference Contrast (DIC) microscopy. Two minor forms showed weak fluorescence on the equatorial segment or no fluorescence on the head. DIC microscopy revealed that both forms were associated with acrosome-lost spermatozoa. WGA labelling showed two forms, one of which consisted of fluorescence on the entire head, albeit more intensely on its anterior segment. Spermatozoa in this form were acrosome-intact by DIC. The other form lacked fluorescence on the acrosomal region, but still showed faint fluorescence in the posterior region. This form was acrosome-lost by DIC. Incubation of fresh spermatozoa with calcium ionophore A23187 for up to 1 h significantly increased the percentage of those forms identified as acrosome-reacted as described above. This was confirmed by the time-dependent accumulation of these forms, as well as by DIC microscopy. At all times, differences among values obtained using these three lectins were not significant. Freezing and thawing led to a decrease of both membrane integrity and acrosomal integrity, irrespective of the lectin used. However, almost all spermatozoa that withstood cryopreservation, as evaluated by Hoechst exclusion, showed intact acrosomes. In this case, no differences between fresh and frozen/thawed samples were observed. These results suggest that the structural integrity of ram spermatozoa is mostly unaffected after cryopreservation, suggesting that it is damage to the plasma membrane that is primarily responsible for the low fertility of cryopreserved samples.

Calcium mobilization and influx during sperm exocytosis

We have previously shown that two intracellular events which occur during capacitation of bovine sperm are the formation of actin filaments on the plasma and outer acrosomal membranes and the attachment of a PIP2-specific phospholipase C (PLC) to this membrane bound F-actin. This PLC plays an essential role in sperm exocytosis (acrosome reaction). In the present report, we further elucidated the role of this PLC using a PIP2-specific PLC of bacterial origin. This PLC is different from the endogenous sperm PLC in that it is calcium independent and not inhibited by neomycin. Here we report using bovine sperm that this bacterial PLC can restore actin release from extracted membranes as well as membrane fusion in a cell-free assay when the endogenous PLC is inhibited by neomycin. The sperm PLC requires 2 microM calcium for half maximal activation, while half maximal actin release from extracted plasma membranes occurs at 80 microM. Extracted sperm membranes were examined for calcium pumps and channels. Sperm plasma membranes were found to possess a thapsigargin insensitive calcium pump and calcium channels which are opened by phosphorylation by protein kinase C. The acrosomal membrane possesses a calcium pump which is inhibited by thapsigargin and calcium channels which are opened by cAMP. These observations are discussed in terms of a model of acrosomal exocytosis which involves a calcium rise that occurs in two stages resulting from calcium mobilization from internal stores followed by influx of extracellular calcium.

Calcium efflux mechanism in sperm mitochondria

This paper reports an investigation on calcium efflux mechanism in ram sperm mitochondria. Energized sperm mitochondria take up Ca2+ via the ruthenium-red sensitive uniporter, and possess a ruthenium-red insensitive efflux mechanism. Extramitochondrial Na+ did not affect the rate of Ca2+ efflux indicating that Na+/Ca2+ exchange mechanism is not involved. Depolarization of inner mitochondrial membrane induced by the uncoupler carbonylcyanide-p-trifluoromethoxyphenyl hydrazone (FCCP) or by the organomercurial SH-reagent mersalyl, causes high stimulation in Ca2+ efflux. This stimulated Ca2+ efflux determined in the presence of ruthenium-red and phosphate, is not inhibited by cyclosporin A (CSA), indicating that mitochondrial permeability transition pore (MTP) is not involved in this Ca2+ efflux mechanism. The stimulated Ca2+ efflux is inhibited by ADP or atractyloside suggesting that the Ca2+ transport mechanism might be intrinsic to the ADP/ATP carrier (AAC). Thus, the data indicate that sperm mitochondria contain a Ca2+ efflux mechanism operated via AAC and regulated by mitochondrial membrane potential and by ADP concentration.

