0013-7227/90/1265-2237$02.00/0 Endocrinology Copyright © 1990 by The Endocrine Society
Vol. 126, No. 5 Printed in U.S.A.
Effect of Reduced Luteinizing Hormone Concentrations on Corpus Luteum Function during the Menstrual Cycle of Rhesus Monkeys* ANTHONY J. ZELEZNIK AND LYNDA L. LITTLE-IHRIG Departments of Physiology and Obstetrics and Gynecology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261
ABSTRACT. To further define the relationship between plasma LH concentrations and progesterone secretion by the primate corpus luteum, we examined luteal function in rhesus monkeys in response to reduced LH concentrations during the luteal phase of the menstrual cycle. Five anovulatory rhesus monkeys received a pulsatile infusion of synthetic GnRH (6 fig/ pulse; one pulse per h, iv) to restore menstrual cyclicity. During the early luteal phase (4-5 days after ovulation), the amount of GnRH administered per pulse was reduced to l/250th or l/750th of the standard GnRH infusion regimen. Plasma LH concentrations, determined by bioassay, were reduced by approximately 50% during cycles maintained by reduced GnRH concentrations compared with the standard GnRH dosage. Serum progesterone concentrations were maintained for 5-6 days after GnRH reduction and declined thereafter, and premature menstruations were
observed in four of seven cycles maintained by the l/250th GnRH reduction and four of six cycles maintained with the 1/ 750th GnRH reduction. These results are consistent with the hypothesis that luteal regression during the nonfertile menstrual cycles of primates is due primarily to an alteration in luteal cell responsiveness to LH, rather than a reduction in the gonadotropic drive to the corpus luteum per se. When plasma LH concentrations were reduced during the early luteal phase to values below those found during the onset of luteal regression in control cycles, luteal function was maintained for 5-6 days. However, as the luteal phase progressed, the reduced LH concentrations were unable to sustain progesterone secretion, and premature menses occurred in some, but not all, animals. (Endocrinology 126: 22372244,1990)
T
HE PRIMATE corpus luteum of nonfertile menstrual cycles produces progesterone for a finite 14to 16-day interval, after which steroid secretion ceases, and the tissue undergoes regression and resorption from the ovary. Although in vitro studies have convincingly demonstrated that the secretion of progesterone by luteal cells is directly regulated by LH (1, 2), the extent to which changes in the pattern of LH secretion in vivo are responsible for the cessation of progesterone secretion and luteal regression at the termination of nonfertile cycles is less clear. Our laboratory has investigated the role of gonadotropins in the secretion of progesterone and the maintenance of the lifespan of the corpus luteum in macaque monkeys, with particular emphasis on determining whether there is a causal relationship between the pattern of gonadotropin secretion during the luteal phase and regression of the corpus luteum. For this purpose we Received November 6,1989. Address all correspondence and requests for reprints to: Anthony J. Zeleznik, Ph.D., Department of Obstetrics and Gynecology, University of Pittsburgh School of Medicine, Magee Womens Hospital, Forbes Avenue at Halket Street, Pittsburgh, Pennsylvania 15213. * This work was supported by NIH Grants HD-16842 and HD-08610 and Research Career Development Award 00531 (to A.J.Z.).
have used rhesus monkeys rendered anovulatory by either placement of radiofrequency lesions in the medial basal hypothalamus or transection of the pituitary stalk, procedures that interrupt the delivery of endogenous GnRH to the pituitary gland (3, 4). Ovulatory menstrual cycles can be restored in these animals by iv infusion of exogenous GnRH in a pulsatile manner. Because gonadotropin secretion in these animals is governed solely by the administration of exogenous GnRH, the absolute pattern of gonadotropin secretion in these animals can be controlled directly by the pattern of administration of exogenous GnRH. Using the above model system, we have demonstrated previously that although the secretion of progesterone by the corpus luteum is absolutely dependent upon LH throughout the entire luteal phase of the menstrual cycle (5), a reduction in LH pulse frequency like such as seen during the mid- to late luteal phase of the menstrual cycle does not directly cause luteal regression (6). Thus, a premature reduction in LH pulse frequency 3 days after ovulation was followed by a pattern of progesterone secretion identical to that observed in spontaneous menstrual cycles, and timely menstruation ensued, suggesting that luteal regression is more likely due to changes in
2237
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LH AND LUTEAL FUNCTION
2238
luteal cell sensitivity to LH than to changes in the pattern of LH secretion per se. In the current study we sought to determine whether a reduction in LH concentrations to values below those normally encountered during the luteal phase of the menstrual cycle would provoke rapid luteal regression. For this purpose, anovulatory rhesus monkeys received pulsatile infusions of synthetic GnRH at concentrations that were reduced compared to the standard GnRH infusion dosage in an attempt to reduce LH concentrations during the luteal phase of the menstrual cycle.
