Pulsatile nature of luteinizing hormone release is maintained during luteinizing hormone-releasing hormone stimulation in normal male rats

The plasma LH concentration is believed to be reasonably steady in normal male rats. We found that LH is released in a regular pulsatile fashion. The overall mean concentration of plasma LH in normal male rats was 46.6 +/- 4.4 (mean +/- SEM) ng/ml. The normal male rats showed periodic LH pulses: the mean pulse amplitude was 144.4 +/- 25.5 ng/ml and the inter-peak interval was 22.5 +/- 2.0 min. Each pulse lasted 9.7 +/- 0.8 min. When LH-RH (1 microgram/kg) was injected as a bolus, the peak concentration was attained in 10-30 min reaching a peak concentration of 279.4 +/- 39.6 ng/ml. Distinct pulsatile bursts of plasma LH were discernible during the period of elevated plasma LH concentration. When a higher dose of LH-RH (5 micrograms/kg) was administered, the LH concentration slowly increased to a peak concentration of 400.2 +/- 38.7 ng/ml in 20-40 min. The pulsatile nature of the LH concentration was recognizable with distinct bursts. We have observed that: (a) normal male rats release LH in a pulsatile fashion with an approximate 20-min inter-peak interval; (b) mean LH pulses last less than 10 min, and (c) the LH pulses are visible even with elevated LH and LH-RH concentrations in the general circulation.     

Effects of acutely increased plasma testosterone concentrations on pulsatile luteinizing hormone secretion in the male rat

To assess the role of testosterone (T) in regulating the minute-to-minute release of pulsatile luteinizing hormone (LH) secretion in the adult male rat, we investigated the negative feedback of acute increases in plasma T concentrations on pulsatile LH secretion in acutely castrated male rats. At the time of castration, we implanted T-filled Silastic capsules, s.c., which maintained plasma T concentrations at approximately 1.8 ng/ml and suppressed LH pulses. On the next day, the capsules were removed; blood sampling (every 6 min) was started 8 h after implant removal, thereby allowing LH pulses to be reinitiated. Immediately following a control bleeding interval of 2 h, either T or vehicle alone was infused s.c., and blood sampling continued for another 4 h. In animals receiving vehicle alone, LH pulse frequency and mean LH levels increased over the 6 h bleeding period. The administration of 200 ng T/min caused a rapid rise in plasma T concentrations of about 4 ng/ml ("physiological") and prevented the increase in pulse frequency that occurred in the control group; it did not, however, reduce pulse frequency over the 4 h infusion period. When T was infused at the rate of 400 ng/ml, plasma T concentrations rose to approximately 18 ng/ml ("supraphysiological") and LH pulse frequency was significantly reduced, but not completely inhibited, during the last 2 h of the infusion. The pulse amplitude of luteinizing hormone did not change significantly in any of the groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of gonadotropin-releasing hormone pulse-frequency modulation on luteinizing hormone, follicle-stimulating hormone and testosterone secretion in hypothalamo/pituitary-disconnected rams