Sperm exocytosis reconstructed in a cell-free system: evidence for the involvement of phospholipase C and actin filaments in membrane fusion

We used a cell-free system to study membrane fusion during sperm exocytosis (acrosome reaction). Extracted bovine sperm plasma and outer acrosomal membranes were labeled with chlorophyll a or DCY, respectively. The occurrence of membrane fusion is indicated by the ability of the probes to diffuse from one membrane species to another which is revealed by resonance energy transfer between the two probes. We have previously shown using this system that the requirement of capacitation for sperm exocytosis is retained in cell-free membrane fusion, and that the pH and calcium dependence of the cell-free fusion mimics those of exocytosis in intact cells. In the present report we further characterize the fusion of sperm membranes which we observe in our assay. Phosphoproteins and phospholipases were found to be involved in the membrane fusion step of sperm exocytosis. Protein kinases, phosphatases, and Gi-like proteins, while involved in exocytosis in intact cells, are not involved specifically in the membrane fusion step of exocytosis. The role of membrane bound F-actin in regulating membrane fusion was also studied using fluorescently labeled phalloidin. The results show that cortical F-actin has two roles in regulating sperm exocytosis. One is to form a scaffolding to hold phospholipase C at the membrane. It also functions as a physical barrier to membrane fusion which is removed by the increases in intracellular calcium and pH which precede fusion.

Calcium ionophore increases amyloid beta peptide production by cultured cells

Amyloid beta peptide (A beta) is released into the media of a variety of cells in culture during normal metabolism. The discovery of several missense mutations within or flanking the A beta region of the beta amyloid precursor protein (beta APP) in familial Alzheimer's disease provides strong evidence for a role of altered processing of beta APP in the pathogenesis of this disorder. The cellular mechanisms that regulate the relative utilization of the secretory pathway, which causes beta APP to be cleaved within the A beta domain, and the alternative proteolytic pathway, which produces intact A beta, are unknown. It is hypothesized that a number of neurodegenerative diseases, including Alzheimer's disease, are characterized by abnormal calcium metabolism. We investigated the effect of disordered calcium homeostasis on A beta production in human kidney 293 cells transfected with beta APP cDNA. A beta immunoprecipitated from the conditioned media of cells was compared to immunoprecipitated full-length and secreted forms of beta APP in both metabolic labeling and pulse-chase labeling paradigms. The calcium ionophore A23187 consistently increased the production of A beta approximately 3-fold. This effect was dependent on the presence of extracellular calcium in intact cells. Caffeine also increased A beta production, possibly through release of calcium from intracellular stores. The increase in A beta was cAMP-independent, and it was not mediated by a protein kinase C-dependent pathway, as treatment with phorbol esters decreased A beta levels. The effects of the ionophore on beta APP maturation and phosphorylation were also established. We conclude that elevation of intracellular calcium levels has an important effect on beta APP maturation and proteolytic processing and substantially enhances the production and release of the amyloidogenic A beta peptide.

Epidermal growth factor induces acrosomal exocytosis in bovine sperm

At the time of fertilization mammalian spermatozoa undergo a Ca(2+)-dependent exocytotic event, which is known as the acrosome reaction (AR). We describe here that EGF-receptor (EGFR) is localized in the head of bull spermatozoa and that epidermal growth factor (EGF) can induce the occurrence of the AR in its typical dose-dependent manner. Previously we showed that protein kinase C (PKC) is involved in the cascade leading to AR in bull spermatozoa. Here, we show that PKC is involved in the mechanism in which EGF exerts its effect on AR. These findings together with our results which show inhibition of AR by tyrosine-phosphorylation inhibitors, indicate that ejaculated bull sperm contain a typical 170-kDa EGFR which is active in the mechanism leading to AR.

The role of anion channels in the mechanism of acrosome reaction in bull spermatozoa

