BIOLOGY OF REPRODUCTION 54, 436-445 (1996)
Inhibition of Nitric Oxide: Effects on Interleukin-lp-Enhanced Ovulation Rate,
Steroid Hormones, and Ovarian Leukocyte Distribution at Ovulation in the Rat'
Nigel Bonello,2 '3 Kylie McKie, 3 Melinda Jasper, 3 Lucy Andrew,3 Nicki Ross, 3
Emily Braybon,3 Mats Brinnstr6m, 3 ' 4 and Robert J. Norman3
Reproductive Medicine Unit,3 Department of Obstetrics and Gynaecology, The University of Adelaide
The Queen Elizabeth Hospital, Woodville, SA 5011, Australia
Departmentof Obstetrics and Gynaecology, 4 The University of GCteborg
Sahlgrenska Sjukhuset, Goteborg S-41345, Sweden
ABSTRACT
The ovulatory process resembles an inflammatory reaction with an infiltration of leukocytes, production of inflammatory mediators
such as cytokines, and a general edema and hyperemia. Nitric oxide (NO), a potent vasodilator and the main mediator of macrophage
tumoricidal and bacteriocidal activities, is known to participate in inflammatory reactions and has been shown to mediate the interleukin1[ (IL-lp)-directed tissue-remodeling events within the ovary. The regulation by NO of ovulation rate, leukocyte distribution, and steroid
release in the rat ovary was investigated through use of a combination of in vivo and in vitro models of ovulation and a competitive
-4
inhibitor, N-omega-nitro-L-arginine methyl ester (L-NAME), of the NO synthase (NOS) enzyme. Subcutaneous L-NAME (1.5 x 10 mol/kg)
administration significantly reduced the in vivo ovulation rate of eCG/hCG-primed rats (L-NAME-treated: 10.6 + 1.8 [mean ± SEMI oocytes
per ovary [O/O, 11.0 ± 1.2 rupture sites per ovary [RS/O]; saline-treated: 18.0 + 1.8 0/O, 19.4 ± 1.1 RS/O; p < 0.01) at 20 h post-hCG.
These results were reflected in vitro, where addition of L-NAME (3.5 x 10-5 mol/L) to LH (0.1 pg/ml)-perfused ovaries decreased ovulation
rate from 8.2 ± 1.6 to 2.7 1ovulations per ovary (p < 0.05) and simultaneously decreased nitrite accumulation at the completion of
perfusions from 16.5 ± 1.9 to 4.1 ± 0.5 nmol/ml (p < 0.001). The addition of L-NAME to LH + IL-1P (4 ng/ml)-perfused ovaries decreased
ovulation rate from 15.2 ± 2.4 to 0.8 ± 0.8 ovulations per ovary (p < 0.001) and simultaneously decreased nitrite accumulation at 22 h
from 22.8 ± 2.2 to 1.9 ±0.6 nmol/ml (p < 0.001). Studies analyzing and manipulating perfusion flow rate indicated that the L-NAME
effects on ovulation rate are primarily due to a reduction in flow rate resulting from inhibition of NO, which may be a consequence of the
known vasoconstrictor effects of NOS inhibitors. The observed reduction of in vivo ovulation rate by NO inhibition at 20 h post-hCG was
associated with a significant reduction in thecal MCA149+ neutrophils at 12 h post-hCG, the expected time of ovulation (L-NAME-treated:
11.5 cells per thecal area; p < 0.001), while ED1 + monocytes/macrophages
98.4 ± 9.2 cells per thecal area; saline-treated: 211.5
underwent similar but nonsignificant changes. Plasma (20 h post-hCG) and perfusate progesterone were not different with L-NAME treatment, while perfusate estradiol levels were markedly reduced upon addition of L-NAME, suggesting a role for NO in ovulation but not in
the process of luteinization. In summary, deprivation of NO by use of the competitive inhibitor, L-NAME, led to fewer ovulations, reduced
accumulation of nitrite, a decreased neutrophil count in the theca of preovulatory follicles, and reduced estradiol secretion, while progesterone release remained unaffected. The NO pathway may therefore play an important role inthe regulation of ovulation and the mediation
of IL-1l's pro-ovulatory effects. There are likely to be primarily vascular effects, but also a nonvascular component, to the NO regulation
of ovulation, with both components indirectly affecting ovulatory leukocyte distribution and steroid secretion.