The effects of changes in pulse frequency of exogenously infused gonadotropin-releasing hormone (GnRH) were investigated in 6 adult surgically hypothalamo/pituitary-disconnected (HPD) gonadal-intact rams. Ten-minute sampling in 16 normal animals prior to HPD showed endogenous luteinizing hormone (LH) pulses occurring every 2.3 h with a mean pulse amplitude of 1.11 +/- 0.06 (SEM) ng/ml. Mean testosterone and follicle-stimulating hormone (FSH) concentrations were 3.0 +/- 0.14 ng/ml and 0.85 +/- 0.10 ng/ml, respectively. Before HPD, increasing single doses of GnRH (50-500 ng) elicited a dose-dependent rise of LH, 50 ng producing a response of similar amplitude to those of spontaneous LH pulses. The effects of varying the pulse frequency of a 100-ng GnRH dose weekly was investigated in 6 HPD animals; the pulse intervals explored were those at 1, 2, and 4 h. The pulsatile GnRH treatment was commenced 2-6 days after HPD when plasma testosterone concentrations were in the castrate range (less than 0.5 ng/ml) in all animals. Pulsatile LH and testosterone secretion was reestablished in all animals in the first 7 days by 2-h GnRH pulses, but the maximal pulse amplitudes of both hormones were only 50 and 62%, respectively, of endogenous pulses in the pre-HPD state. The plasma FSH pattern was nonpulsatile and FSH concentrations gradually increased in the first 7 days, although not to the pre-HPD range. Increasing GnRH pulse frequency from 2- to 1-hour immediately increased the LH baseline and pulse amplitude. As testosterone concentrations increased, the LH responses declined in a reciprocal fashion between Days 2 and 7. FSH concentration decreased gradually over the 7 days at the 1-h pulse frequency. Slowing the GnRH pulse to a 4-h frequency produced a progressive fall in testosterone concentrations, even though LH baselines were unchanged and LH pulse amplitudes increased transiently. FSH concentrations were unaltered during the 4-h regime. These results show that 1) the pulsatile pattern of LH and testosterone secretion in HPD rams can be reestablished by exogenous GnRH, 2) the magnitude of LH, FSH, and testosterone secretion were not fully restored to pre-HPD levels by the GnRH dose of 100 ng per pulse, and 3) changes in GnRH pulse frequency alone can influence both gonadotropin and testosterone secretion in the HPD model.     

Effect of luteinizing hormone releasing hormone pulse characteristics on comparative luteinizing hormone and follicle stimulating hormone secretion from superfused rat anterior pituitary cell cultures

We have shown that 4 ng luteinizing hormone releasing hormone (LHRH) pulses induced significantly greater luteinizing hormone (LH) release from proestrous rat superfused anterior pituitary cells with no cycle related differences in follicle stimulating hormone (FSH). Current studies gave 8 ng LHRH in various pulse regimens to study amplitude, duration and frequency effects on LH and FSH secretion from estrous 0800, proestrous 1500 and proestrous 1900 cells. Regimen 1 gave 8 ng LHRH as a single bolus once/h; regimen 2 divided the 8 ng into 3 equal 'minipulses' given at 4 min intervals to extend duration; regimen 3 gave the 3 'minipulses' at 10 min intervals, thereby further extending duration: regimen 4 was the same as regimen 2, except that the 3 'minipulses' were given at a pulse frequency of 2 h rather than 1 h. In experiment 1, all four regimens were employed at proestrus 1900. FSH was significantly elevated by all 8 ng regimens as compared to 4 ng pulses; further, 8 ng divided into 3 equal 'minipulses' separated by 4 min at 1 and 3 h frequencies (regimens 2 and 4) resulted in FSH secretion that was significantly greater than with either a single 8 ng bolus (regimen 1) or when the 'minipulses' were separated by 10 min (regimen 3). In experiment 2, at proestrus 1500, FSH response to the second pulse of regimen 4 was significantly greater than in regimen 2; LH release was significantly suppressed at pulse 2 compared to regimen 2 accentuating divergent FSH secretion. At estrus 0800, FSH response to the second pulse of regimen 4 was significantly stimulated FSH at proestrus 1900, 1500 and estrus 0800, FSH divergence was most marked at proestrus 1500. These data indicate a potential role for hypothalamic LHRH secretory pattern in inducing divergent gonadotropin secretion in the rat.     

Pituitary receptors for gonadotropin-releasing hormone in relation to changes in pituitary and plasma luteinizing hormone in ovariectomized-hypothalamo pituitary disconnected ewes. I. Effect of changing frequency of gonadotropin-releasing hormone pulses

