Department of Biological Sciences, University of Warwick, Coventry CV4 7AL, UK
1 Present address: Bourn Hall, Bourn, Cambridge CB3 7TR, UK
2 To whom correspondence should be addressed. e-mail: geraldine.hartshorne{at}warwick.ac.uk
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Abstract |
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Key words: mouse follicles/nitric oxide/nitric oxide synthase/oocytes/ovulation
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Introduction |
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NO is implicated in various aspects of reproductive function, including uterine contractility, penile erection, ovulation and fertilization (Burnett et al., 1996; Kuo et al., 2000
). NO might affect follicle growth and development via a vasodilatory action influencing blood flow to the follicle (Jansen, 1975
; Bonello et al., 1996
), or via a direct effect on local cellular interactions, like other reactive oxygen species. For example, NO might be involved in follicle wall breakdown at ovulation (Jablonka-Shariff and Olson, 1997
) or have effects on apoptotic processes required for tissue remodelling (Chun et al., 1995
).
The role of free radicals such as NO in the reproductive system is complex, as they may influence cellular processes via a variety of mechanisms and with both positive and negative effects. Free radicals must be closely controlled to avoid unplanned tissue destruction; however, little is known about how this may occur in an integrated system such as the follicle. Transferrin is one factor found in follicles that may suppress the generation of reactive oxygen species. It is often added to culture media as a supplement, usually in combination with insulin and selenium. Although beneficial effects of the combination have been observed in human and bovine follicle cultures (Katska and Rynska, 1998; Wright et al., 1999
), in mouse follicle cultures, insulin was detrimental (Eppig et al., 1998
). The role of transferrin alone in follicle culture has not been examined previously, so we took this opportunity to perform a pilot study examining the possible interaction of transferrin with the NO free radical pathway in follicle culture.
There is uncertainty as to which NOS isoform(s) may be involved in NO production during follicle development in mammalian ovaries in vivo. In cultured follicles, a functional vascular system is lacking, although some carry-over of endothelial cells resident in the nascent thecal layers may occur. This model offers an opportunity to study any local effects of the NO pathway on follicle growth. If NO is involved, then significant changes in at least one of the NOS isoforms might occur during follicle growth and ovulation, which seems to be the case with iNOS and eNOS, though not with nNOS (Srivastava et al., 1997).
We set out to study the location and role of NO in follicle development using an in vitro model of mouse follicle growth and ovulation (Cortvrindt et al., 1996; Mitchell et al., 2002
). In this system, manually dissected individual follicles remain associated with thecal cells, but lack a blood supply and innervation as would be present in vivo. However, the flattened follicular architecture enables clear observations of follicle development, and results in viable oocytes (Cortvrindt et al., 1996
). We assessed the presence of iNOS and eNOS in unstimulated immature mouse ovaries, and in cultured follicles from the same source, using immunocytochemistry. The NO pathway was then manipulated in cultured mouse follicles to determine if the precursor (L-arg), intermediate (NG-OH-L-arg) and end product (L-cit) of NOS action affected mouse follicle development, survival and ovulation in vitro. A preliminary assessment of the possible interaction of apo-transferrin with the NO system is also presented.
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Materials and methods |
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The methods of follicle culture were as described and extensively characterized previously (Mitchell et al., 2002). Briefly, small preantral follicles were dissected manually from quartered ovaries using 29 gauge needles. Follicles selected for culture were spherical, had an intact basement membrane and a centrally located oocyte. The average diameter of follicles at the start of culture was
135 µm. Each mouse yielded
3060 follicles. Individual follicles were cultured in microdrops under oil in minimum essential medium (MEM)
(Gibco, UK) containing 5% fetal calf serum (FCS; Gibco), penicillin (50 µg/ml), streptomycin (50 µg/ml) and FSH (100 mIU/ml; hpMetrodin; Serono, UK). The medium was refreshed every second day. Follicles were considered to have survived when the oocyte was retained, they appeared round in shape, and the follicle cells were not overtly degenerate. These criteria had been previously validated using fluorescent vital dyes (Mitchell et al., 2002
). hCG (1.5 IU/ml; Profasi; Serono) was added on day 10 and ovulation assessed 16, 24, 40 and 48 h later. The response to hCG, the promptness of ovulation, and the appearance and expansion of cumulus of the ovulated oocyte were used as indicators of follicular health, as previously described (Mitchell et al., 2002
). Normal ovulation was considered to be ovulation within 16 h after hCG of a spherical translucent oocyte surrounded by mucified cumulus cells. The total ovulation rate was calculated as the number of follicles which ovulated as a proportion of the total number originally cultured. The timely ovulation rate was calculated as the number of follicles ovulating within 16 h of the hCG dose, as a proportion of the total number originally cultured.