INTRODUCTION
diverse cell types in both physiological and pathophysiological conditions, have all been shown to play a role in the
process of follicle rupture [8]. Of particular interest is the
cytokine interleukin-lp (IL-1p), which has been shown to
be present in the ovary at both the mRNA and protein levels
[9, 10] and which undergoes an hCG-dependent rise in
mRNA just before ovulation [11]. IL-1P enhances ovulation
rate in perfused rat ovaries when administered alone, but
especially so in the presence of LH [12], and it modulates
the activity of ovulatory mediators such as those within the
plasminogen activator system [13, 14]. Furthermore, local
administration of the natural inhibitor of the IL-1 system, the
IL-1 receptor antagonist, is known to markedly reduce the
hCG-induced ovulation rate of the rat in vivo [15].
Nitric oxide (NO), a potent vasodilator, is formed from the
oxidation of L-arginine by NO synthase (NOS), which has
constitutive calcium/calmodulin-dependent and cytokineinducible calcium/calmodulin-independent forms. Other
Ovulation is a complex process involving not only gonadotropins and steroid hormones but also many mediators
common to inflammatory reactions [1]. At the cellular level,
white blood cells-especially neutrophils and monocytes/
macrophages-have been shown to infiltrate the ovary just
before ovulation [2-4] and to enhance the LH-induced ovulation rate of in vitro-perfused rat ovaries [5], while their
depletion from the peripheral blood is associated with a
reduction in ovulation rate [6]. At the molecular level, products of inflammatory leukocytes such as cytokines [7], prostaglandins, leukotrienes, plasminogen activators, histamine,
and bradykinin, which are now known to be secreted by
Accepted September 27, 1995.
Received June 30, 1995.
'This research was funded by the NH &MRC (Australia). N.B. is supported by a University of Adelaide Medical Faculty Postgraduate Scholarship and a Queen Elizabeth Hospital
Scholarship. M.B. was supported by the Swedish Medical Research Council (B96-17X).
2Correspondence. FAX: 61 8 2687978.
436
OVULATORY EFFECTS OF NITRIC OXIDE INHIBITION
physiological functions of NO include smooth muscle relaxation, neurotransmission, and the mediation of macrophage tumoricidal and bacteriocidal actions [16, 17]. An important regulatory role for NO in follicle rupture is indicated
in a recent study revealing a reduction in the in vivo ovulation rate of rats administered inhibitors of the NOS enzyme intrabursally [18]. Detection of NOS in ovarian cells
[19-21] and in hypothalamic neurons that regulate LHRH
secretion [22] and the observation of cyclicity in the concentration of serum nitrite (a stable metabolic breakdown
product of NO) [23] during a woman's menstrual cycle, have
further implied a role for NO in ovarian function. Furthermore, NO mediates the cytotoxic actions of IL-I 3 on ovarian
cells [24], suggesting that through remodeling of ovarian tissue NO may be involved in the pro-ovulatory pathway of
IL-1.
The present study addresses several questions concerning the relative importance of NO in the ovulatory process and
the mechanisms through which it may act. By using a wellcharacterized NOS competitive inhibitor [25, 26], N-omeganitro-L-arginine methyl ester (L-NAME), we wished to confirm that NO is indeed important for ovulation in vivo and
to investigate whether the same effect can be reproduced
by using the in vitro rat ovary perfusion model-a system
that is devoid of systemic influences. The level of nitrite
accumulation by perfused ovaries and the changes in steroid hormone production attributable to L-NAME treatment
were also examined. In addition, we studied the role of NO
in mediating the known effects of IL-1i on ovulation rate
and steroid release, and we investigated the ovulatory
effects of NO ascribed to its known vasodilatory actions.
Finally, given the increasing direct and indirect evidence
supporting a role for leukocytes in ovulation, we used immunohistochemistry to examine the effect of NO deprivation on the distribution of neutrophils and monocytes/macrophages in the ovary.
MATERIALS AND METHODS
Chemicals and Monoclonal Antibodies
L-NAME, sulfanilamide, and N-(1-naphthyl) ethylamide
dihydrochloride were from Sigma Chemical Company (St.
Louis, MO); isopentane was from BDH (Poole, UK); and
Tissue-Tek (OCT) compound was from Miles Inc. (Elkhart,
IN). Ovine LH (NIH LH s-25; specific activity [SA] 2.3 U/mg)
was provided by the NIDDK and the National Hormone and
Pituitary Program, University of Maryland School of Medicine (Baltimore, MD). Equine CG was from Intervet (Boxmeer, The Netherlands), and hCG was from Organon (Oss,
The Netherlands). Monoclonal antibodies MCA149 and ED1
were from Serotec (Oxford, UK), while human recombinant
IL-13 (spec. act. 5 X 108 U/mg) was from Genzyme (Boston, MA).
437
Animals
Immature female Sprague-Dawley rats (from the University of Adelaide colony) weighing 50-75 g were maintained
under controlled conditions (14L:10D) with free access to
pelleted food and water. At 27 days of age, all rats received
an s.c. injection of eCG (in vivo study, 16 IU; in vitro study,
20 IU) at 1200 h on experimental Day -2 to promote the
growth and maturation of a first generation of antral follicles.
