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Evidence for an inherent rhythm in pulsatile LH release in ovariectomized cows
THERIOGENOLOGY EVIDENCE PULSATILE C.
H.
FOR
LH b
Rahe
J.
IN
Animal
and
Science
P.
G.
Experiment
Station,
Received
COWS a Harms
’
Department
A&M University
College
IN
Section
Reproduction
Agricultural
Texas
RHYTHM
OVARIECTOMIZED
Fleeger
of
Texas
INHERENT
L.
Physioligy The
AN
RELEASE
77843
Texas
for
Station
System
Publication:
May
Accepted:
26,
1982
August
31,
1982
ABSTRACT Luteinizing collected the
hormone
at
a.m.
.5 minute
and
p.m.
ovariectomized manner rapidly more
all
to
gradual
suggest
that
pattern
of
hormones
.
each
LH
cow
both
may
the
of
pulsatile increased
increase, The
and
days
results
we
consistent an
a
interval
between
the
inherent
absence
a
levels
observed. From
samples during
long-term
rapid
within
an
(hr)
LH
the
cows.
have
in
blood
in
pulses,
was
within
in hours
from
Following
consistent
3
fluctuated
the
decline
release
for days
of
ng/ml).
collection LH
measured
consecutive During
6.0 was
sample
were
intervals
Levels
exponential
pulses
blood
3
animals.
(2.5
between
of
cows.
in
levels
(min)
of
rhythmic
ovarian
source
of
INTRODUCTION Several tration
investigators
of
tuates
LH
in
a
is
not
have
demonstrated
maintained
pulsatile
at
manner
in
a
that
constant
plasma
of
the
level
concenbut
normally
fluc-
cycling
cows
ewes (2) and rats (3) and in women with normal menstrual (I), Furthermore, cycles (4 ). the amplitude and frequency of the pulses of LH are dependent upon the period of the cycle and are p robab
Iy
influenced
pulse
(a)
by
ovariectomy,
Following
amp 1 itude
Technical G.
NIH
for
LH
(5-8).
No.
Niswender for
levels
increased
Station. D.
hormone
systemic
Article
Experiment Dr.
is
ovarian
17702 The
and
(l-4). elevated
LH are
Pulses
from
authors the
secretion of
the
of
Texas
thank
Dr.
Endocrinology
iodination,
LH antisera
and
LH occur
at
Agricultural L.
E.
Study
Reichert, Section
and
LH
and
Dairy
of
standards,
respectively. (b)
(c)
:
Present
address
Auburn
University,
AL
requests:
Dr.
Reprint Texas
NOVEMBER
Department P.
A&M University,
1982
VOL. 18 NO.
of
Animal
Sciences,
36849 G.
College
5
Harms, Station,
Animal TX
Science
Dept.,
77843.
573
THERIOGENOLOGY
approximately 20-25 min intervals in at approximately 60 min intervals in monkeys (5), ewes (7) and cows (8,9). designed to further characterize the of pulsatile LH using ovariectomized
MATERIALS
ovariectomized ovariectomized The present frequency and cows.
rats (6) rhesus study was repeatability
and
AND METHODS
Animals Three Hereford cows (approximately four years of age) which had been ovariectomized 14 years prior to the experiment were used in the study. The animals were maintained on bermudagrass pasture, supplemented with hay and grain to meet all nutrient requirements. The animals were docile (i.e., trained to lead) and were conditioned to the experimental facility two days prior to blood sample collection during March and April. One day prior to blood sample collection, the cows were fitted with an indwelling jugular vein cannula (Silastic medical grade; 0.76 mm, 1.65 mm, o.d., Dow-Corning Corp., Midland, Ml). i.d.; Blood samples (5 ml) were collected at 5 min intervals on three consecutive days for a period of three hr each a.m. (0800-1055 hr) and p.m. (1300-1555 hr). Blood samples were subsequently placed into heparinized tubes andocentrifuged at Recovered plasma was stored at -20 C. 1,500 x g. Radioimmfunoassay Plasma LH was measured by the homologous double-antibody RIA described by Niswender et al. (10) with modifications describea by Forrest et al.Tpr Plasma samples (200 ~1) were -Results were expressed by the bovine assayed in duplicate. reference standard NIH-LH-B9 (biological potency, 0.70 Sensitivity of the assay was such NIH-LH-Sl units/mg). The that 0.2 rig/assay tube could be reliably detected. coefficients of variations for interand intraassay variability were 10.73% and 6.45%, respectively. Statistical
Analysis
Due to the samll number of pulses per period of blood sample collection and the dependent nature of the data, time series analysis (Rahe -et al. (1) and conventional methods t-test, ANOV) are not possible and unreliable, respec(i.e., data from each animal were tively. Therefore, a.m. and p.m. superimposed and described by visual appraisal.