Materials and Methods Animals
Endogenous gonadotropin secretion was abolished in four female rhesus monkeys (5-8 kg BW) by sectioning the pituitary stalk. The pituitary stalk was approached transorbitally and transected under direct visualization, as described by Carmel et al. (7). A 4 X 7-mm oval piece of Teflon sheeting (0.4 mm thick) was inserted between the proximal and distal ends of the cut stalk to prevent revascularization of the pituitary gland and diffusion of endogenous GnRH from the median eminence to pituitary gonadotrophs (4). Surgery was performed under pentobarbital anesthesia. Immediately before surgery, animals received 50 mg hydrocortisone (Solucortef, Upjohn, Kalamazoo, MI) and 500 mg cefamandol nafate (Mandol, Eli Lilly, Indianapolis, IN) as prophylactic antibiotic therapy. During the immediate postoperative period, animals received cortisone acetate (Cortone, Eli Lilly Co., Indianapolis, IN; 10 mg/day, im); the dosage was then tapered to 3 mg/kg, im, during the remainder of the study period. Pituitary stalk transections were considered complete if the animals failed to maintain progesterone production after exogenous GnRH replacement therapy was discontinued during the luteal phase (see Results). Pituitary stalk-sectioned monkeys were fitted with two jugular or femoral catheters, which were exteriorized through a head cap and connected to a swivel mounted to the cage roof, as previously described (8). Synthetic GnRH was dissolved in 0.01 M glacial acetic acid in 0.9% NaCl (1 mg/ml), diluted further as described below, and infused through one catheter as a 6-min pulse. The infusion rate was controlled by an eightchannel peristaltic pump (Minipulse 2, Gilson Medical Electronics, Inc., Middleton, WI). The pulse frequency was regulated by a programable electronic timer (Chontrol DIL-4, Lindberg Enterprises, Inc., San Diego, CA). Blood samples were withdrawn from the other chronically implanted venous catheter. In addition to the four animals with pituitary stalk sections, we studied one prepubertal female rhesus monkey (8 months old) with an intact hypothalamic-pituitary axis. Because ovarian cyclicity can be initiated in prepubertal monkeys by pulsatile GnRH treatment (9), we reasoned that prepubertal animals may serve as another model of a GnRH-deficient state such as that of hypothalamic-lesioned or pituitary stalk-sectioned animals, but would not require extensive neurosurgery as did the latter animals. The responses of the prepubertal animal were
Endo • 1990 Vol 126 • No 5
identical to those of the stalk-sectioned animals, and data obtained from this animal are included in the analysis. Pituitary stalk-sectioned monkeys exhibit elevated plasma PRL concentrations (10). In some hyperprolactinemic animals, low plasma progesterone concentrations have been shown to persist beyond the time of typical luteolysis and delay the onset of menstruation (11). Weekly im injections of bromocriptine (14-16 mg; 2-bromo-a-ergocriptine, CB-154 mesylate, Sandoz Pharmaceuticals, East Hanover, NJ) were shown to suppress PRL concentrations to the normal range in hyperprolactinemic monkeys and prevent the persistent production of progesterone beyond the normal lifespan of the corpus luteum. To avoid the possibility of persistent production of progesterone in response to hyperprolactinemia, all stalk-sectioned monkeys in the current study received weekly bromocriptine treatment to maintain plasma PRL concentrations within the normal range (11). Previous studies using GnRH-driven monkeys have not shown any difference in luteal phase duration or progesterone concentrations in untreated normoprolactinemic animals or hyperprolactinemic animals treated with CB-154 (5, 6, 11). Experimental design Folliculogenesis and ovulation were reestablished in all monkeys by administering exogenous GnRH at a frequency of one 6-min pulse per h at the standard dosage of 1 /ig GnRH/min (5, 6). Daily blood samples were obtained between 0800-0930 h, and ovulation was confirmed by the rapid fall in preovulatory estradiol levels and the appearance of plasma progesterone in excess of 1 ng/ml within 2 days of the fall in estrogen concentrations. Day 1 of the luteal phase was defined as the day on which serum estradiol concentrations fell from preovulatory values to less than 75 pg/ml (5). Once an individual animal was confirmed to have ovulatory menstrual cycles with the standard