The involvement of anion channels in the mechanism of the acrosome reaction (AR) was investigated. The AR was induced by Ca2+ or by addition of the Ca2+ ionophore A23187. The occurrence of AR was determined by following the release of acrosin from the cells. In order to investigate the role of anion channels in the AR, several anion-channel inhibitors were tested, mainly DIDS (4,4'-diisothiocyanostilbene-2,2'-disulfonic acid). Other blockers, like SITS (4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid), furosemide, probenecid and pyridoxal 5-phosphate, were also tested. We found that DIDS binds covalently to sperm plasma membrane in a time- and concentration-dependent manner. Maximal binding occurs after 2 h with 0.3 mM DIDS. DIDS and SITS inhibit AR in a concentration-dependent manner. The IC50 of DIDS and SITS in the presence of A23187 is 0.15 and 0.22 mM, respectively. Tributyltin chloride (TBTC), an Cl-/OH- exchanger, partially overcomes DIDS inhibition of the AR. HCO3- is required for a maximal acrosin release and Ca(2+)-uptake, in the presence or absence of A23187. It is known that HCO3- activates adenylate cyclase and therefore, increases the intracellular level of cAMP. The inhibition of the AR by DIDS decreases from 95 to 50% when (dibutyryl cyclic AMP (dbcAMP) was added, i.e., HCO3- is no longer required while elevating the level of cAMP in an alternative way. Moreover, we show that the stimulatory effect of HCO3- on Ca(2+)-uptake is completely inhibited by DIDS. We conclude that DIDS inhibits AR by blocking anion channels, including those that transport HCO3- into the cell.

Loss of acid phosphatase from rat spermatozoa as a method for assessing the acrosome reaction

A method is presented for evaluating the extent of the acrosome reaction by measuring the release of acrosomal acid phosphatase from rat spermatozoa during incubation under capacitating conditions. Treatment of spermatozoa with lysophosphatidylcholine or Triton X-100 released the acid phosphatase from the sperm cell. Using this enzymatic method we could not detect an alteration in enzyme activity following 5 h incubation under capacitating conditions. The effect of in vitro capacitation for 5 h in the absence or presence of heparin or ionophore A23187 was studied. Incubation in the presence of heparin (10 micrograms ml-1) caused a 32% increase in enzyme activity. After exposure of the spermatozoa to ionophore A23187 (0.5 microM) 16% increase of enzyme activity could be detected.

Role of protein kinase C in the acrosome reaction of mammalian spermatozoa

Mammalian spermatozoa undergo a Ca(2+)-dependent exocytotic event before fertilization which is known as the acrosome reaction. The process of exocytosis in several cell systems is mediated by a protein kinase C (PKC)-catalysed phosphorylation. Addition of phorbol 12-myristate-13-acetate or the membrane-permeant diacylglycerol analogue 1-oleoyl-2-acetylglycerol, which are potent activators of PKC, to bovine spermatozoa resulted in stimulation of the acrosome reaction. This stimulation was inhibited by low concentrations (50% inhibition at 0.7 nM) of the PKC inhibitor staurosporine. PKC specific activity in bovine spermatozoa is extremely low in comparison with other cells; however, it is comparable with the activity found in human spermatozoa. Immunohistochemical analysis using anti-PKC antibodies revealed staining in the equatorial segment, the post-acrosomal region and the upper region of the head. We propose that PKC is involved in the mammalian sperm acrosome reaction.

Role of spermine in mammalian sperm capacitation and acrosome reaction

The binding properties of seminal polyamines to ram spermatozoa and their possible role in sperm capacitation and the acrosome reaction were studied. Binding and release of [14C]spermine from ram spermatozoa occurred at a rate faster than in somatic cells and were not energy-dependent. Release of bound spermine was further facilitated by heparin, a constituent of the female reproductive tract which was reported to induce capacitation and the acrosome reaction. High- and low-affinity polyamine-binding sites were identified, of which the high-affinity site was specific to polyamines with three or more amino groups. We also found that spermine inhibited the acrosome reaction and propose that it is the major seminal decapacitating factor. Since precise timing of capacitation and the acrosome reaction are critical for successful fertilization, it is suggested that the role of seminal spermine is to prevent premature capacitation and the acrosome reaction.