574
NOVEMBER
1982 VOL. 18 NO. 5
THERIOGENOLOGY
6
PM
8
6
Time figure
1.
Patt
erns
of
day 2 (t.*-*) co11 ect ion
NOVEMBER
p I asma and
1982 VOL. 18 NO. 5
LH day
in 3
(HR)
cow
( w)
1 on
day 1 (m) of blood sample
575
THERIOGENOLOGY
AM
5 PM
I
6\ 5- ‘\*
0
2 Time
Figure
576
2:
3
b-d
Patterns of plasma LH from cow 2 on day 1 (-1, day 2 (o-----o) and day 3 (n ) of blood sample collection.
NOVEMBER
1982 VOL. 18 NO. 5
THERIOGENOLOGY
RESULTS AND DISCUSSION Levels of LH fluctuated in a pulsatile manner during each collection period in all cows (Fig. 1, 2 and 3). During pulses LH levels increased rapidly (from 8.5 to 6.0 ng/ml) and then decreased gradually to levels observed before the pulse. The interval between pulses ranged from about 40 to 60 min depending on the cow. The characteristic shape and rhythm of the pulses closely resemble those previously observed in ovariectomized rhesus monkeys (5), ewes (7)) and cows (8,g). The consistent rhythm of the LH pulses within cows may be observed in the superimposed daily patterns (a.m. and p.m.) for In addition, patterns of LH during each cow (Fig. 1, 2 and 3). the a.m.’ appeared similar to the patterns in blood collected during the p.m. Such a consistent pattern within cows suggests rhythmic pattern of that each cow may possess an inherent pituitary LH release. Such a finding is in agreement with previous reports (6, 8, 9) with less extensive data per cow, in which the interval between pulses was more consistent within than between animals. It is likely that the pulsatile release of LH in ovariectomized cows is due to a pulsatile release of LHRH from the hypothalamus. Fluctuations in the concentration of CHRH have been reported in the hypophyseal portal blood of rhesus monkeys Carmel et al * (12) found a high correlation between (11). hypophyseal stalk LHRH levels and peripheral plasma LH levels secretion of LH is in rhesus monkeys. MOreove r , the pulsatile abolished by administration of antiserum to LHRH (13, 14). Further, evidence in support of the hypothesis that pulsatile LH release in regulated by pulsatile release of hypothalamic LHRH has been presented by.Belchtz et al (15) ., -_* In their studv, ovariectomized rhesus monkevs wrth hvoothalamic lesions were either constantly or intermittently infuied with equivalent amounts of LHRH. Animals receiving the constant infusion of LHRH failed to manifest a rhythmic pattern of LH whereas animals receiving LHRH at hourly intervals exhibited studies using pituirhythmic patterns of plasma LH. --In vitro taries from ovariectomized rats have also shown that pulsatile release of LH is achieved only after the pulsatile administration of LHRH (16). The range of intervals ovariectomized cows in this previously reported (1) for (day 3) of the estrous cycle
NOVEMBER
1982 VOL. 18 NO. 5
observed between LH pulses in the study is similar to that which we cows during the early luteal stage when progesterone levels were low.
577
THERIOGENOLOGY
AM
0
Time Figure
578
3:
Patterns of day 2 (t--o) co1 lect ion.
3
2
I
(HR)
plasma LH from cow 3 on day and day 3 ( M) of blood
NOVEMBER
1 (m), sample
1982 VOL. 18 NO. 5
THERIOGENOLOGY
In conclusion, the results from the present study indicate that each cow may have an inherent rhythmic pattern of pituitary LH release. Furthermore, it is likely that hypothalamic LHRH is also released in a pulsatile manner (15-16). When progesterone levels were elevated during the mid-luteal period, the interval between LH pulses was extended (1). Such data suggest that the hypothalamus of each cow may have a unique rhythm for LHRH release which is altered by ovarian hormones. The physiofor the proposed inherent rhythm in LH logic role, if any, release in reproductive efficiency of the cow remains to be determined.