GnRH regimen (6 /ttg/pulse at a frequency of one pulse per h), the following experimental protocol was followed: cycle 1, terminate standard GnRH infusion on days 5-6 of the luteal phase to confirm the lack of endogenous GnRH secretion; cycle 2, maintenance of the standard GnRH regimen (6 ^g/pulse; one pulse per h) throughout the entire luteal phase (control cycle); cycle 3, reduce the amount of GnRH delivered per pulse to l/250th the standard dosage (24 ng/pulse; one pulse per h) on days 4-5 of the luteal phase; cycle 4, reduce the amount of GnRH delivered per pulse to l/750th the standard dosage (8 ng/pulse; one pulse per h) on days 4-5 of the luteal phase; and cycle 5, terminate the standard GnRH infusion on days 5-6 of the luteal phase to reconfirm the absence of endogenous GnRH secretion. All animals included in the analysis underwent at least one complete series of studies. Daily blood samples were collected between 0800-0930 h, and plasma was stored at -20 C until assayed for progesterone and LH concentrations. Luteal phase length is defined as the number of days between the midcycle estrogen peak (day 0) and the day of the onset of menses, which was checked twice daily. Hormone measurements Plasma estradiol and progesterone concentrations were measured by RIAs, as described previously (12). Plasma LH
The Endocrine Society. Downloaded from press.endocrine.org by [${individualUser.displayName}] on 22 December 2016. at 19:23 For personal use only. No other uses without permission. . All rights reserved.
LH AND LUTEAL FUNCTION concentrations were measured by bioassay, using the mouse interstitial cell assay of Van Damme et al. (13), which yields parallel dose-response curves when mouse and monkey standard LH preparations are used (14). Plasma samples, a serum pool from hypophysectomized animals, a serum pool from an intact male, and LH standard (WDP-XV-20, described in Ref. 15) were diluted 1:1 (vol/vol) with 12% polyethylene glycol, incubated at 4 C for 30 min, and centrifuged at 900 X g for 5 min, as described by Jai and Hsueh (16), and supernatants were saved for bioassay. All unknown samples and serum pools were run in triplicate at dose levels of 10 and 15 ^1/300 ^1 assay volume containing an interstitial cell fraction from 6-week-old male Swiss mice (Hilltop Laboratories, Scottdale, PA) in Eagle's Minimum Essental Medium with Earle's salts, 2 mM Lglutamine, and 2% calf serum (Gibco, Grand Island, NY). LH standards were run in triplicate at the beginning of each assay and in duplicate at the end of each assay to control for potential assay drift. Samples were incubated for 3 h at 34 C, after which the assay was terminated by boiling for 20 min. One milliliter of 0.1 M PBS and 1% BSA, pH 7.2, were added to each tube, and 50-/xl aliquots were assayed for testosterone content by RIA (17). LH concentrations in unknown samples and standard serum pools were calculated from the testosterone production rates in response to known concentrations of the macaque LH standard. The intra- and interassay coefficients of variation of a serum pool that measured 11.8 ng/ml were 9.2% and 20.6%, respectively, for 37 assays. Statistics Differences in serum LH concentrations and serum progesterone concentrations between control cycles, GnRH stop cycles, and GnRH reduction cycles were analyzed for statistical significance by unpaired t tests (18) on values from consecutive days of the luteal phase after alteration of the GnRH infusion.
Results GnRH: stop studies Figure 1 illustrates serum bioactive LH concentrations in rhesus monkeys during the luteal phase of control menstrual cycles during which animals were maintained on a fixed GnRH pulse frequency of one pulse per h and menstrual cycles in which the pulsatile GnRH infusion was terminated during the early luteal phase (days 5-6 after ovulation). Serum LH concentrations were significantly (P < 0.05) reduced on the first day after termination of the GnRH infusion and remained so throughout the period of GnRH withdrawal. Figure 2 illustrates the pattern of serum progesterone concentrations during control cycles and cycles in which the pulsatile GnRH infusion was terminated during the early luteal phase. Serum progesterone concentrations were significantly (P < 0.01) depressed by the second day after the termination of the GnRH infusion, fell to undetectable concentrations (
Vol. 126, No. 5 Printed in U.S.A.
