Temporal relationships among peripheral blood concentrations of corticosteroids, luteinizing hormone and testosterone in bulls

Interrelationships among peripheral blood concentrations of corticosteroids (CS), luteinizing hormone (LH) and testosterone (T) were evaluated over a 24-hr period in four Angus bulls (18 months of age and 450 kg in body weight). Concentrations of LH and T were determined by radioimmunoassay and concentrations of CS by competitive protein binding assay of blood samples collected via jugular cannula at hourly intervals for 24 consecutive hr. A positive temporal relationship was observed between LH and T as significant positive correlations were obtained between concentrations of LH at one hour and concentrations of T at the subsequent hour in 3 of 4 bulls. Although LH peaks preceded T peaks by 1 hr, variation in this temporal relationship was observed as LH peaks occurred which were not accompanied by T peaks in some bulls. LH peaks were usually preceded by basal or declining concentrations of CS and prolonged elevations in concentrations of CS were often coincident with basal concentrations of LH and T. Negative correlations were obtained between concentrations of CS at one hour and concentrations of LH and T at the subsequent hour. These data describe the positive regulatory role of LH in testicular T production in the bull and suggest that alterations in endogenous concentrations of CS may influence peripheral concentrations of LH and T in the bull.     

Long-term testosterone supplementation augments overnight growth hormone secretion in healthy older men

Circulating testosterone (T) and GH/IGF-I are diminished in healthy aging men. Short-term administration of high doses of T augments GH secretion in older men. However, effects of long-term, low-dose T supplementation on GH secretion are unknown. Our objective was to evaluate effects of long-term, low-dose T administration on nocturnal GH secretory dynamics and AM concentrations of IGF-I and IGFBP-3 in healthy older men (65-88 yr, n = 34) with low-normal T and IGF-I. In a double-masked, placebo-controlled, randomized study we assessed effects of low-dose T supplementation (100 mg im every 2 wk) for 26 wk on nocturnal GH secretory dynamics [8 PM to 8 AM, Q(20) min sampling, analyzed by multiparameter deconvolution and approximate entropy (ApEn) algorithms]. The results were that T administration increased serum total T by 33% (P = 0.004) and E(2) by 31% (P = 0.009) and decreased SHBG by 17% (P = 0.002) vs. placebo. T supplementation increased nocturnal integrated GH concentrations by 60% (P = 0.02) and pulsatile GH secretion by 79% (P = 0.05), primarily due to a twofold increase in GH secretory burst mass (P = 0.02) and a 1.9-fold increase in basal GH secretion rate (P = 0.05) vs. placebo. There were no significant changes in GH burst frequency or orderliness of GH release (ApEn). IGF-I levels increased by 22% (P = 0.02), with no significant change in IGFBP-3 levels after T vs. placebo. We conclude that low-dose T supplementation for 26 wk increases spontaneous nocturnal GH secretion and morning serum IGF-I concentrations in healthy older men.     

Prolactin involvement in regulation of testicular 5 alpha-reductase activity in the immature rat

Treatment of intact immature (25-day-old) rats with bromoergocryptine (BR), which suppressed prolactin (Prl) secretion, decreased testicular 5 alpha-reductase activity, whereas treatment with Prl increased the enzyme activity in BR-treated animals. Serum luteinizing hormone (LH) concentrations were not reduced by BR treatment or elevated by Prl, suggesting that the BR and Prl effects on enzyme activity were not due to alterations in LH secretion. Hypophysectomy (at 21 days of age) caused a dramatic decrease in testicular 5 alpha-reductase activity, and treatment with LH partially reversed this effect. Treatment of hypophysectomized animals with Prl alone had no effect on the enzyme activity but enhanced the effect of LH. Testosterone propionate, given to hypophysectomized animals in a regimen that increased testicular testosterone to concentrations at least as high as those in intact (sham-hypophysectomized) controls, had no effect on enzyme activity, whether given alone or in combination with LH. These results indicate that Prl is involved, along with LH, in maintaining the high 5 alpha-reductase activity of the prepubertal rat testis; the action of Prl, apparently requiring the presence of LH, may be to decrease the rate of degradation of the enzyme. The data also suggest that the action of LH on testicular 5 alpha-reductase activity is not mediated by its stimulation of testosterone production.     