Studies were undertaken to determine if changes in the amplitude of luteinizing hormone (LH) pulses that occur in response to changes in the frequency of gonadotropin-releasing hormone (GnRH) pulses are due to an alteration in the number of GnRH receptors. Ewes were ovariectomized (OVX) and the hypothalamus was disconnected from the pituitary (HPD). Ewes were then given pulses of GnRH at a frequency of 1/h or 1/3 h. Two control groups were included: OVX ewes not subjected to HPD, and HPD ewes that were not OVX. At the end of one week of treatment, blood samples were collected to determine the amplitude of LH pulses. The treated ewes were killed just before the next scheduled pulse of GnRH, and the content of LH and number of GnRH receptors were measured in each pituitary. The amplitude of LH pulses was highly correlated with the amount of LH in the pituitary gland (r = 0.71, p less than 0.01), and both LH content and pulse amplitude (mean + SEM) were higher in ewes receiving GnRH once per 3 h (189.7 +/- 39.3 microgram/pituitary, 10.3 +/- 1.1 ng/ml, respectively) than in ewes receiving GnRH once per h (77.8 +/- 11.4 microgram/pituitary, 5.2 +/- 1.3 ng/ml). The pituitary content of LH was highest in the OVX ewes (260.2 +/- 57.4 micrograms/pituitary) and lowest in the nonpulsed HPD ewes (61.7 +/- 51.2 micrograms/pituitary). The number of GnRH receptors was similar in all groups, and was not correlated with any other variable.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pituitary responsiveness to luteinizing hormone-releasing hormone in prepubertal and postpubertal male ferrets

The pituitary response to three different doses of exogenously administered LHRH was examined in prepubertal (9-wk-old) and postpubertal (32-wk-old) male ferrets. The doses of 5, 10, and 15 ng LHRH/kg body weight tested in this study produced dose-related increases in circulating LH concentrations in both pre- and postpubertal groups. In addition, a significant effect of age on LH response was observed, with the prepubertal animals demonstrating significantly greater serum LH values in response to the two higher doses than the postpubertal males. Prepubertal ferrets also exhibited a significant increase in endogenous LH pulse amplitude in sampling periods following exogenous administration of LHRH compared to baseline pulse amplitudes in periods prior to the LHRH infusions. These results suggest that the low frequency of endogenous LH pulses previously observed in prepubertal ferrets is not due to unresponsiveness of the pituitary gland to LHRH. Thus, suppression of the hypothalamo-hypophyseal axis observed in the prepubertal ferret is probably mediated at the level of the hypothalamus.     

Changes in circulating levels of immunoreactive follicle stimulating hormone, luteinizing hormone, and testosterone during sexual development in the rhesus monkey, Macaca mulatta

Basal serum levels of follicle stimulating hormone (FSH), luteinizing hormone (LH), and testosterone (T) and the responsiveness of these hormones to a challenge dose of luteinizing hormone releasing hormone (LHRH), were determined in juvenile, pubertal, and adult rhesus monkeys. The monkey gonadotrophins were analyzed using RIA reagents supplied by the World Health Organization (WHO) Special Programme of Human Reproduction. The FSH levels which were near the assay sensitivity in immature monkeys (2.4 +/- 0.8 ng/ml) showed a discernible increase in pubertal animals (6.4 +/- 1.8 ng/ml). Compared to other two age groups, the serum FSH concentration was markedly higher (16.1 +/- 1.8 ng/ml) in adults. Serum LH levels were below the detectable limits of the assay in juvenile monkeys but rose to 16.2 +/- 3.1 ng/ml in pubertal animals. When compared to pubertal animals, a two-fold increase in LH levels paralleled changes in serum LH during the three developmental stages. Response of serum gonadotrophins and T levels to a challenge dose of LHRH (2.5 micrograms; i.v.) was variable in the different age groups. The present data suggest: an asynchronous rise of FSH and LH during the pubertal period and a temporal correlation between the testicular size and FSH concentrations; the challenge dose of LHRH, which induces a significant rise in serum LH and T levels, fails to elicit an FSH response in all the three age groups; and the pubertal as compared to adult monkeys release significantly larger quantities of LH in response to exogenous LHRH.     