Effect of Reduced Luteinizing Hormone Concentrations on Corpus Luteum Function during the Menstrual Cycle of Rhesus Monkeys* ANTHONY J. ZELEZNIK AND LYNDA L. LITTLE-IHRIG Departments of Physiology and Obstetrics and Gynecology, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania 15261
ABSTRACT. To further define the relationship between plasma LH concentrations and progesterone secretion by the primate corpus luteum, we examined luteal function in rhesus monkeys in response to reduced LH concentrations during the luteal phase of the menstrual cycle. Five anovulatory rhesus monkeys received a pulsatile infusion of synthetic GnRH (6 fig/ pulse; one pulse per h, iv) to restore menstrual cyclicity. During the early luteal phase (4-5 days after ovulation), the amount of GnRH administered per pulse was reduced to l/250th or l/750th of the standard GnRH infusion regimen. Plasma LH concentrations, determined by bioassay, were reduced by approximately 50% during cycles maintained by reduced GnRH concentrations compared with the standard GnRH dosage. Serum progesterone concentrations were maintained for 5-6 days after GnRH reduction and declined thereafter, and premature menstruations were
observed in four of seven cycles maintained by the l/250th GnRH reduction and four of six cycles maintained with the 1/ 750th GnRH reduction. These results are consistent with the hypothesis that luteal regression during the nonfertile menstrual cycles of primates is due primarily to an alteration in luteal cell responsiveness to LH, rather than a reduction in the gonadotropic drive to the corpus luteum per se. When plasma LH concentrations were reduced during the early luteal phase to values below those found during the onset of luteal regression in control cycles, luteal function was maintained for 5-6 days. However, as the luteal phase progressed, the reduced LH concentrations were unable to sustain progesterone secretion, and premature menses occurred in some, but not all, animals. (Endocrinology 126: 22372244,1990)
T
HE PRIMATE corpus luteum of nonfertile menstrual cycles produces progesterone for a finite 14to 16-day interval, after which steroid secretion ceases, and the tissue undergoes regression and resorption from the ovary. Although in vitro studies have convincingly demonstrated that the secretion of progesterone by luteal cells is directly regulated by LH (1, 2), the extent to which changes in the pattern of LH secretion in vivo are responsible for the cessation of progesterone secretion and luteal regression at the termination of nonfertile cycles is less clear. Our laboratory has investigated the role of gonadotropins in the secretion of progesterone and the maintenance of the lifespan of the corpus luteum in macaque monkeys, with particular emphasis on determining whether there is a causal relationship between the pattern of gonadotropin secretion during the luteal phase and regression of the corpus luteum. For this purpose we Received November 6,1989. Address all correspondence and requests for reprints to: Anthony J. Zeleznik, Ph.D., Department of Obstetrics and Gynecology, University of Pittsburgh School of Medicine, Magee Womens Hospital, Forbes Avenue at Halket Street, Pittsburgh, Pennsylvania 15213. * This work was supported by NIH Grants HD-16842 and HD-08610 and Research Career Development Award 00531 (to A.J.Z.).
have used rhesus monkeys rendered anovulatory by either placement of radiofrequency lesions in the medial basal hypothalamus or transection of the pituitary stalk, procedures that interrupt the delivery of endogenous GnRH to the pituitary gland (3, 4). Ovulatory menstrual cycles can be restored in these animals by iv infusion of exogenous GnRH in a pulsatile manner. Because gonadotropin secretion in these animals is governed solely by the administration of exogenous GnRH, the absolute pattern of gonadotropin secretion in these animals can be controlled directly by the pattern of administration of exogenous GnRH. Using the above model system, we have demonstrated previously that although the secretion of progesterone by the corpus luteum is absolutely dependent upon LH throughout the entire luteal phase of the menstrual cycle (5), a reduction in LH pulse frequency like such as seen during the mid- to late luteal phase of the menstrual cycle does not directly cause luteal regression (6). Thus, a premature reduction in LH pulse frequency 3 days after ovulation was followed by a pattern of progesterone secretion identical to that observed in spontaneous menstrual cycles, and timely menstruation ensued, suggesting that luteal regression is more likely due to changes in
2237
The Endocrine Society. Downloaded from press.endocrine.org by [${individualUser.displayName}] on 22 December 2016. at 19:23 For personal use only. No other uses without permission. . All rights reserved.