To facilitate histological processing, some follicles were cultured individually in 8-well chamber slides (Nunc; Gibco) in 0.5 ml medium without oil. The morphological appearance of follicles in these vessels was similar to that in standard culture.
For immunocytochemistry, isolated ovaries were fixed at room temperature in neutral buffered formalin for 2472 h, dehydrated in ethanol and cleared in xylene before embedding in paraffin wax. Sections of 6 µm were mounted on Vectabond-coated slides (Vector Laboratories, UK).
Cultured follicles in chamber slides were fixed in 150 µl of neutral buffered formalin for 16 h at room temperature. The fixative was removed and 10% molten agar at 65°C was added and left to harden for 30 min at room temperature. The chambers were then removed and the agar block containing the follicle was gently dislodged from the slide before trimming and processing for histology as above. Due to difficulties with cutting and processing sections from these follicles which became flattened in culture, only iNOS immunocytochemistry was attempted.
NOS immunocytochemistry
iNOS detection was performed using one of two primary antibodies: (i) polyclonal rabbit anti-mouse iNOS (PA3-030; Affinity Bioreagents, UK) and (ii) polyclonal rabbit anti-rat iNOS (N52920; Transduction Laboratories, USA). Dilutions of 1/400 were found to be optimal. eNOS was detected using polyclonal rabbit anti-bovine eNOS (Affinity Bioreagents) using an optimal concentration of 1/300. This antibody was reported by the manufacturer to cross-react with mouse eNOS.
Deparaffinized, rehydrated slides were incubated in 1.5% normal blocking serum in phosphate-buffered saline (PBS) for 20 min. Blotted slides were incubated in primary antibody for 1 h at room temperature. Detection was performed using the anti-rabbit IgG Vectastain Elite ABC kit, according to the manufacturers instructions, producing a redbrown colour in regions of primary antibody binding. Harris haematoxylin provided a blue counterstain.
For each experiment, the negative control comprised sections incubated without primary antibody. In some experiments, iNOS knockout mouse sections of lung and uterus provided a second negative control. Samples from iNOS knockout mice were kindly provided by Dr Andrew Thomson of Glasgow Royal Infirmary, Scotland.
In situ hybridization for iNOS mRNA
In situ hybridization was carried out using a variation of the methods described by Heidaran et al. (1988) and Shih and Kleene (1992
). A digoxigenin (DIG)nucleic acid detection system was used according to the manufacturers instructions.
In situ hybridization was performed using an anti-sense iNOS RNA probe. The corresponding sense probe was used as a negative control. The RNA probes were produced from a partial cDNA clone for rat iNOS (95% homologous to mouse iNOS), generously provided by Dr Bruce Kone, University of Texas, Houston, USA. The probes produced were 250300 base pairs in length and free from contamination (data not shown).
Whole mouse ovaries were fixed immediately after dissection by overnight incubation in 4% paraformaldehyde, pH7, at 4°C. Ovaries were then equilibrated in fresh 0.5 mol/l sucrose in PBS at room temperature for 1 h before placing in 0.85% (w/v) NaCl for 15 min at 4°C. Ovaries were then dehydrated in an ethanol series, cleared in xylene and embedded in paraffin wax for sectioning at 6 µm thickness.
DIG-UTP-labelled probes
The partial cDNA clone of rat iNOS was cloned 5' to 3' into the EcoRI and BamHI sites of pBluescript KS+. The sample was centrifuged for 15 min at 13 000 g, the supernatant was removed and the pellet was washed with 100 µl 75% ethanol. The ethanol was removed and discarded, and remaining ethanol was allowed to evaporate from the DNA pellet at room temperature for 20 min. DNA was resuspended in 10 µl of dH2O and the concentration determined spectrophotometrically.