In Vivo Study
The animals were divided into two groups; the experimental group received L-NAME s.c. (1.5 X 10 - 4 mol/kg),
and the control group received a matched volume of saline
s.c. Animals were first administered L-NAME or saline at
1100 h on Day 0. Human CG was administered i.p. at 1200
h on Day 0, followed by further L-NAME or saline administrations at 1400 h, 1700 h, 2000 h, and 2300 h on Day 0.
Animals required for ovarian immunohistochemistry were
killed at 0000 h on Day 1, and their ovaries were frozen.
Animals required for ovulation assessment were killed at
0800 h on Day 1. A cardiac blood sample was collected
within 30 sec for subsequent plasma progesterone assay,
and the ovaries and oviducts were immediately examined.
In Vitro (Rat Ovary Perfusion) Study
Ovary perfusions in a 35-ml recirculating system were
performed on the morning of Day 0 through use of surgery
and perfusion procedures that have been described in detail
previously [27]. The perfusion pressure was maintained at
80 mm Hg (except in the reduced-pressure group), resulting
in an average flow of 0.9 ml/min in untreated and LH-perfused ovaries. The ovaries were initially perfused for 1 h to
allow metabolic stabilization of the tissue before treatment
was commenced. Treatments of L-NAME (3.5 X 10- 5 mol/
L), LH (0.1 lg/ml), and IL-13 (4 ng/ml), alone or in combination, were added to perfusion medium; in the case of
controls, the ovaries were perfused with Medium 199
(Gibco, Grand Island, NY) alone. Samples of the circulating
medium were obtained at 0, 1, 2, 3, 4, 8, and 22 h following
treatment administration. Samples were stored at - 20°C for
subsequent steroid hormone and NO assays.
Assessment of Ovulation Rate
The oocytes in the ampulla region of each oviduct were
counted at 20 h post-hCG (i.e., 0800 h on Day 1). These
data were supported by surface examination of the corresponding ovary for rupture sites. In vitro-perfused ovaries
were removed from the perfusion system and washed gently with saline to dislodge any adherent oocytes before the
number of oocytes found in the perfusion chamber was
counted. All counting was coded and conducted by investigators blind to the treatment protocol.
438
BONELLO ET AL.
Perfusion Flow Rate Analysis
To assess the effects of L-NAME on ovarian vessel constriction and flow, the perfusion flow rate was measured in
LH- and LH + L-NAME-treated ovaries. The flow rate of the
LH + L-NAME-treated ovaries was then reproduced in LH
+ IL-1S3 (reduced pressure; RP)-perfused ovaries by reducing the perfusion pressure to achieve the required flow rate.
Progesteroneand EstradiolAssays
Progesterone levels in perfusion media and rat plasma
were analyzed through use of RIA kits from Amersham (Buckinghamshire, UK) with inter- and intraassay coefficients of variation (CVs) of < 7% and < 5%, respectively. Estradiol levels
in perfusion media were analyzed with an RIA kit from Diagnostic Systems Laboratories (Webster, TX) with inter- and
intraassay CVs of < 10% and < 9%, respectively.
NO Assay
Accumulation of NO in perfusion samples was analyzed
by measuring nitrite as described previously [28]. Briefly,
Greiss reagent was prepared; this consisted of one part 0.1%
naphthylethylenediamine dihydrochloride in distilled water
plus one part 1% sulfanilamide in 5% concentrated H3 PO 4.
Fifty microliters of Greiss reagent was added to wells in a
96-well plate (Disposable Products, Adelaide, Australia),
followed by 50 ll sample or standard (2.5, 5, 10, 20, 40, 80
gIM NaNO 2 ). After a 10-min incubation at room temperature, the plate was read for absorbance in an ELISA plate
reader set at a wavelength of 540 nm.
Immunohistochemistry
The ovaries of animals killed at 0000 h on Day 1 were
embedded in OCT and frozen in isopentane in liquid nitrogen before being stored at - 85°C. Air-dried 5-jm-thick cryostat sections then underwent immunohistochemical staining using the murine antibodies MCA149 (1:800 dilution),
which detects rat neutrophilic granulocytes [29], and ED1
(1:400 dilution), which detects rat monocytes/macrophages
[30], exactly as described previously [14] except that normal
sheep serum (nonimmune serum of the species from which
the linking antibody was derived) was used to block nonspecific binding. For negative controls, the mAb was replaced with either the antibody diluent or mouse IgG1.
Only stained cells in the theca of follicles > 0.6 mm (average
of 4.4 follicles per ovary) were counted, as it has previously
been shown that follicles of this size at proestrus can be
considered to have escaped atresia and are destined to ovulate [31]. Two independent observers (interobserver difference < 16%) counted the number of positively stained cells
within an entire thecal region and in 1
1-mm areas of
medullary stroma (average of 1.9 fields per ovary).