NOVEMBER
1982 VOL. 18 NO. 5
579
THEFUOGENOLOGY REFERENCES 1.
Rahe, C. H., Owens, R. E., Fleeger, J. L., Harms, P. G. Pattern of plasma luteinizing cycl ic cow: Dependence upon the period of crinology lo7:4g8-503 (1980).
2.
Hauger, R. L., Karsch, F. J., Foster, D. L. A new concept for control of the estrous cycle of the ewe based on the temporal relationships .between luteinizing hormone, estradiol and progesterone in peripheral serum and evidence that progesterone inhibits tonic LH secretion. Endocrinology 101:807-817 (1977).
3.
Gallo, R. V. Pulsatile level LH secretion in (1981). -24~771-777
4.
Yen, S. C., Tsai, C. C., Naftol in, F., L. Pulsatile patterns of gonadotropin J. with and without ovarian function. Metab. (1972). -34:671-675
5.
Dierschke, D. J., Bhattacharya, Knobil, E. Circhoral oscillations ovariectomized rhesus monkey.
6.
Gay, V. L. and Sheth, N. A. of LH in castrated male and
-go:158-162
Newton, H. J. and hormone in the the cycle. Endo-
LH release during periods of low the rat estrous cycle. Reprod. Biol.
Vandenberg, G., Ajabor, release in subjects Clin. Endocrinol.
A.
N., Atkinson, L. E. of plasma LH levels Endocrinology -87:850-853
Evidence for female rats.
a periodic release Endocrinology
(1972).
7.
Roche, J. F., Foster, D. L., Karsch, F. J. and Effect of castration and infusion of melatonin luteinizing hormone in sera and pituitaries of Endocrinology (1970). -87: 1205-1210
8.
Beck, T. W. and Convey, J. Anim. ovariectomy.
9.
Forrest, D. W., Fleeger, J. L., Long, C. R., Sorensen, Effect of exogenous prolactin A. M. and Harms, P. G. peripheral luteinizing hormone levels in ovariectomized Biol. Reprod. 22: 197-201 (1980). cows. -
E. Sci.
Serum LH in M. Abstr. -43:274,
Dziuk, P. on levels ewes.
heifers (1976).
10.
Niswender, G. D., Reichert, Jr., L. E., Midgley, Radioimmunoassay for bovine and Nalbandov, A. V. Endocrinology luteinizing hormone. -84:1166-1173
11.
Neil 1, J. D., Patton, J. Luteinizing Tindall ~ G. T. in pituitary stalk blood to level of LH release.
580
and in the (1970)
J. of
after
Jr., on
Jr., A. R. and ovine (1969).
M.,
Dailey, R.A., Tsou, R. C. and hormone releasing hormone (LHRH) Relationship of rhesus monkeys: Endocrinology 101:430-434 (1977).
NOVEMBER
1982 VOL. 18 NO. 5
THERIOGENOLOGY
12.
13.
Pituitary stalk Carmel, P. W., Araki, S. and Ferin, M. Evidence for pulsati portal collection in rhesus monkeys: Endorelease of gonadotropin-releasing hormone (GnRH). crinology (1976). -Yy:?_43-248
le
Chobsieng, P., Zor, U., Fridkin, M., and Linder, Koch, Y., H. R. Suppression of gonadotropin secretion and prevention of ovulation in the rat by antiserum to synthetic gonadoBiochem. Biophys. Res. Commun. tropin-releasing hormone.
s:623-629
(1973).
14.
McCormack, J. T., Plant, T. M., Hess, D. L. and Knobil, E. The effect of luteinizing hormone releasing hormone (LHRH) antiserum administration on gonadotropin secretion in the rhesus monkey. Endocrinology 100:66X-667 (1977).
15.
Belchetz, Knobil, mittent hormone.
16.
In vitro Gallo, R. V. and Wi 11 iams, J. Osland, R. B., release of luteinizing hormone from anterior pituitary fragments super-fused with constant or pulsatile amounts of Endocrinology 96: luteinizing hormone-releasing factor. 1210-1216 (1975).
NOVEMBER
P. E., Plant, T. M., Nakai, Y., Keogh, E. Hypophysial responses to continuous and E. delivery of hypothalamic gonadotropin-releasing Science 202:631-633 (1978).