Restriction enzyme digests were performed according to the manufacturers instructions. Plasmid DNA (4 µg) was digested with 1 µl BamHI (Gibco, 10 IU/µl) or 1 µl EcoRI (Gibco, 10 IU/µl) with 1 µl reaction buffer (Gibco) and 4 µl distilled water. This was mixed thoroughly and left for 60 min at 37°C. Linearized DNA was cleaned using QIA Quick Gel Extraction kit (Qiagen) and run in 1xTBE (Tris, borate, EDTA buffer) on a 1% agarose gel with standards to determine the amount of DNA present.
Plasmids were then transcribed in a 20 µl reaction which contained 1 µg DNA, 50 IU of T3 or T7 (Boehringer, UK), 1xtranscription buffer (Boehringer), 10 IU RNase inhibitor (Gibco) and 0.5 mmol/l of DIG-UTP labelled nucleotides (Boehringer). The solution was thoroughly mixed and incubated for 2 h at 37°C. The reaction was stopped at 65°C for 1015 min and then DNase (RNase-free; Boehringer) was added at 1 IU/µg of DNA and incubated at 37°C for 15 min. This reaction was stopped by the addition of 2 µl of 0.2 mol/l EDTA (BDH, UK) pH 8.0. RNA was precipitated at 70°C for
30 min with 0.5 volumes of 10 mol/l ammonium acetate (BDH) and 2.5 volumes of 100% ethanol. RNA was recovered by centrifugation at 3000 g for 20 min at 4°C, washed in 100 µl 70% ethanol and centrifuged again for 5 min. The ethanol was then removed carefully and the pellet resuspended in 20 µl dH2O and placed on ice. One microlitre of RNA preparation was run on a 1% agarose gel (Gibco), together with known amounts of
DNA and photographed under UV light. The RNA probe was stored at 70°C until needed.
Hybridization and detection of probe
Sections were deparaffinized in xylene and hydrated in an ethanol series, placed into PBS and finally into 2xsaline sodium citrate (SSC). Sections were digested with 10 µg/ml proteinase K (Sigma) in distilled water with 0.1 mol/l TrisHCl (BDH, pH 8) and 50 mmol/l EDTA for 5, 10 and 30 min at 37°C (Inderdeo et al., 1996; Berruti et al., 1998
). Sections were treated with 0.25% (v/v) acetic anhydride (Sigma) in 0.1 mol/l triethanolamide (BDH) for 10 min and dehydrated in an ethanol series. Sections were then prehybridized in hybridization buffer for 60 min at 40°C. Hybridization buffer comprised 5 ml 50% formamide, 100 µl 1 mol/l TrisHCl pH 8, 20 µl 0.5 mol/l EDTA, 200 µl 50xDenharts solution, 500 µl 10 µg/ml tRNA (Sigma), 4.18 ml dH2O. Dextran sulphate (0.5 g; Sigma) was added to 5 ml hybridization buffer, then 5 ng/µl probe was added to a sufficient volume of hybridization buffer resulting in 20 µl probe solution/slide. This mixture was heated to 80°C, chilled on ice and applied to the sections. A parafilm covering was applied and the slides were heated to 80°C for 10 min and then incubated for 16 h at 40°C in a humidity box. Sections were washed in 2xSSC at room temperature for 30 min and then treated with 100 µg/ml RNase A (Boehringer) in 2xSSC for 60 min at 37°C. The sections were finally washed in 0.5xSSC for 30 min at 50°C. Sections were rinsed in Tris-buffered saline (TBS) and blocking solution applied [8 ml TBS, 0.3 g bovine serum albumin, 10 µl Triton X-100 (BDH), and 2 ml normal sheep serum (Sigma)] for 30 min at room temperature. Sections were rinsed in TBS, then 1/500 dilution of anti-DIG alkaline phosphatase conjugate (Boehringer) in TBS was applied to each section and incubated for 3 h at room temperature. Slides were rinsed twice in TBS and then equilibrated in substrate buffer (18 ml distilled water, 2 ml Tris pH 9, 0.22 g MgCl2 and 0.12 g NaCl) for 5 min at room temperature. Alkaline phosphatase substrate (Boehringer, 10 ml substrate buffer, 80 µg/ml nitro-blue tetrazolium and 175 µg/ml of 5-bromo-4-chloro-3-indolyl-phosphate) was applied to the sections, covered with parafilm and left for 3 days in the dark. Slides were rinsed under running water for 15 min, dehydrated in an ethanol series, cleared in xylene and mounted with DPX.