Electroporation of bovine spermatozoa to carry foreign DNA in oocytes

In the present study, electroporation was used to test the ability of spermatozoa to carry foreign DNA into the bovine oocytes. Frozen-thawed bovine spermatozoa (10(7)/ml) were electroporated using six different combinations of voltage (500, 1,000, or 1,500 V) and capacitance (1 or 25 microFarads) in the presence of 1 mg/ml of plasmid pRGH527. The portions of plasmids retained by sperm cells after three washings (stable for ten washings) were 4.3, 5.5, 5.1, 6.0, 6.8, and 5.8% for 1 microFarad, 500, 1,000, and 1,500 V and 25 microFarads, 500, 1,000, and 1,500 V, respectively. Nonelectroporated cells have retained only 1% of plasmids. In the same experiment, electroporated spermatozoa were acrosome reacted by treatment with ionophore A23187 to evaluate the fraction of marked plasmids joined at the acrosomal membrane. The results show that 3.5, 5.0, 4.4, 5.0, 6.3, and 4.4% remain tied to the ionophore-treated sperm. Only 0.7% of plasmid was retained after removal of the acrosome of nonelectroporated cells. Acrosome reaction was not significantly induced by the electrical field (EF) (P less than 0.005). EF decrease motility significantly for greater than 100 V in 0.3 M mannitol (M) and mannitol-TALP (MT) (1/1) media and greater than or equal to 500 V (P less than 0.05) in TALP medium. The retained plasmid rate was compared between TALP medium M and MT media and resulted in a percentage of 1.0, 2.5, 6.5 at 1 microFarads, 100 V, and 0.9, 3.8, and 3.8 at 25 microFarads, 100 V in TALP, MT, and M medium, respectively. Sperm cells electroporated at 1 microFarad, 500 or 1,000 V, 25 microFarad, 500 V or 1,000 in TALP medium hold plasmids in proportion of 5.2, 5.4, 7.4, and 6.0%. Electroporation above 100 V in M and MT killed the cells. In a part of this experiment, spermatozoa electroporated in the presence of radiolabeled plasmids have been treated with DNase I and results revealed that 35, 28, 54, 58, and 3% of marked DNA remains in sperm cells following digestion after electroporation in TALP (1,000 V, 1 and 25 microFarads), M medium (100 V, 1 and 25 microFarads), and control, respectively. Using in vitro matured bovine oocytes, the electroporation conditions were correlated with the fertilization rate (85% for control and 55% for electroporated spermatozoa). Autoradiography of embryos following fertilization indicated the presence of plasmids in the cytoplasm and in the zona pellucida.(ABSTRACT TRUNCATED AT 400 WORDS)

Stimulation of Ca2+ uptake into epididymal bull spermatozoa by analogues of amiloride

Certain amiloride analogues 3',4'-dichlorobenzamil 2',4'-dimethylbenzamil and alpha',2'-benzobenzamil hydrochloride (ATBB) stimulate calcium accumulation and motility by epididymal bovine spermatozoa. This stimulation can be seen at a range of 0.1-0.4 mM, while at higher concentration there is inhibition of calcium uptake by these amiloride analogues. The amiloride derivative 5-(4-chlorobenzyl)-2',4'-dimethylbenzamil (CBDMB), which bears a 4-chlorobenzyl substituent on the 5-amino nitrogen atom, did not stimulate calcium uptake. The amiloride analogue 3',4'-dichlorobenzamil inhibits the Na+/Ca2(+)-exchange activity in isolated plasma membrane vesicles, and the stimulatory effect of 3',4'-dichlorobenzamil on calcium uptake into epididymal sperm could be seen in Na(+)-free medium. Thus, the stimulation of Ca2+ accumulation in the cells caused by 3',4'-dichlorobenzamil is not a result of inhibiting the Na(+)-dependent Ca2+ clearance. There is no stimulation of Ca2+ uptake into ejaculated cells by adding 3',4'-dichlorobenzamil, which is not due to the presence of the calcium-transport inhibitor (caltrin) in these cells [Rufo, G.A., Schoff, P.K. & Lardy, H.A. (1984) J. Biol. Chem. 259, 2547-2552]. The stimulatory effect of 3',4'-dichlorobenzamil on Ca2+ uptake is inhibited by the voltage-dependent Ca2(+)-channel blockers nifedipin and diltiazem. This indicates that the stimulation of Ca2+ uptake by the amiloride analogues is due to the activation of a voltage-dependent Ca2+ channel of the plasma membrane.