LH AND LUTEAL FUNCTION
2238
luteal cell sensitivity to LH than to changes in the pattern of LH secretion per se. In the current study we sought to determine whether a reduction in LH concentrations to values below those normally encountered during the luteal phase of the menstrual cycle would provoke rapid luteal regression. For this purpose, anovulatory rhesus monkeys received pulsatile infusions of synthetic GnRH at concentrations that were reduced compared to the standard GnRH infusion dosage in an attempt to reduce LH concentrations during the luteal phase of the menstrual cycle.
Materials and Methods Animals
Endogenous gonadotropin secretion was abolished in four female rhesus monkeys (5-8 kg BW) by sectioning the pituitary stalk. The pituitary stalk was approached transorbitally and transected under direct visualization, as described by Carmel et al. (7). A 4 X 7-mm oval piece of Teflon sheeting (0.4 mm thick) was inserted between the proximal and distal ends of the cut stalk to prevent revascularization of the pituitary gland and diffusion of endogenous GnRH from the median eminence to pituitary gonadotrophs (4). Surgery was performed under pentobarbital anesthesia. Immediately before surgery, animals received 50 mg hydrocortisone (Solucortef, Upjohn, Kalamazoo, MI) and 500 mg cefamandol nafate (Mandol, Eli Lilly, Indianapolis, IN) as prophylactic antibiotic therapy. During the immediate postoperative period, animals received cortisone acetate (Cortone, Eli Lilly Co., Indianapolis, IN; 10 mg/day, im); the dosage was then tapered to 3 mg/kg, im, during the remainder of the study period. Pituitary stalk transections were considered complete if the animals failed to maintain progesterone production after exogenous GnRH replacement therapy was discontinued during the luteal phase (see Results). Pituitary stalk-sectioned monkeys were fitted with two jugular or femoral catheters, which were exteriorized through a head cap and connected to a swivel mounted to the cage roof, as previously described (8). Synthetic GnRH was dissolved in 0.01 M glacial acetic acid in 0.9% NaCl (1 mg/ml), diluted further as described below, and infused through one catheter as a 6-min pulse. The infusion rate was controlled by an eightchannel peristaltic pump (Minipulse 2, Gilson Medical Electronics, Inc., Middleton, WI). The pulse frequency was regulated by a programable electronic timer (Chontrol DIL-4, Lindberg Enterprises, Inc., San Diego, CA). Blood samples were withdrawn from the other chronically implanted venous catheter. In addition to the four animals with pituitary stalk sections, we studied one prepubertal female rhesus monkey (8 months old) with an intact hypothalamic-pituitary axis. Because ovarian cyclicity can be initiated in prepubertal monkeys by pulsatile GnRH treatment (9), we reasoned that prepubertal animals may serve as another model of a GnRH-deficient state such as that of hypothalamic-lesioned or pituitary stalk-sectioned animals, but would not require extensive neurosurgery as did the latter animals. The responses of the prepubertal animal were
Endo • 1990 Vol 126 • No 5
identical to those of the stalk-sectioned animals, and data obtained from this animal are included in the analysis. Pituitary stalk-sectioned monkeys exhibit elevated plasma PRL concentrations (10). In some hyperprolactinemic animals, low plasma progesterone concentrations have been shown to persist beyond the time of typical luteolysis and delay the onset of menstruation (11). Weekly im injections of bromocriptine (14-16 mg; 2-bromo-a-ergocriptine, CB-154 mesylate, Sandoz Pharmaceuticals, East Hanover, NJ) were shown to suppress PRL concentrations to the normal range in hyperprolactinemic monkeys and prevent the persistent production of progesterone beyond the normal lifespan of the corpus luteum. To avoid the possibility of persistent production of progesterone in response to hyperprolactinemia, all stalk-sectioned monkeys in the current study received weekly bromocriptine treatment to maintain plasma PRL concentrations within the normal range (11). Previous studies using GnRH-driven monkeys have not shown any difference in luteal phase duration or progesterone concentrations in untreated normoprolactinemic animals or hyperprolactinemic animals treated with CB-154 (5, 6, 11). Experimental design Folliculogenesis and ovulation were reestablished in all monkeys by administering exogenous GnRH at a frequency of one 6-min pulse per h at the standard dosage of 1 /ig GnRH/min (5, 6). Daily blood samples were obtained between 0800-0930 h, and ovulation was confirmed by the rapid fall in preovulatory estradiol levels and the appearance of plasma progesterone