Control sections were incubated without probe or with sense probe. In addition, a section of lung from an iNOS knockout mouse was incubated with anti-sense probe.
Modulation of the NO pathway
In order to modulate the NO pathway, various concentrations of NOS substrates were applied in vitro. Follicles were cultured in L-arg-free MEM (special order, Gibco) with other culture supplements unaltered and using standardized culture and analysis techniques that we have previously validated for this system (Mitchell et al., 2002
). Each experiment was repeated a minimum of six times with
30 follicles per experiment and the results of the experiments collated to produce the results tables. Follicle survival and ovulation rates were compared with control cultures in complete MEM
(L-arg concentration 600 µmol/l) or L-arg-free medium with L-arg (Sigma) added back at 600 µmol/l. The influence of apo-transferrin (10 µg/ml) on follicle development was assessed in the presence and absence of L-arg.
To determine the relative effects of different intermediates of the NO pathway, L-cit (0, 6, 60, 600 µmol/l; Sigma) or NG-hydroxy-L-arg (Sigma, 0, 6, 60, 600 µmol/l) were added as supplements to L-arg-free MEM. Ovulation and survival rates in vitro were compared with follicles cultured with or without L-arg.
Prior to use, L-cit and NG-hydroxy-L-arg were made up as x100 stock concentrations in media lacking L-arg and FCS. These stock reagents were added in volumes of 1% to the culture medium under test. This avoided the possibility of carried-over L-arg affecting the results.
Experiments modulating the NO pathway were carried out in the absence of apo-transferrin.
Statistics
Survival and ovulation rates were assessed using 2x2 contingency tables, with 2-test. P < 0.05 was considered significant.
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Results |
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Detection of iNOS mRNA
A positive signal for iNOS mRNA was detected in granulosa cells and oocytes (Figure 1e). Granulosa cells and oocytes in follicles of various sizes from preantral to antral were stained. There was weak, irregular staining of thecal cells (Figure 1f). No signal was produced in the negative controls; hybridization buffer only (with no probe, Figure 1g), iNOS sense probe (Figure 1h) or using anti-sense probe on sections of liver from an iNOS knockout mouse (not shown).
Detection of eNOS
Positive staining for eNOS was detected in blood vessels, as expected, and also in the oocyte and thecal cells. Staining was also evident in the granulosa cells of some follicles of various sizes from preantral to antral, as shown in Figure 1i. Slight non-specific staining occurred in negative control sections incubated without primary antibody (Figure 1j).
Follicle culture
Effect of absence of L-arg
Figure 2 shows that omission of L-arg significantly reduced follicle survival and ovulation. Follicles cultured in standard MEM had a 77% survival rate with 71% ovulating and in the absence of L-arg, 36% survived, 15% ovulated (P < 0.001). When 600 µmol/l L-arg was added back to the L-arg-free medium, the survival rate of 88% was significantly greater than in complete medium (P < 0.05) and medium lacking L-arg (P < 0.001). In medium with L-arg added back, 75% of follicles ovulated which was not different to standard MEM
, but was significantly greater than in the absence of L-arg (P < 0.001). Eighty-five per cent of follicles that ovulated normally, with L-arg present, released oocytes with cumulus cells attached, compared with 47% of those in L-arg-free medium (P < 0.001).
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The majority of follicles considered to have survived using our previously established criteria showed features of abnormal development and deterioration (Table I). There was an unexpected and highly significant difference (P < 0.001) in the proportions of follicles in which the granulosa structure, normally contained within the basement membrane, remained intact at the end of culture (68% with apo-transferrin versus 12% without). Interestingly, 3% of follicles cultured without L-arg or apo-transferrin spontaneously released oocytes surrounded by cumulus cells, without exposure to hCG. A total of eight follicles released their oocytes in this way (6 on day 8, 1 on day 9 and 1 on day 10) in five separate experiments, but only when media lacked both L-arg and apo-transferrin. Such oocyte release was visibly indistinguishable from that considered to be ovulation in vitro when it occurred in response to hCG.
In contrast to the results when L-arg was absent, follicles cultured in medium containing L-arg but lacking apo-transferrin had significantly higher total (86 versus 75%, P < 0.01) and timely (<16 h) ovulation rates (78 versus 59%, P < 0.001), although survival rates were not significantly different, compared with those containing both L-arg and apo-transferrin (Table I). The majority of follicles cultured with L-arg were healthy in appearance, regardless of whether apo-transferrin was present. Apo-transferrin therefore had a positive effect on follicle survival when L-arg was absent, but a detrimental effect upon ovulation when L-arg was present.