Carbohydrates and fertilization in animals

A frequently used mechanism for sperm-egg recognition in many species involves complementary protein-carbohydrate interaction. The usual paradigm includes complex glycoconjugates in reproductive tract fluids or on the eggs which are recognized by carbohydrate-binding proteins on the sperm surface. Various glycoconjugates are utilized in the steps of sperm capacitation, sperm binding to the egg extracellular matrix and vitelline membrane and induction of the acrosome reaction. Several types of complex glycoconjugates are involved in these processes, including proteoglycans, lactosaminoglycans, sulfated fucose-containing glycoconjugates, and glycoproteins. There appear to be some structural similarities between active glycoconjugates; they are large in molecular weight and complex, and they are often sulfated, fucosylated, and attached to a protein through serine or threonine residues. In some species, the protein core of the glycoconjugates also participates in the interaction by limiting the binding of carbohydrates to sperm only of the relevant species, likely by providing the proper steric arrangement for the interaction. In other cases the protein core seems to serve more as a crosslinker of the carbohydrate moieties. This review discusses the types of glycoconjugates implicated in fertilization and the complementary lectin-like proteins found on sperm.

Role of lipoxygenase in the mechanism of acrosome reaction in mammalian spermatozoa

The acrosome reaction (AR) in bull spermatozoa was induced by the Ca2(+)-ionophore A23187, by dilauroylphosphatidylcholine or by arachidonic acid in the presence of Ca2+ in the incubation medium. The occurrence of AR was determined by following the release of acrosin from the cells. Nordihydroguaiaretic acid (NDGA), an inhibitor of both lipoxygenase and prostaglandin-synthetase, caused 35%, 43% and 69% inhibition of AR at concentrations of 1, 10 or 100 microM, respectively. Eicosatetraynoic acid (ETYA), an analogue of arachidonic acid, caused 17%, 61% and 77% inhibition of AR at concentrations of 20, 40 or 80 micrograms/ml, respectively. When AR was induced by arachidonic acid, ETYA, causes 36% and 58% inhibition at concentrations of 2 or 20 micrograms/ml, respectively. Under identical conditions, 100 microM indomethacin, a specific inhibitor of prostaglandin-synthetase, showed no inhibition but rather 35% stimulation at acrosin release rate. The fact that AR is inhibited by NDGA and not by indomethacin indicates that the lipoxygenase, rather than prostaglandin-synthetase, is involved in the mechanism of AR. Since the inhibition by NDGA is seen in the presence of the Ca-ionophore, we suggest that lipoxygenase activity is not involved in enhancing calcium transport into the cell, but rather at other steps in AR mechanism. A thin-layer chromatography revealed the presence of 15-HETE, the classical product of 15-lipoxygenase activity, which was identified by HPLC. Under AR conditions, there is an elevation of lipoxygenase products and the addition of NDGA caused a reduction in their levels. The inhibition of acrosin release by NDGA can be eliminated by adding 15-HETE or 15-HPETE to the incubation medium. In conclusion, we suggest here for the first time, a physiological role for 15-lipoxygenase in the mechanism of AR in mammalian spermatozoa.

Regulation of mammalian sperm motility

The physiological regulation of sperm motility has become more amendable to investigation since the demonstration that cAMP and calcium play a role in modulating the functioning of the flagellar axoneme. Although the external triggering mechanisms that initiate motility and capacitation are still unknown, evidence supports a modification of the calcium balance by gated Ca2+ channels, accompanied by shifts in the internal pH. Ca2+ and pH may in turn act indirectly through cAMP and cAMP-dependent kinase (kinase(a] to control the phosphorylation state of functional proteins in the flagellar axoneme. The role of calcium is of central importance, but it is clear that several separate Ca2+-dependent mechanisms are involved. Ca2+ controls the curvature of the sperm flagellum and, so, can change the motility of the sperm from progressive swimming to tumbling. Under the appropriate conditions, calcium appears to have the capacity to deactivate motility by activating phosphodiesterase and phosphatase. The deactivating effect of Ca2+ may be offset under some circumstances by coactivation of adenyl cyclase, so phosphorylation of the axoneme and the motility are maintained. The specific factors determining the predominant calcium effect are not yet known, but internal pH of the sperm may play a major role.