StatisticalAnalysis
Unpaired Student's t-tests were utilized to compare the
means of the saline- and L-NAME-treated groups in the in
vivo component of the study, while one-way ANOVAs were
used to examine differences between group variances (at
each sample point in the time courses) in the in vitro-perfused ovary studies. A Tukey-Kramer Multiple Comparisons
post-hoc test then detected the specific groups with different
variances. In all cases p < 0.05 was considered significant.
RESULTS
In Vivo Study
Ovulation rate (Fig. 1). Examination of oocyte numbers
in the oviducts of rats at 20 h post-hCG revealed that the LNAME-treated rats ovulated significantly (p < 0.01) fewer
oocytes per ovary than rats in the control group (mean +
SEM, 10.6 ± 1.8 vs. 18.0 + 1.8, respectively). When follicle
rupture points on the surface of ovaries were counted and
used as the parameter for ovulation having occurred, almost
identical results were obtained (L-NAME 11.0 ± 1.2; control
group 19.4
1.1 rupture sites per ovary; p < 0.001). Numbers of ovulated oocytes correlated well with numbers of
observed rupture sites for each ovary, with correlation coefficients of r = 0.86 and r = 0.84 in the control and LNAME-treated groups, respectively. Mean levels of progesterone in the plasma of rats at 20 h post-hCG were not
significantly different (p = 0.65) between the L-NAMEtreated group (24.3 + 3 pmol/ml) and the control group
(22.3
3.3 pmol/ml). Estradiol was not measured because
of lack of plasma.
Ovarian leukocyte distribution (Figs. 2 and 3). Untreated animals had more MCA149 + neutrophil cells than
ED1 + monocyte/macrophage cells in the thecal region of
follicles greater than 0.6 mm in diameter and had greater
numbers of ED1 + compared to MCA149+ in 1 X -mm
areas of medullary stroma in ovaries at the expected time
of ovulation. The effect of L-NAME was most noticeable
with regard to changes in neutrophil distribution. There was
a significant 2-fold reduction in thecal neutrophils and a
corresponding 2-fold rise in medullary neutrophils. Negative controls revealed negligible nonspecific cell staining:
0.5 + 0.2 cells per thecal area and 0.2
0.2 cells per medullary stroma when the antibody diluent was used and 1.6
+ 0.8 cells per thecal area and 0.8 ± 0.3 cells per medullary
stroma when the matching isotype, mouse IgG1, was used
as the negative control.
In Vitro Study
Perfusionflow rate (Fig. 4). L-NAME had an immediate
effect on the flow rate of LH-perfused ovaries, resulting in
a 46% decrease in the initial flow rate within the first 15 min
after addition of the compound. This decreased level was
OWVULATORY EFFECTS OF NITRIC OXIDE INHIBITION
250
439
_
]1 Control IS L-NAME
200
_
. 150
E
0
THE,CA
MEDULLA
z
CI
100
0
50
I_
n ay
Neutrophils
.i:
Neutrophils
Monocyte/
Macrophages
I
Monocyte/
Macrophages
FIG. 1. In vivo ovulation rate (mean + SEM ovulations per ovary, n = 12 rats per
group) at 20 h post-hCG. Effects of L-NAME treatment were analyzed by counting
the number of oocytes recovered from the ampulla region of the oviduct and assessing the number of separate follicle rupture sites found on the corresponding
ovary. **/***Significantly (p < 0.01/p < 0.001, respectively) different from untreated controls.
FIG. 2. Ovarian leukocyte distribution at 12 h post-hCG. Effects of L-NAME treatment on ovarian leukocyte distribution (mean SEM cells per unit area, n = 6 rats
per group) in the thecal area of unruptured follicles with a diameter greater than
0.6 mm (designated preovulatory), and in 1 x 1-mm areas of the medullary stroma
were immunohistochemically analyzed through use of mAbs MCA149 (specific for
rat neutrophils) and ED1 (specific for rat monocytes/macrophages). */***Significantly (p < 0.05/p < 0.001, respectively) different from untreated controls.
maintained for the first 2 h of perfusion, normalizing by 3
h into the perfusions and until their completion. The flow
rate profile of LH + IL-13 (RP)-perfused ovaries was subsequently made to exactly mimic that of the LH + L-NAME
group by adjusting perfusion pressure between 82.8 and
53.3 mm Hg. IL-i1I was included with LH in this comparison
group because of its known stimulation of ovulations [121,
therefore facilitating detection of any reduction in ovulation
rate attributable to flow rate depression caused by L-NAME.