Addition of L-cit to L-arg-free medium
Table II shows the response of follicles to culture in conditions where L-arg was replaced with L-cit at concentrations between 0 and 600 µmol/l, compared with complete medium containing 600 µmol/l L-arg, all in the absence of apo-transferrin. Follicle survival and ovulation rates were significantly lower in the absence of L-arg compared with complete medium (P < 0.001), confirming previous results. The inclusion of progressively greater concentrations of L-cit resulted in improvements in follicle survival and ovulation rates, as well as the appearance of the cumulus at ovulation, although the maximum concentration of 600 µmol/l did not fully compensate for the absence of L-arg.
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Discussion |
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In this study, eNOS protein was detected in oocytes, theca and in the granulosa cells of some follicles in immature mouse ovaries. eNOS has been previously reported in mature oocytes of rats and mice (Nishikimi et al., 2001), and its presence in the granulosa cell layer increases during stimulated follicle growth (Jablonka-Shariff and Olson, 1997
). Others have found eNOS located on the surface of pre-ovulatory and ovulated oocytes and in the theca and stroma of healthy growing follicles, while staining in granulosa cells was punctate (Powers et al., 1996
; Jablonka-Shariff and Olson, 1998
).
In sections of ovary and cultured follicles, iNOS protein and mRNA, and eNOS protein were localized most clearly to the ooplasm. The presence of iNOS in immature oocytes in growing and small follicles had not been previously demonstrated, although NOS involvement in oocyte maturation has been inferred from experiments using NOS inhibitors (Sengoku et al., 2001). iNOS is known to increase substantially after fertilization in ovulated mouse oocytes and embryos (Nishikimi et al., 2001
) and its presence is stimulated by hCG in rats (Jablonka-Shariff and Olson, 1997
).
It is interesting that iNOS protein was located mainly in the oocytes and theca of immature mouse ovaries, while iNOS mRNA was detected in granulosa cells and the oocyte. Inter-communication between the cell types of the follicle may partly explain this finding. Ribonucleotides are transferred into the oocyte via gap junctions with the granulosa cells (Brower and Schulz, 1982) and increased oocyte growth and RNA polymerase activity are among the first signs of follicle growth (Gosden et al., 1993
). Others, working in rats, have shown cycle-dependent variation in mRNA and protein levels. In the immature rat ovary before ovulation, high levels of iNOS mRNA (Van Voorhis et al., 1995
; Srivastava et al., 1997
) but low levels of iNOS protein (Zackrisson et al., 1996
) occur in granulosa cells. After ovulation, iNOS mRNA decreases, and iNOS protein increases (Jablonka-Shariff and Olson, 1997
; Srivastava et al., 1997
). Various transcription factors and stimuli, including NO itself, can regulate iNOS gene expression transcriptionally, post-transcriptionally and translationally (Nathan and Xie, 1994b
; Lysiak et al., 1995
). Many factors present in follicles have the capacity to influence iNOS mRNA levels, such as interleukin (IL)-4, IL-10 and transforming growth factor-
(Roy and Kole, 1998
). Therefore, the lack of coincidence of iNOS mRNA and protein detected in our study seems to be in keeping with what is known of iNOS in other situations, and suggests a dissociation between transcription and translation.
The presence of iNOS protein in cultured follicles provided the possibility of studying NO actions on follicle development in vitro by substrate manipulation. Evidence for NO involvement in rodent ovulation arises from many in vivo or perfused ovary studies (e.g. Hesla et al., 1997; Mitsube et al., 1999
). NO has also been found to inhibit steroidogenesis of cultured rat granulosa cells (Ahsan et al., 1997
; Dave et al., 1997
); however, no previous studies have examined the role of NO in individual follicle cultures.