Testosterone modulates the differential release of luteinizing hormone and follicle-stimulating hormone that occurs in response to changing gonadotropin-releasing hormone pulse frequency in the male monkey, Macaca fascicularis

Selective elevations of plasma follicle-stimulating hormone (FSH) levels are characteristic of some physiological conditions, such as the early stages of human puberty, and in some disorders of testicular function, such as idiopathic oligospermia. We tested the hypotheses that a slow gonadotropin-releasing hormone (GnRH) pulse frequency favors a selective elevation of plasma FSH and that this is influenced by the circulating steroidal milieu. We administered exogenous GnRH at frequencies of once every 90 min (q 90 min) and once every 240 min (q 240 min) to castrated prepubertal male monkeys who had received either empty (sham) or testosterone (T)-filled Silastic capsules at the time of castration. At the end of each experimental frequency period, mean plasma levels of luteinizing hormone (LH) and FSH were measured. Plasma T levels were also measured. Animals with T implants had plasma levels of this hormone that were in the adult range (approximately equal to 8 ng/ml), whereas those with sham implants had plasma T levels in the prepubertal range (less than or equal to 4 ng/ml). In animals with sham implants, mean plasma FSH levels were markedly elevated at the slower GnRH pulse frequency (39.5 +/- 3.6 ng/ml following GnRH q 240 min compared with 23.7 +/- 2.8 ng/ml following GnRH q 90 min). This selective FSH elevation was not apparent in animals with T implants. Mean plasma LH levels were similar (approximately equal to 8 micrograms/ml) at the two GnRH pulse frequencies, in both T-treated and sham-implanted animals.(ABSTRACT TRUNCATED AT 250 WORDS)     

Photoperiod and luteinizing hormone secretion in domestic and wild pigs

During seasonal anoestrus (long-days), oestradiol can effectively inhibit the pulsatile secretion of luteinizing hormone (LH) in sheep. The aim of our trial was to determine whether the same regulatory mechanism exists in the pig. Altogether, 20 ovariectomized and oestradiol-implanted gilts (16 domestic pigs, 4 European wild boars) were randomly allocated to two treatment groups. The first group was kept under a short-day light-dark cycle of 8L:16D, and the second group under a long-day light regime of 16L:8D. After a 6-week treatment period, blood samples were taken at 20-min intervals for 12h. After sampling, the light regimens were switched. Sampling was then repeated following another 6 weeks of treatment. In both treatment groups, 2.3 LH pulses occurred every 12h. The basal LH level was 0.7+/-0.4 ng/ml for the short-day group and 1.0+/-0.5 ng/ml for the long-day group. The mean LH level was 0.9+/-0.4 and 1.3+/-0.6 ng/ml and the LH pulse amplitude 0.5+/-0.4 and 0.6+/-0.5 ng/ml, respectively. The basal and mean LH levels were therefore lower in short-day gilts (P<0.05), while LH pulse amplitude and frequency remained unaffected by treatment. In conclusion, the 6-week period under two different light regimes resulted in higher basal LH concentration in long-day gilts but was not able to produce changes in LH frequency in prepubertal gilts.     

Effects of pulsatile infusion of luteinizing hormone-releasing hormone on luteinizing hormone secretion and ovarian function in hypophysial stalk-transected beef heifers

Hypothalamic regulation of luteinizing hormone (LH) secretion and ovarian function were investigated in beef heifers by infusing LH-releasing hormone (LHRH) in a pulsatile manner (1 microgram/ml; 1 ml during 1 min every h) into the external jugular vein of 10 hypophysial stalk-transected (HST) animals. The heifers were HST approximately 30 mo earlier. All heifers had increased ovarian size during the LHRH infusion. The maximum ovarian size (16 +/- 2.7 cm3) was greater (P less than 0.01) than the initial ovarian size (8 +/- 1.4 cm3). Ovarian follicular growth occurred in 4 of 10 HST heifers in response to pulsatile LHRH infusion. In 2 heifers, an ovarian follicle developed to preovulatory size, but ovulation occurred in only 1 animal after the frequency of LHRH was increased (1 microgram every 20 min during 8 h). In blood samples obtained at 20-min intervals every 5th day, LH concentrations in peripheral serum remained consistently low (0.9 ng/ml) and nonepisodic in the 10 HST heifers during infusion of vehicle on the day before beginning LHRH. In 7 of 10 HST animals, episodic LH secretion occurred in response to pulsatile infusion of LHRH. In 3 of these long-term HST heifers, however, serum LH remained at basal levels and the isolated pituitary seemingly was unresponsive to pulsatile infusion of LHRH as indicated by sequential patterns of gonadotropin secretion obtained at 5-day intervals. These results indicate that pulsatile infusion of LHRH induces LH release in HST beef heifers.     