Ovulation rate (Fig. 5). The reduction of ovulation rate
in rats administered L-NAME in vivo was similarly reflected
in vitro. Ovaries co-perfused with LH and L-NAME ovulated
significantly less (2.7 + 1 ovulations per ovary, p < 0.05)
than their LH-perfused counterparts (8.2 + 1.6), while coperfusion of LH + IL-1, resulted in an ovulation rate (15.2
+ 2.4) that was significantly greater than that of the LHperfused group. Adding L-NAME to the ovaries perfused
with LH + IL-1 led to a dramatic attenuation (p < 0.001)
in the ovulatory response, resulting in 0.8
0.8 ovulations
per ovary. A major portion of this effect could be attributed
to the decrease in ovarian flow as demonstrated by the LH
+ IL-1 (RP)-perfused group, which ovulated 4.2 + 1.5
oocytes per ovary. Control ovaries perfused with medium
alone showed no ovulations.
NO accumulation (Table 1). Nitrite levels as a measure
of NO accumulation during the perfusion period were measured in samples of the perfused ovaries obtained at 0, 1,
8, and 22 h after addition of the treatments. No significant
difference in nitrite concentration existed at 0 h between
the different treatment groups. Comparison of the rise in
nitrite levels, standardized to each ovary's initial (0 h) nitrite
concentration, showed only an approximately 2-fold rise in
nitrite accumulation during the entire perfusion period of
ovaries treated with LH + L-NAME and LH + IL-1P + L-
TABLE 1. Nitrite accumulation (mean t SEM nmol/ml) in perfusion medium at 0, 1, 8, and 22 h of in vitro rat ovary perfusions.
Hours of perfusion
Treatment'
Control
LH
LH + L-NAME
LH + IL-1p
LH + IL-15 + L-NAME
LH + IL-11 (RP)
0
3.7
4.6
2.4
1.4
1.1
3.2
+ 1.1
+ 1.4
± 0.2
± 0.1
+ 0.2
+ 0.5
1
4.6
3.1
2.6
1.9
1.8
2.3
+ 1.6
± 0.5
± 0.2
± 0.4
±+0.4
± 0.3
8
6.4
4.7
2.6
6.2
2.4
4.5
± 2.2
+ 0.8
± 0.3
± 1.0
± 0.9
+ 1.0
22
11.9
16.5
4.1
22.8
1.9
17.6
+ 0.7
± 1.9
+ 0.5***
+ 2.2
+ 0.6*** a
+ 1.9* **b
Fold rise'
4.7
5.4
1.8
17.2
2.1
6.1
+ 1.7
+ 1.0
+ 0.3
+ 2.1**
± 1.0* **a
± 1.3***a
tDoses and experimental numbers as shown in Figure 5 legend.
'Fold rise at 22 h of perfusion. Standardized by dividing 22-h nitrite concentration by initial (0 h) concentration for each ovary
within each treatment.
***/***a/***bSignificantly different (p < 0.001) from LH, LH + IL-1p, or LH + IL-1p+ L-NAME, respectively.
440
BONELLO ET AL.
441
OVULATORY EFFECTS OF NITRIC OXIDE INHIBITION
0
5
10
15
20
25
Control
LH
LH +L-NAME
LH +IL-18
Hours of Perfusion
FIG. 4. Flow rate (mean ± SEM ml/min, n = 4 ovaries per group) of perfusate
passing through in vitro-perfused ovaries treated with LH (0.1 Vlg/ml) ± L-NAME
(3.5 x 10-5 mol/L). Flow rate measurements were obtained every 15 min for the
first 2 h of perfusion and then at 3, 4, 8, and 22 h after commencement of the
perfusions. ***All samples of LH + L-NAME-treated ovaries within 2 h of commencing perfusions were significantly different (p < 0.001) from ovaries treated
with LH alone.
NAME, demonstrating the effectiveness of L-NAME at inhibiting the production of NO in the system. An approximately
4.7-fold increase in nitrite accumulation occurred at the 22h time point in ovaries perfused with medium alone in comparison to L-NAME-perfused ovaries, while there were 5.4and 6.1-fold increases in nitrite levels in LH- and LH + IL1P (RP)-perfused ovaries over the same time period and a
highly significant 17-fold increase in the LH + IL-13-perfused group; this indicated that IL-1P is a potent stimulator
of NO production in perfused rat ovaries.