1982 VOL. 18 NO. 5
J. and inter-
581
H.
FOR
LH b
Rahe
J.
IN
Animal
and
Science
P.
G.
Experiment
Station,
Received
COWS a Harms
’
Department
A&M University
College
IN
Section
Reproduction
Agricultural
Texas
RHYTHM
OVARIECTOMIZED
Fleeger
of
Texas
INHERENT
L.
Physioligy The
AN
RELEASE
77843
Texas
for
Station
System
Publication:
May
Accepted:
26,
1982
August
31,
1982
ABSTRACT Luteinizing collected the
hormone
at
a.m.
.5 minute
and
p.m.
ovariectomized manner rapidly more
all
to
gradual
suggest
that
pattern
of
hormones
.
each
LH
cow
both
may
the
of
pulsatile increased
increase, The
and
days
results
we
consistent an
a
interval
between
the
inherent
absence
a
levels
observed. From
samples during
long-term
rapid
within
an
(hr)
LH
the
cows.
have
in
blood
in
pulses,
was
within
in hours
from
Following
consistent
3
fluctuated
the
decline
release
for days
of
ng/ml).
collection LH
measured
consecutive During
6.0 was
sample
were
intervals
Levels
exponential
pulses
blood
3
animals.
(2.5
between
of
cows.
in
levels
(min)
of
rhythmic
ovarian
source
of
INTRODUCTION Several tration
investigators
of
tuates
LH
in
a
is
not
have
demonstrated
maintained
pulsatile
at
manner
in
a
that
constant
plasma
of
the
level
concenbut
normally
fluc-
cycling
cows
ewes (2) and rats (3) and in women with normal menstrual (I), Furthermore, cycles (4 ). the amplitude and frequency of the pulses of LH are dependent upon the period of the cycle and are p robab
Iy
influenced
pulse
(a)
by
ovariectomy,
Following
amp 1 itude
Technical G.
NIH
for
LH
(5-8).
No.
Niswender for
levels
increased
Station. D.
hormone
systemic
Article
Experiment Dr.
is
ovarian
17702 The
and
(l-4). elevated
LH are
Pulses
from
authors the
secretion of
the
of
Texas
thank
Dr.
Endocrinology
iodination,
LH antisera
and
LH occur
at
Agricultural L.
E.
Study
Reichert, Section
and
LH
and
Dairy
of
standards,
respectively. (b)
(c)
:
Present
address
Auburn
University,
AL
requests:
Dr.
Reprint Texas
NOVEMBER
Department P.
A&M University,
1982
VOL. 18 NO.
of
Animal
Sciences,
36849 G.
College
5
Harms, Station,
Animal TX
Science
Dept.,
77843.
573
THERIOGENOLOGY
approximately 20-25 min intervals in at approximately 60 min intervals in monkeys (5), ewes (7) and cows (8,9). designed to further characterize the of pulsatile LH using ovariectomized
MATERIALS
ovariectomized ovariectomized The present frequency and cows.
rats (6) rhesus study was repeatability
and
AND METHODS
Animals Three Hereford cows (approximately four years of age) which had been ovariectomized 14 years prior to the experiment were used in the study. The animals were maintained on bermudagrass pasture, supplemented with hay and grain to meet all nutrient requirements. The animals were docile (i.e., trained to lead) and were conditioned to the experimental facility two days prior to blood sample collection during March and April. One day prior to blood sample collection, the cows were fitted with an indwelling jugular vein cannula (Silastic medical grade; 0.76 mm, 1.65 mm, o.d., Dow-Corning Corp., Midland, Ml). i.d.; Blood samples (5 ml) were collected at 5 min intervals on three consecutive days for a period of three hr each a.m. (0800-1055 hr) and p.m. (1300-1555 hr). Blood samples were subsequently placed into heparinized tubes andocentrifuged at Recovered plasma was stored at -20 C. 1,500 x g. Radioimmfunoassay Plasma LH was measured by the homologous double-antibody RIA described by Niswender et al. (10) with modifications describea by Forrest et al.Tpr Plasma samples (200 ~1) were -Results were expressed by the bovine assayed in duplicate. reference standard NIH-LH-B9 (biological potency, 0.70 Sensitivity of the assay was such NIH-LH-Sl units/mg). The that 0.2 rig/assay tube could be reliably detected. coefficients of variations for interand intraassay variability were 10.73% and 6.45%, respectively. Statistical
Analysis
Due to the samll number of pulses per period of blood sample collection and the dependent nature of the data, time series analysis (Rahe -et al. (1) and conventional methods t-test, ANOV) are not possible and unreliable, respec(i.e., data from each animal were tively. Therefore, a.m. and p.m. superimposed and described by visual appraisal.