Simultaneous measurement of luteinizing hormone-releasing hormone and luteinizing hormone during estradiol-induced luteinizing hormone surges in the ovariectomized ewe

Sequential bleeding and push-pull perfusion of the hypothalamus were used to characterize luteinizing hormone (LH) and LH-releasing hormone (LHRH) release in ovariectomized (OVX) ewes after injection of corn oil or estradiol benzoate (EB). Push-pull cannulae were surgically implanted into the stalk median eminences of 24 OVX ewes. Seven to 14 days later each of 20 animals was given an i.m. injection of 50 micrograms EB. Blood samples and push-pull perfusate were collected at 10-min intervals for 6-12 h beginning 12-15 h after EB injection. Four OVX ewes were given i.m. injections of corn oil 7 days after implantation of push-pull cannulae. Blood samples and push-pull perfusate were collected at 10-min intervals for 4 h between 18 and 22 h after injection of corn oil. Luteinizing hormone remained below 2 ng/ml throughout most of the sampling periods in 9 of 20 EB-treated ewes. In 5 of these 9 LHRH also was undetectable, whereas in 4 LHRH was detectable (1.84 +/- 0.29 pg/10 min), but did not increase with time. Preovulatory-like surges of LH occurred in 11 EB-treated ewes, but LHRH was undetectable in 5. In 4 of 6 ewes showing LH surges and detectable LHRH, sampling occurred during the onset of the LH surge.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pulsatile infusion of luteinizing hormone-releasing hormone: Effects on luteinizing hormone secretion and ovarian function in prepuberal gilts

Ten intact and hypophysial stalk-transected (HST), prepuberal Yorkshire gilts, 112–160 days old, were subjected to a pulsatile infusion regimen of luteinizing hormone-releasing hormone (LHRH) to investigate secretion profiles of luteinizing hormone (LH) and ovarian function. A catheter was implanted in a common carotid artery and connected to an infusion pump and recycling timer, whereas an indwelling external jugular catheter allowed collection of sequential blood samples for radioimmunoassay of LH and progesterone. In a dose response study, intracarotid injection of 5 μg LHRH induced peak LH release (5.9 ± 0.65 ng/ml; mean ± SE) within 20 min, which was greater (P < 0.001) than during the preinjection period (0.7 ± 0.65 ng/ml). After HST, 5 μg LHRH elicited LH release in only one of three prepuberal gilts. Four intact animals were infused with 5 μg LHRH (in 0.1% gel phosphate buffer saline, PBS) in 0.5-ml pulses (0.1 ml/min) at 1.5-h intervals continuously during 12 days. Daily blood samples were obtained at 20-min intervals 1 h before and 5, 10, 20, 40, 60 and 80 min after one LHRH infusion. Plasma LH release occurred in response to pulsatile LHRH infusion during the 12-day period; circulating LH during 60 min before onset of LHRH infusion was 0.7 ± 0.16 ng/ml compared with 1.3 ± 0.16 ng/ml during 60 min after onset of infusion (P < 0.001). Only one of four intact gilts ovulated, however, in response to LHRH infusion. This animal was 159 days old, and successive estrous cycles did not recur after LHRH infusion was discontinued. Puberal estrus occurred at 252 ± 7 days in these gilts and was confirmed by plasma progesterone levels. These results indicate that intracarotid infusion of 5 μg LHRH elicits LH release in the intact prepuberal gilt, but this dosage is insufficient to cause a consistent response after HST.     