Steroidsecretion (Fig. 6). In all groups of perfused ovaries except the group perfused with medium alone, there
was the expected dramatic increase in secreted progesterone within the first 1-2 h after the addition of LH + treatments (Fig. 6, a and b). The peak progesterone production
in ovaries perfused with LH + IL-113 + L-NAME occurred
a little later, however, at 4 h after the beginning of treatment;
but in this group, as in all groups exhibiting an initial peak,
detection of progesterone in the medium gradually declined
over the remaining period of perfusion because of the
known binding of progesterone with the glass walls of the
perfusion apparatus [27]. Inhibition of NO via the addition
of L-NAME had no real effect on either LH- or LH + IL-13-
FIG. 3. Effects of NO inhibition on immunohistochemical localization of neutrophils (MCA149+) and monocytes/macrophages (ED1+) in the rat ovary at the expected time of ovulation (12 h post-hCG). MCA149+ cells (darkly stained) were
localized to the thecal region of preovulatory follicles with significantly larger cell
numbers inthe thecal area of saline-treated controls (a)than of L-NAME-treated rats
(b). ED1 + cells were present in large numbers in the medullary stroma region of
the ovary, while their density did not vary significantly between saline-treated controls (c)and L-NAME-treated rats (d). Magnification: a-b, x 100; c-d, x200.
LH+ IL-I1
+ L-NAME
LH
IL-IL
(RP)
FIG. 5. In vitro ovulation rate. Oocytes (mean + SEM) ovulated from untreated
perfused ovaries or from ovaries treated with LH (0.1 ig/ml) + IL-1I(4 ng/ml) LNAME (3.5 x 10- 5 mol/L). Experimental numbers: LH, LH + L-NAME, n = 10; LH
+ IL-1p (RP), n = 6; LH + IL-1p, n = 5; controls, LH + IL-1i + L-NAME, n = 4.
*/***Significantly different (p < 0.05/p < 0.001) from the LH-perfused group and
the LH + IL-lp-perfused group, respectively.
perfused ovaries, resulting in a noticeably (p < 0.05) reduced progesterone level only at the 22-h time point in LH
+ IL-1 3-perfused ovaries.
Estradiol secretion did seem to be at least partly regulated
by NO (Fig. 6, c and d). L-NAME treatment of LH-perfused
ovaries resulted in a reduction in estradiol secretion (p
< 0.05), while treating LH + IL-13-perfused ovaries with
the NO inhibitor invoked an even more substantial attenuation of estradiol release (p < 0.01) over most time points.
Peak secretion of estradiol in each group generally occurred
between 4 and 8 h after the commencement of perfusions
and then plateaued for the remainder of the perfusion.
DISCUSSION
Administration of the inducible NOS inhibitors aminoguanidine and NG-methyl-L-arginine, either i.p. or into the
ovarian bursal cavity, has previously been shown to significantly reduce ovulation rate in rats, an effect that could be
reversed with the NO generator, sodium nitroprusside [18].
In the current study we reduced ovulation rate to similar
levels in vivo, and to an even greater extent in the in vitroperfused rat ovary model, by treating rats, or their excised
and perfused ovaries, with the NOS competitive inhibitor LNAME. The reduction in ovulation rate was associated with
altered steroid secretion profiles in vitro and altered leukocyte distribution in vivo at the time of ovulation.
The finding of a good correlation between follicle rupture sites and oocytes collected from the oviduct suggested
that while L-NAME was inhibiting ovulation rate in vivo, the
treatment was having no effects on transportation of the
oocyte from the site of rupture to the ampullary region of
442
BONELLO ET AL.
Hours of Perfusion
Hours of Perfusion
CA
-2
E
S
0
4
-0
to
a.
5
5
Hours of Perfusion
Hours of Perfusion
FIG. 6. Time course of steroid secretion (mean + SEM pmol/ml) throughout the period of perfusion. a-b) Progesterone secretion by perfused ovaries treated as shown. cd) Estradiol secretion by perfused ovaries treated as shown. Treatment doses and experimental numbers as described in Figure 5 legend. b) */**/***Significantly different
(p < 0.05/p < 0.01 /p < 0.001, respectively) from LH + IL-1 (RP) group; *"Significantly different (p < 0.05) from LH + IL-1p group. c) */**Significantly different (p < 0.05/
p < 0.01) from LH group. d) */**/***Significantly different (p < 0.05/p < 0.01/p < 0.001) from LH + IL-1I group.
the rat oviduct. A similar reduction of the ovulation rate in
vitro by L-NAME indicated that the in vivo results were not
attributable solely to indirect systemic effects of L-NAME but
resulted from direct effects of the NOS inhibitor on the
ovary. Interestingly, our in vitro results indicate that L-NAME
was particularly effective in inhibiting the IL-13-mediated
enhancement of the LH-induced ovulation rate reported by
us previously [12]; this suggested that a major component
of the increase in ovulation rate caused by IL-11 may be
mediated by NO. Indeed it is noteworthy that the maximal
production of NO occurred in the group co-perfused with
LH and IL-13.