The depletion and reintroduction of L-arg is widely used as a method of studying the role of NOS and hence NO on cellular systems in vitro. This assumes that there is no other effect of L-arg withdrawal apart from those mediated by the NO pathway. The literature does not provide any evidence of other mechanisms being involved, but this cannot currently be ruled out as a possible contributory factor in the observations that we present. Information on physiological levels of NOS substrates is available only for endothelial cells. Intracellular L-arg concentrations up to 2 mmol/l may occur in freshly isolated endothelial cells (Hecker et al., 1990b), whereas in cultured cells, levels range from 100 to 800 µmol/l (Gold et al., 1989
; Baydoun et al., 1990
; Hecker et al., 1990b
) and 200400 µmol/l after 24 h of culture without L-arg (Baydoun et al., 1990
; Hecker et al., 1990a
). L-cit levels of 150 µmol/l were found in cultured bovine aortic endothelial cells, decreasing to 50 µmol/l after 24 h of L-arg depletion (Baydoun et al., 1990
). Omission of L-arg led rapidly to a decrease in the total intracellular amino acid pool, but amino acid levels began to normalize within 24 h, except for L-arg and L-cit.
If intracellular L-arg is regulated similarly in cultured follicular cells, then follicles without L-arg could maintain similar development to control follicles by utilizing intracellular L-arg until exhaustion. Follicles cultured without L-arg appeared healthy until day 5, which probably indicates the time taken to reach critically low levels of L-arg. Our doseresponse data suggest that L-cit may be recycled into L-arg, if L-arg was lacking, as occurs in endothelial cells and macrophages (Hecker et al., 1990a; Nussler et al., 1994
). Follicles cultured with L-cit may produce L-arg from it via enzymes with activities similar to argininosuccinate lyase and argininosuccinate synthetase. The synthesis of L-arg in this way in low L-arg conditions could play a significant role in regulating NO production (Nussler et al., 1994
); however, the efficiency of this process is likely to be low unless the prevailing concentrations of L-cit are
60 µmol/l, as shown in Figure 5. NG-Hydroxy- L-arg, if present, would be a more effective substrate, but serum levels in rats were reported as 3.7 µmol/l in basal and 15.8 µmol/l in NOS-stimulated conditions (Hecker et al., 1995
). While follicular levels of NG-hydroxy-L-arg are unknown, the levels used in our experiments are supraphysiological and the availability of NG-hydroxy- L-arg may be limited in the physiological situation.
A curious observation in this study was the apparent maintenance of an intact basement membrane in more than half the follicles cultured in medium where L-arg was lacking but apo-transferrin was supplemented. Transferrin readily enters follicles from the circulation and is active in iron transport, although it may have local actions also (Tilly, 1998). For example, transferrin may suppress the generation of reactive oxygen species and may have specific functions in granulosa cells, although little is known of its role in follicle development (Briggs et al., 1999
). Transferrin is expressed in mouse granulosa cells, where its receptor is also present (Briggs et al., 1999
). This may be particularly relevant for the follicle culture system, where extrafollicular transferrin is unavailable unless added to culture media. Omitting L-arg has been shown to increase free radicals by formation of superoxide and NO which forms peroxynitrite, causing cellular injury (Xia et al., 1996
). Transferrin may protect cells by acting as a chelator of contaminating toxic metals, as a competitive substrate for proteolytic enzymes (Orly et al., 1980
), as well as in reducing oxidative stress from free radicals (Cairo et al., 2002
). Our observation may therefore provide some support for the idea that basement membrane breakdown involves free radicals (Luck et al., 1995
). The addition of apo-transferrin would tend to reduce free radicals which may have been increased by the omission of L-arg (Xia et al., 1996
), possibly contributing to follicular demise. Alternatively, it is possible that transferrin may exert an inhibitory effect upon cell proliferation, reducing the tendency for the basement membrane to rupture under the culture conditions applied. We do not have evidence on the mechanism by which this effect is occurring, but this observation warrants further examination, since the inhibition of ovulation would be potentially useful clinically, and embryos and gametes can be adversely affected by an excess of free radicals (Dinara et al., 2001
). Experiments in this series will be continued using specific inhibitors of NOS, to assess their effects upon follicle growth in vitro.
In conclusion, these results demonstrate the importance and relevance of L-arg, the precursor of NO, in normal follicle development in vitro. Without L-arg in the culture medium, follicle survival and ovulation were inhibited. The addition of other NO pathway components (NG-hydroxy-L-arginine, L-citrulline and NO) did not fully replace L-arg as survival and ovulation rates did not reach control levels.
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Acknowledgements |
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Submitted on January 22, 2003; resubmitted on June 6, 2003; accepted on September 24, 2003.