574
NOVEMBER
1982 VOL. 18 NO. 5
THERIOGENOLOGY
6
PM
8
6
Time figure
1.
Patt
erns
of
day 2 (t.*-*) co11 ect ion
NOVEMBER
p I asma and
1982 VOL. 18 NO. 5
LH day
in 3
(HR)
cow
( w)
1 on
day 1 (m) of blood sample
575
THERIOGENOLOGY
AM
5 PM
I
6\ 5- ‘\*
0
2 Time
Figure
576
2:
3
b-d
Patterns of plasma LH from cow 2 on day 1 (-1, day 2 (o-----o) and day 3 (n ) of blood sample collection.
NOVEMBER
1982 VOL. 18 NO. 5
THERIOGENOLOGY
RESULTS AND DISCUSSION Levels of LH fluctuated in a pulsatile manner during each collection period in all cows (Fig. 1, 2 and 3). During pulses LH levels increased rapidly (from 8.5 to 6.0 ng/ml) and then decreased gradually to levels observed before the pulse. The interval between pulses ranged from about 40 to 60 min depending on the cow. The characteristic shape and rhythm of the pulses closely resemble those previously observed in ovariectomized rhesus monkeys (5), ewes (7)) and cows (8,g). The consistent rhythm of the LH pulses within cows may be observed in the superimposed daily patterns (a.m. and p.m.) for In addition, patterns of LH during each cow (Fig. 1, 2 and 3). the a.m.’ appeared similar to the patterns in blood collected during the p.m. Such a consistent pattern within cows suggests rhythmic pattern of that each cow may possess an inherent pituitary LH release. Such a finding is in agreement with previous reports (6, 8, 9) with less extensive data per cow, in which the interval between pulses was more consistent within than between animals. It is likely that the pulsatile release of LH in ovariectomized cows is due to a pulsatile release of LHRH from the hypothalamus. Fluctuations in the concentration of CHRH have been reported in the hypophyseal portal blood of rhesus monkeys Carmel et al * (12) found a high correlation between (11). hypophyseal stalk LHRH levels and peripheral plasma LH levels secretion of LH is in rhesus monkeys. MOreove r , the pulsatile abolished by administration of antiserum to LHRH (13, 14). Further, evidence in support of the hypothesis that pulsatile LH release in regulated by pulsatile release of hypothalamic LHRH has been presented by.Belchtz et al (15) ., -_* In their studv, ovariectomized rhesus monkevs wrth hvoothalamic lesions were either constantly or intermittently infuied with equivalent amounts of LHRH. Animals receiving the constant infusion of LHRH failed to manifest a rhythmic pattern of LH whereas animals receiving LHRH at hourly intervals exhibited studies using pituirhythmic patterns of plasma LH. --In vitro taries from ovariectomized rats have also shown that pulsatile release of LH is achieved only after the pulsatile administration of LHRH (16). The range of intervals ovariectomized cows in this previously reported (1) for (day 3) of the estrous cycle
NOVEMBER
1982 VOL. 18 NO. 5
observed between LH pulses in the study is similar to that which we cows during the early luteal stage when progesterone levels were low.
577
THERIOGENOLOGY
AM
0
Time Figure
578
3:
Patterns of day 2 (t--o) co1 lect ion.
3
2
I
(HR)
plasma LH from cow 3 on day and day 3 ( M) of blood
NOVEMBER
1 (m), sample
1982 VOL. 18 NO. 5
THERIOGENOLOGY
In conclusion, the results from the present study indicate that each cow may have an inherent rhythmic pattern of pituitary LH release. Furthermore, it is likely that hypothalamic LHRH is also released in a pulsatile manner (15-16). When progesterone levels were elevated during the mid-luteal period, the interval between LH pulses was extended (1). Such data suggest that the hypothalamus of each cow may have a unique rhythm for LHRH release which is altered by ovarian hormones. The physiofor the proposed inherent rhythm in LH logic role, if any, release in reproductive efficiency of the cow remains to be determined.