Influence of gonadotropin-releasing hormone pulse amplitude, frequency, and treatment duration on the regulation of luteinizing hormone (LH) subunit messenger ribonucleic acids and LH secretion

The effects of GnRH pulse amplitude, frequency, and treatment duration on pituitary alpha and LH beta subunit mRNA concentrations were examined in castrate-testosterone replaced male rats. Experimental groups received iv GnRH pulses (5, 25, or 125 ng) at 7.5-, 30-, or 120-min intervals for 8, 24, or 48 h. Saline pulses were given to control rats. Acute LH secretion was measured in blood drawn before and 20 min after the last GnRH pulse. In saline controls, alpha and LH beta mRNAs (150 +/- 14, 23 +/- 2 pg cDNA bound/100 micrograms pituitary DNA) fell to 129 +/- 14 and 18 +/- 2, respectively, after 48 h. In animals receiving GnRH pulses (7.5-min intervals), the 125-ng dose stimulated a slight increase (P less than 0.01) in alpha mRNA levels after 8 and 24 h and both LH subunit mRNAs were increased by the 25- and 125-ng doses after 48 h. The 30-min pulse interval injections (25- and 125-ng doses) increased LH beta mRNA levels after 8 h, but alpha mRNAs were not elevated until after 24 h. Maximum (3-fold) increases in alpha and LH beta mRNAs were seen in rats receiving 25-ng pulses every 30 min for 48 h. Using 120-min pulses, LH subunit mRNAs were not increased by any GnRH dose through 48 h. Acute LH release was not seen in rats receiving 5 ng GnRH pulses at any pulse interval.(ABSTRACT TRUNCATED AT 250 WORDS)     

Seasonal pattern of luteinizing hormone and testosterone pulsatile secretion in young adult red deer stags (Cervus elaphus) and its association with the antler cycle.

Blood from stages aged 15 months (n = 6) was sampled at monthly intervals every 30 min for 24 h for 12 months, at 45 degrees S in New Zealand. Three extra samplings each for 24 h were carried out at about the anticipated time of antler casting. All samples were analysed for luteinizing hormone (LH) and testosterone and the resulting data further analysed by the Pulsar pulse detection routine. The animals were kept indoors under natural daylength and were fed ad libitum. All animals were weighed, antler status and size recorded and testes diameter was measured on each sampling day. Mean LH and testosterone pulsatily and plasma concentration varied seasonally. LH pulse frequency was low during autumn (2.5 pulses in 24 h), winter (1.0-1.5 pulses in 24 h) and early spring (1 pulse in 24 h) and lowest in late spring (0.2 pulse in 24 h) before rising in summer (1.0-4.0 pulses in 24 h). LH pulse amplitude and mean plasma concentration were low (< 1 ng ml-1) from March to November (autumn-spring); both rose to a peak in January (summer) of 3.4 and 1.6 ng ml-1, respectively. Testosterone pulse frequency was generally similar to LH except that slightly more pulses of testosterone than of LH were detected from March to November and more pulses of LH from November to February (summer). Testosterone pulse amplitude fell from March to November (5.3 ng ml-1 to undetectable) although there was a conspicuous peak in July (midwinter) of almost 5 ng ml-1.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of pulse amplitude of luteinizing hormone, duration and rate of change on progesterone secretion from rat corpora lutea.

Corpora lutea (CL) were obtained from immature rats primed with pregnant mares' serum gonadotrophin followed by human chorionic gonadotrophin (hCG). Two days after hCG, CL were isolated, placed in perifusion culture and exposed to control medium or specific pulses of luteinizing hormone (LH). In Expt 1, a frequency of 1 pulse LH/h (amplitude 500 pg/ml, duration 40 min, 30 ng/min) increased progesterone secretion compared with control values (P less than 0.05). In Expt 2, LH rate was held constant and the amplitude and duration of a single LH pulse varied; 250 and 500 ng LH/ml initially stimulated progesterone secretion equivalently, but increasing the duration of the LH pulse prolonged high progesterone secretion. These observations suggest that at less than or equal to 500 ng LH/ml, once a stimulatory amplitude is obtained, higher amplitudes do not further increase progesterone secretion, while increasing pulse duration further enhances progesterone secretion. In Expt 3, the LH pulse amplitude was 250 ng/ml and the rate set at 0, 5 or 30 ng LH/min; only 30 ng LH/min resulted in sustained stimulation of progesterone (P less than 0.05). Taken together, these data demonstrate that the characteristics which determine whether an LH pulse will be stimulatory include not only amplitude and duration but also the rate at which an amplitude is obtained.     