Previous studies using ovarian dispersates from immature
rats have shown the ability of IL-I1 to induce production of
NO in the ovary [24, 32]. The present study indicates that this
effect persists in the rat, after exposure of whole perfused
ovaries to LH and up until and including the time of rupture,
as shown in a similar study on whole ovarian dispersates of
the gonadotropin-primed rat [331. IL-1p activates the inducible NOS (iNOS) in the ovary [24, 32], presumably requiring
protein synthesis; this may account for the time delay in ni-
trite accumulation we observed in IL-l1-perfused ovaries.
Furthermore, the induction by IL-13 of ovarian NO is receptor-mediated, and IL-i1 requires the participation of both
granulosa and theca-interstitial cells to exert its cytotoxic and
other effects [24, 32]. Hence a complex intrafollicular cellular
communication system comprising IL-13B, its receptors, NOS,
and NO is likely to exist at ovulation. While mRNA for IL- 1p
and the IL-1 receptor type 1 (IL-IRtl) have been found in
aspirated granulosa-luteal cells in the human [9, 13], rodent
mRNA and protein for IL-10 and IL-1Rtl are known to be
concentrated in the thecal-interstitial layer before ovulation
[10, 11]. The mRNA and protein for a constitutive NOS
(eNOS) have been localized to preovulatory human granulosa-luteal cells [19], and NOS activity has been shown in
luteal phase thecal, granulosa, and CL cells of cynomolgus
monkeys [20]; but to our knowledge no localization of iNOS
in the ovary has yet been published.
The nitrite data also confirm that i.-NAME effectively inhibits NO production throughout the course of perfusions
and that NO production is not significantly increased in LHperfused ovaries as compared to ovaries perfused with me-
OVULATORY EFFECTS OF NITRIC OXIDE INHIBITION
dium only; this indicates that as shown previously [32], gonadotropin alone is not sufficient to induce NO production
in vitro. However, the ovarian infiltration of leukocytes (a
potent source and site of action for NOS-activating cytokines such as IL- 13) that follows hCG stimulation of rats in
vivo [4] (but obviously cannot occur in the in vitro-perfused
rat ovary) may well increasingly stimulate NO production
as ovulation approaches. Perfusions with IL-13 increased
LH-induced ovulations to a frequency comparable to that
for hCG-induced ovulations in vivo in this study, perhaps
indicating that exogenous IL-113 compensates for the lack of
infiltrating leukocytes in vitro. The fact that the LH-perfused
ovaries ovulated while untreated ovaries did not, when both
groups demonstrated similar nitrite accumulation, is probably attributable to factors other than NO (such as the eicosanoids, serine proteases, kinins, and histamine) that are
activated by LH and required in addition to basal NO levels
in order for ovulation to occur. LH-perfused ovaries deprived of this apparently necessary background level of NO
by L-NAME had very few ovulations, indicating that NO is
obligatory for the ovulatory process.
Human ovarian vein rings have been shown to constrict
in vitro in response to stimulation with L-NAME [341. Hence,
inhibitory effects on ovulation induced by the use of NOS
inhibitors may be due to inhibition of NO-derived relaxation
of ovarian vascular smooth muscle and unmasking of the
vasopressor response to endothelin [35], leading to decreases
in ovarian blood flow or, in the case of the in vitro-perfused
ovaries, decreases in perfusate flow. Ovarian blood flow normally increases as ovulation approaches [36, 371 and contributes to the hyperemia and macroscopically observable
"follicular blushing" that accompany the general increase in
vascular permeability and edema observed in ovulating ovaries. By comparing the flow rates of LH-perfused and LH +
L-NAME-perfused ovaries we have shown that there is an
approximately 46% decrease in flow rate upon addition of
the NOS inhibitor at a dose of 3.5 X 10 -5 mol/L; as mimicked
in optimally stimulated ovaries, this resulted in a decreased
ovulation rate that accounted for approximately 76% of the
effect observed in this group when perfused with L-NAME.
This result implies that there are vascular and nonvascular
components to the reduced ovulation rate observed with NO
inhibition and that the vascular effects are probably dominant. NO also promotes vascular permeability and edema
formation [38]. Hence, it seems likely that NO is important
for maintaining the state of ovarian vasorelaxation required
to accommodate the ovulatory increase in blood flow, blood
volume, and plasma exudation, and that at least part of this
effect is regulated by IL-113.
The importance of the role of leukocytes in ovulation has
been established [5, 6]. Immunohistochemical localization
of ovarian leukocytes in the current study confirmed the
previously reported abundance of neutrophils over monocytes/macrophages in the theca and its reversal in the med-
443
ullary stroma of ovulating ovaries in the rat [4]. The significant depression in thecal and the concomitant increase in
medullary neutrophils upon L-NAME treatment, which may
be reflected by changes in the distribution of monocytes/
macrophages, may indicate a direct role for NO in modifying leukocyte trafficking at ovulation. NO has previously
been implicated in promoting monocyte chemotaxis in humans [39]; this may explain the decreased numbers of thecal
leukocytes found in animals treated with the NOS inhibitor.