NOVEMBER
1982 VOL. 18 NO. 5
579
THEFUOGENOLOGY REFERENCES 1.
Rahe, C. H., Owens, R. E., Fleeger, J. L., Harms, P. G. Pattern of plasma luteinizing cycl ic cow: Dependence upon the period of crinology lo7:4g8-503 (1980).
2.
Hauger, R. L., Karsch, F. J., Foster, D. L. A new concept for control of the estrous cycle of the ewe based on the temporal relationships .between luteinizing hormone, estradiol and progesterone in peripheral serum and evidence that progesterone inhibits tonic LH secretion. Endocrinology 101:807-817 (1977).
3.
Gallo, R. V. Pulsatile level LH secretion in (1981). -24~771-777
4.
Yen, S. C., Tsai, C. C., Naftol in, F., L. Pulsatile patterns of gonadotropin J. with and without ovarian function. Metab. (1972). -34:671-675
5.
Dierschke, D. J., Bhattacharya, Knobil, E. Circhoral oscillations ovariectomized rhesus monkey.
6.
Gay, V. L. and Sheth, N. A. of LH in castrated male and
-go:158-162
Newton, H. J. and hormone in the the cycle. Endo-
LH release during periods of low the rat estrous cycle. Reprod. Biol.
Vandenberg, G., Ajabor, release in subjects Clin. Endocrinol.
A.
N., Atkinson, L. E. of plasma LH levels Endocrinology -87:850-853
Evidence for female rats.
a periodic release Endocrinology
(1972).
7.
Roche, J. F., Foster, D. L., Karsch, F. J. and Effect of castration and infusion of melatonin luteinizing hormone in sera and pituitaries of Endocrinology (1970). -87: 1205-1210
8.
Beck, T. W. and Convey, J. Anim. ovariectomy.
9.
Forrest, D. W., Fleeger, J. L., Long, C. R., Sorensen, Effect of exogenous prolactin A. M. and Harms, P. G. peripheral luteinizing hormone levels in ovariectomized Biol. Reprod. 22: 197-201 (1980). cows. -
E. Sci.
Serum LH in M. Abstr. -43:274,
Dziuk, P. on levels ewes.
heifers (1976).
10.
Niswender, G. D., Reichert, Jr., L. E., Midgley, Radioimmunoassay for bovine and Nalbandov, A. V. Endocrinology luteinizing hormone. -84:1166-1173
11.
Neil 1, J. D., Patton, J. Luteinizing Tindall ~ G. T. in pituitary stalk blood to level of LH release.
580
and in the (1970)
J. of
after
Jr., on
Jr., A. R. and ovine (1969).
M.,
Dailey, R.A., Tsou, R. C. and hormone releasing hormone (LHRH) Relationship of rhesus monkeys: Endocrinology 101:430-434 (1977).
NOVEMBER
1982 VOL. 18 NO. 5
THERIOGENOLOGY
12.
13.
Pituitary stalk Carmel, P. W., Araki, S. and Ferin, M. Evidence for pulsati portal collection in rhesus monkeys: Endorelease of gonadotropin-releasing hormone (GnRH). crinology (1976). -Yy:?_43-248
le
Chobsieng, P., Zor, U., Fridkin, M., and Linder, Koch, Y., H. R. Suppression of gonadotropin secretion and prevention of ovulation in the rat by antiserum to synthetic gonadoBiochem. Biophys. Res. Commun. tropin-releasing hormone.
s:623-629
(1973).
14.
McCormack, J. T., Plant, T. M., Hess, D. L. and Knobil, E. The effect of luteinizing hormone releasing hormone (LHRH) antiserum administration on gonadotropin secretion in the rhesus monkey. Endocrinology 100:66X-667 (1977).
15.
Belchetz, Knobil, mittent hormone.
16.
In vitro Gallo, R. V. and Wi 11 iams, J. Osland, R. B., release of luteinizing hormone from anterior pituitary fragments super-fused with constant or pulsatile amounts of Endocrinology 96: luteinizing hormone-releasing factor. 1210-1216 (1975).
NOVEMBER
P. E., Plant, T. M., Nakai, Y., Keogh, E. Hypophysial responses to continuous and E. delivery of hypothalamic gonadotropin-releasing Science 202:631-633 (1978).
1982 VOL. 18 NO. 5
J. and inter-
581