The acute effect of bull presence on plasma profiles of luteinizing hormone in postpartum, anoestrous dairy cows

The objective of this study was to investigate whether bull exposure affects LH profiles in postpartum, anoestrous dairy cows. Eight cows between 10 and 17 days after parturition were used. On Day 1, blood samples were taken at 10 min intervals for 8 h. On Day 2, blood sampling continued at 10 min intervals and after 2 h a bull was introduced behind a fence, and blood sampling continued for another 8 h. Time of resumption of luteal activity was between 25 and more than 80 days after parturition for these animals and was not related (P>0.1) with frequency of LH pulses, amplitude of pulses and basal LH concentration on either Day 1 or Day 2. In 6 of the 8 cows, average and basal LH concentration were greater (P<0.001) during the 8 h of bull presence (0.56 +/- 0.33 and 0.39 +/- 0.26 ng/ml, respectively) compared to the 8 h without a bull (0.50 +/- 0.30 and 0.35 +/- 0.24 ng/ml, respectively). Pulse amplitude did not differ (P=0.85) between Day 2 (0.45 +/- 0.24 ng/ml) or Day 1 (0.45 +/- 0.14 ng/ml). LH pulse frequency was greater (P<0.1) on Day 2 (5.3 pulses/8h) compared to the Day 1 (4.6 pulses/8h). In conclusion, fenceline bull exposure early postpartum seems to have an acute effect on LH-release in anoestrous dairy cows. Whether sustained bull exposure can hasten first ovulation after calving through an effect on LH release in dairy cows is an interesting area of research.     

Effect of glucose availability on pulsatile luteinizing hormone release in rams before and after castration

The hypothesis tested was that availability of glucose modulates the control of luteinizing hormone (LH) release. A second objective was to determine the role of testicular hormones in the control of pulsatile LH secretion during depressed blood glucose. Serial blood samples were collected at 15 min intervals for 8 h from intact pubertal Suffolk rams (n = 8; 7 months old) on consecutive days (Days 1, 2 and 3). Rams were castrated after sampling on Day 3 and samples were collected 3 weeks later on consecutive days (Days 4, 5 and 6). Insulin (120 units, iv) was given at Hour 4 of each of the six days to lower blood glucose. On Days 1 and 4, no other treatments were given (Control). On Days 2 and 5, LH releasing hormone (LHRH; 5 ng/kg, iv) was given at Hours 5, 6 and 7 to assess the ability of the pituitary to release LH. On Days 3 and 6, N-methyl-D,L-aspartate (NMA; 5 mg/kg, iv) was given at Hours 5, 6 and 7 to assess the ability of the hypothalamus to release LHRH. Insulin reduced plasma glucose by 52% for at least 3 h (P < 0.001). Insulin reduced the mean LH concentration (P < 0.05) and tended to reduce the LH response area (P < 0.10) in castrated animals during the control period. LHRH increased LH pulse number (P < 0.001) in intact rams and increased mean LH concentration (P < 0.01), LH pulse amplitude (P < 0.05) and LH response area (P < 0.01) in castrated animals compared to respective control periods. NMA increased mean LH concentration in intact rams (P < 0.0001) but did not affect mean LH in castrates. NMA increased LH pulse number in rams (P < 0.0001) but decreased number of pulses in castrates (P < 0.0001) compared to control periods. NMA increased LH pulse amplitude in both intact (P < 0.001) and castrated animals (P < 0.05). In conclusion, these results support the hypothesis that blood glucose concentrations influence the control of LH release in sheep. In addition, LH release in response to the LHRH secretagogue, NMA, is positively influenced by testicular hormones.     