in excess of 1 ng/ml within 2 days of the fall in estrogen concentrations. Day 1 of the luteal phase was defined as the day on which serum estradiol concentrations fell from preovulatory values to less than 75 pg/ml (5). Once an individual animal was confirmed to have ovulatory menstrual cycles with the standard GnRH regimen (6 /ttg/pulse at a frequency of one pulse per h), the following experimental protocol was followed: cycle 1, terminate standard GnRH infusion on days 5-6 of the luteal phase to confirm the lack of endogenous GnRH secretion; cycle 2, maintenance of the standard GnRH regimen (6 ^g/pulse; one pulse per h) throughout the entire luteal phase (control cycle); cycle 3, reduce the amount of GnRH delivered per pulse to l/250th the standard dosage (24 ng/pulse; one pulse per h) on days 4-5 of the luteal phase; cycle 4, reduce the amount of GnRH delivered per pulse to l/750th the standard dosage (8 ng/pulse; one pulse per h) on days 4-5 of the luteal phase; and cycle 5, terminate the standard GnRH infusion on days 5-6 of the luteal phase to reconfirm the absence of endogenous GnRH secretion. All animals included in the analysis underwent at least one complete series of studies. Daily blood samples were collected between 0800-0930 h, and plasma was stored at -20 C until assayed for progesterone and LH concentrations. Luteal phase length is defined as the number of days between the midcycle estrogen peak (day 0) and the day of the onset of menses, which was checked twice daily. Hormone measurements Plasma estradiol and progesterone concentrations were measured by RIAs, as described previously (12). Plasma LH
The Endocrine Society. Downloaded from press.endocrine.org by [${individualUser.displayName}] on 22 December 2016. at 19:23 For personal use only. No other uses without permission. . All rights reserved.
LH AND LUTEAL FUNCTION concentrations were measured by bioassay, using the mouse interstitial cell assay of Van Damme et al. (13), which yields parallel dose-response curves when mouse and monkey standard LH preparations are used (14). Plasma samples, a serum pool from hypophysectomized animals, a serum pool from an intact male, and LH standard (WDP-XV-20, described in Ref. 15) were diluted 1:1 (vol/vol) with 12% polyethylene glycol, incubated at 4 C for 30 min, and centrifuged at 900 X g for 5 min, as described by Jai and Hsueh (16), and supernatants were saved for bioassay. All unknown samples and serum pools were run in triplicate at dose levels of 10 and 15 ^1/300 ^1 assay volume containing an interstitial cell fraction from 6-week-old male Swiss mice (Hilltop Laboratories, Scottdale, PA) in Eagle's Minimum Essental Medium with Earle's salts, 2 mM Lglutamine, and 2% calf serum (Gibco, Grand Island, NY). LH standards were run in triplicate at the beginning of each assay and in duplicate at the end of each assay to control for potential assay drift. Samples were incubated for 3 h at 34 C, after which the assay was terminated by boiling for 20 min. One milliliter of 0.1 M PBS and 1% BSA, pH 7.2, were added to each tube, and 50-/xl aliquots were assayed for testosterone content by RIA (17). LH concentrations in unknown samples and standard serum pools were calculated from the testosterone production rates in response to known concentrations of the macaque LH standard. The intra- and interassay coefficients of variation of a serum pool that measured 11.8 ng/ml were 9.2% and 20.6%, respectively, for 37 assays. Statistics Differences in serum LH concentrations and serum progesterone concentrations between control cycles, GnRH stop cycles, and GnRH reduction cycles were analyzed for statistical significance by unpaired t tests (18) on values from consecutive days of the luteal phase after alteration of the GnRH infusion.
Results GnRH: stop studies Figure 1 illustrates serum bioactive LH concentrations in rhesus monkeys during the luteal phase of control menstrual cycles during which animals were maintained on a fixed GnRH pulse frequency of one pulse per h and menstrual cycles in which the pulsatile GnRH infusion was terminated during the early luteal phase (days 5-6 after ovulation). Serum LH concentrations were significantly (P < 0.05) reduced on the first day after termination of the GnRH infusion and remained so throughout the period of GnRH withdrawal. Figure 2 illustrates the pattern of serum progesterone concentrations during control cycles and cycles in which the pulsatile GnRH infusion was terminated during the early luteal phase. Serum progesterone concentrations were significantly (P < 0.01) depressed by the second day after the termination of the GnRH infusion, fell to undetectable concentrations (