Similarly, ovarian NO may enhance leukocyte adhesion in
thecal vessels during the periovulatory period. Evidence exists, however, that NO is an inhibitor of platelet and leukocyte adhesion rather than a promoter of the process [17].
An alternative explanation for the considerable trafficking
of neutrophils from the follicular theca to the ovarian medullary stroma is preferential vasoconstriction of capillaries
in the thecal area of follicles as a result of inhibition of the
vasodilatory response to NO.
Analysis of the effect of NO inhibition on steroidogenesis
revealed a significant reduction in the secretion of estradiol
(but not progesterone) in those perfused ovaries that exhibited a marked reduction in nitrite accumulation, suggesting that NO may have a role in modulation of ovarian
estradiol secretion. This result complements findings of another study showing that serum nitrite levels increase during
the follicular phase of the ovarian cycle in women and decrease after ovulation, correlating with serum estradiol but
not with progesterone, FSH, or LH [231. It has previously
been shown that inhibitors of the NOS enzyme significantly
increase secretion of estradiol from human granulosa-luteal
cells [19]; however, these experiments on isolated and cultured cells, lacking an intact vasculature and from another
species, may not serve as an appropriate comparison to this
study. The opposing view that estradiol regulates NO production has also been presented [23, 40, 41]. A similar study
investigating the role of NO in regulating flow rate and steroidogenesis in the in vitro-perfused rat adrenal gland revealed that the marked decrease in flow rate resulting from
L-NAME treatment was similarly associated with a drop in
hormone secretion [42]. Hence, it is possible that a component of the decrease in estradiol secretion observed with
L-NAME treatment may be explained by the resultant reduced flow rate. This possibility is supported by the partial
reduction in secretion of estradiol observed in the LH + IL13 (RP) group. The specific importance of estradiol to the
rupture process is not currently understood [43], although a
possible role for estrogen in augmenting blood pressure
control by NO in the rat has been suggested and may apply
during the dramatic vascular changes that occur in the ovulating ovary [44].
Progesterone output during the periovulatory period was
not altered by NO inhibition of perfused ovaries. Plasma
progesterone was similarly unchanged during the early
luteal period (20 h post-hCG) of in vivo-treated rats, sug-
444
BONELLO ET AL.
gesting that while follicle rupture was affected by L-NAME
treatment, luteinization was proceeding normally, possibly
because of the phenomenon of luteinized unruptured follicles [45]. We are unable to account for the marked increase
in progesterone levels observed in LH + IL-p3 (RP) ovaries
perfused with the artificially decreased flow rate, and further investigations are required to establish its physiological
significance.
There are alternative mechanisms by which NO may control ovulation. The physical processes of rupture and cellular reorganization accompanying the transformation of
ovulatory follicles into young CL require extensive tissue
remodeling and cell death. For example, NO has already
been found to mediate the cytotoxic actions of IL-1 3 within
the ovary [24] and so may be involved with the degradative
events of ovulation. As with other free radicals, NO can
damage DNA by base deamination, causing activation of
ATP-consuming DNA repair enzymes, which can lead to cell
death by energy depletion [17]. NO can also destroy cells
by binding the heme- or non-heme iron of numerous enzymes to catalyze inhibition of vital DNA synthesis or can
combine with free oxygen to produce a variety of cytotoxic
factors [46]. Furthermore, a neural pathway for the NO effects on ovulation may exist as evidenced by reports identifying hypothalamic NOS-containing neurons that regulate
LHRH secretion via guanylate cyclase [22] and NOS-containing nerve fibers present in the rat [21] but not the mouse
ovary [47].
This study has shown that inhibition of NO significantly
reduced the rate of ovulation in rats both in vivo and in
vitro, and suggests that NO is likely to mediate the proovulatory actions of IL-1P3 that enhance LH-induced ovulations in vitro. Control of ovarian vessel relaxation to accommodate the necessary changes in blood flow, volume, and
plasma exudation that accompany rupture is likely to be the
most important role of NO in ovulation; but effects on steroidogenesis, leukocyte trafficking, and cytotoxicity may
also exist and warrant further investigation.
ACKNOWLEDGMENTS
We wish to thank Dr. Hideo Tozawa and Vivian Pascoe for helpful advice and Jane
Lomas, Maria Bellis, and Suzie Dosljak for expert technical assistance. We are also grateful
to The National Hormone and Pituitary Program, University of Maryland, and NIDDK for the
generous gift of LH.
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