Luteinizing hormone secretion in hypophysial stalk-transected pigs given progesterone and pulsatile gonadotropin-releasing hormone

This study was conducted to determine whether progesterone inhibits luteinizing hormone (LH) secretion in female pigs by a direct action on the pituitary gland. Eight ovariectomized, hypophysial stalk-transected gilts were given 1-microgram pulses of gonadotropin-releasing hormone iv every 45 min from Day 0 to 12. On Days 5-12, each of four gilts received either progesterone or oil vehicle im at 12-hr intervals. Serum progesterone concentrations in steroid-treated gilts reached 70 +/- 6.8 ng/ml (mean +/- SE) by Day 8 and remained elevated thereafter, whereas serum progesterone concentrations in oil-treated controls were less than 1 ng/ml for the entire study. Daily serum LH concentrations were not different between gilts treated with progesterone or oil. The 1-microgram pulses of gonadotropin-releasing hormone reliably evoked pulses of LH in both treatment groups. The LH pulse frequency and amplitude, assessed from samples collected every 15 min for 6 hr on Day 12, were similar for progesterone- and oil-treated gilts. These results provide evidence that progesterone does not act at the pituitary gland to alter LH secretion in pigs.     

Testosterone inhibits gonadotropin-releasing hormone pulse frequency in the male sheep

The objective was to determine the effect of chronic testosterone (T) treatment on GnRH and LH secretion in wethers. Rams were either castrated only or castrated and immediately treated with Silastic implants containing T. Several weeks later, a device for collecting hypophyseal-portal blood was surgically implanted. Six to seven days later, blood samples were collected simultaneously and continuously from the portal vessels and jugular vein of pairs of conscious animals. Samples were divided at 10-min intervals for 6-12 h. One hour before the end of collection, all animals received i.v. injections of 250 ng of GnRH. In samples collected simultaneously from 6 pairs of animals, T reduced the frequency of both GnRH pulses (1.8 +/- 0.2 vs. 0.9 +/- 0.3/h, p less than 0.03) and LH pulses (1.6 +/- 0.1 vs. 0.8 +/- 0.3/h, p less than 0.03). T did not alter amplitude of either GnRH or LH pulses. Testosterone reduced mean GnRH (9.7 +/- 0.6 vs. 7.9 +/- 0.5 pg/ml, p less than 0.05), whereas mean LH was not significantly reduced (9.6 +/- 1.4 vs. 6.1 +/- 1.8 ng/ml, p = 0.16). These results support the hypothesis that T reduces GnRH pulse frequency.     

The induction of divergent gonadotropin secretion in vitro by variations in luteinizing hormone releasing hormone pulse regimen

The effects of luteinizing hormone releasing hormone (LHRH) pulse amplitude, duration, and frequency on divergent gonadotropin secretion were examined using superfused anterior pituitary cells from selected stages of the rat estrous cycle. Cells were stimulated with one of five LHRH regimens. With low-amplitude LHRH pulses (regimen 1) in the presence of potentially estrogenic phenol red, LH response in pituitary cells from proestrus 1900, estrus 0800, and diestrus 1,0800 were all significantly larger (P less than 0.05) than the other stages tested. In the absence of phenol red, responsiveness at proestrus 1900 was significantly larger than proestrus 0800, proestrus 1500, and estrus 0800 (P less than 0.01, 0.05, and 0.05, respectively); other cycle stages tested were smaller. No significant differences were observed between cycle stages for follicle-stimulating hormone (FSH) secretion in the presence or absence of phenol red. Because pituitary cells at proestrus 1900 were the most responsive to low-amplitude 4 ng LHRH pulses, they were also used to study the effects of LHRH pulses of increased amplitude or duration and decreased frequency. Increasing the amplitude (regimen 2) or the duration (regimens 3 to 5) increased FSH secretion; this effect was greatest with regimens 3 and 5. When regimens 3 and 5 were studied in pituitary cells obtained at proestrus 1500, FSH was significantly increased by both regimes, but most by regimen 5; furthermore, LH release was significantly reduced. When regimens 3 and 5 were studied in pituitary cells obtained at estrus 0800, FSH release was elevated most significantly by regimen 5. Thus, variations in LHRH pulse regimen were found to be capable of inducing significant divergence in FSH release from superfused anterior pituitary cells derived from specific stages of the estrous cycle.