Profiles of luteinizing hormone, follicle-stimulating hormone, testosterone and prolactin in rams of diverse breeds: effects of contrasting short (8L:16D) and long (16L:8D) photoperiods

Mature rams of Polled Dorset, Finnish Landrace, Rambouillet and Suffolk breeding were maintained in a temperature-controlled environment and exposed to two consecutive cycles of short (8L:16D) followed by long (16L:8D) days. Serum hormone concentrations were determined in weekly samples and in 24-h profiles characterized at the end of each lighting schedule (i.e., 12, 24, 36 and 48 weeks). In all four breeds, the pituitary-testicular axis was more active during short days as compared with long days and the magnitudes of changes in serum luteinizing hormone (LH), follicle-stimulating hormone (FSH) and testosterone concentrations were greater for the two most seasonal breeds, Finnish Landrace and Suffolks. In comparison to other breeds, Finnish Landrace rams had significantly (P less than 0.05) higher mean LH levels, showed the greatest number of LH peaks/24 h, and had the highest mean testosterone levels at the end of both periods of short days, while Rambouillet rams had significantly (P less than 0.05) lower testosterone. Rambouillets also showed the smallest changes in pulsatile LH and testosterone secretion and displayed the least number of LH peaks/24 h following short days. Serum FSH levels were significantly (P less than 0.05) higher in Finnish Landrace and Suffolk rams than in Polled Dorsets and Rambouillets after 12 weeks of short days. Breed differences in serum LH, FSH and testosterone were not apparent following long days. Prolactin levels in Rambouillet rams were significantly (P less than 0.05) lower than in the other breeds following both periods of long days. These results indicate that breed differences exist in mature rams with regard to hormone secretory profiles. Breed differences in serum gonadotropin and testosterone are only apparent during short days when the hypothalamo-pituitary-testicular axis in rams is considered most active. Likewise, breed differences in prolactin are noticeable only during long days when secretion of this hormone is enhanced. Breed differences in LH, FSH and testosterone secretion in rams during short days might be related to seasonality of mating and/or fecundity of breed types.     

The effects of intracerebroventricular infusion of prolactin in luteinizing hormone, testosterone and growth hormone secretion in male sheep

This study tested a hypothesis that the enhancement of the prolactin (PRL) concentration within the central nervous system (CNS) disturbs pulsatile luteinizing hormone (LH) and growth hormone (GH) secretion in rams that are in the natural breeding season. A 3h long intracerebroventricular (icv.) infusion of ovine PRL (50 microg/100 microl/h) was made in six rams during the daily period characterized by low PRL secretion in this species (from 12:00 to 15:00 h); the other six animals received control infusions during the same time. Blood samples were collected from 9:00 to 18:00 h at 10 min intervals. A clear daily pattern of LH secretion was shown in control animals, with the lowest concentration at noon and an increasing basal level around the time of sunset (P < 0.001). No significant changes in LH concentration occurred in PRL-infused animals and the concentration noted after infusion of PRL was significantly (P < 0.05) lower than after the control infusion. The frequency of LH pulses tended to decrease in rams after PRL treatment. The changes in LH secretion clearly carried over to the secretion of testosterone in the rams of both groups. The GH concentrations changed throughout the experiment in both groups of rams, being higher after the infusions (P < 0.001). However, the mean GH concentration and GH pulse amplitude noted after PRL infusion were significantly lower (P < 0.001 and P < 0.05, respectively) from those recorded in the control. The continued fall in PRL secretion observed in rams following PRL infusion (P < 0.05 to P < 0.001) indicates a high degree of effectiveness of exogenous PRL at the level of the CNS. In conclusion, maintenance of an elevated PRL concentration within the CNS leads to disturbances in the neuroendocrine mechanisms responsible for pulsatile LH and GH secretion in sexually active rams.     

Effects of hyper- and hypoprolactinemia on gonadotropin secretion, rat testicular luteinizing hormone/human chorionic gonadotropin receptors and testosterone production by isolated Leydig cells

The effect of prolactin (Prl) on gonadotropin secretion, testicular luteinizing hormone (LH)/human chorionic gonadotropin (hCG) receptors, and testosterone (T) production by isolated Leydig cells has been studied in 60-day-old rats treated for 4 days, 4 and 8 weeks with sulpiride (SLP), a dopaminergic antagonist, or for 4 days and 4 weeks with bromocriptine (CB), a dopaminergic agonist. Plasma Prl concentrations were significantly greater in the SLP groups (204 +/- 6 ng/ml) and lower in the CB groups (3.0 +/- 0.2 ng/ml) than those measured in the control groups (54 +/- 6 ng/ml). The plasma concentrations of gonadotropin were not affected by a 4-day treatment with SLP or CB, nor were they after a 4-week treatment with CB. However, the hyperprolactinemia induced by an 8-week treatment with SLP was associated with a reduced secretion of gonadotropin (LH, 16 +/- 4 vs. 35 +/- 6 ng/ml; FSH, 166 +/- 12 vs. 307 +/- 14 ng/ml). In SLP-induced hyperprolactinemia, a 30% increase in the density of the LH/hCG testicular binding sites was observed (178 +/- 12 fmol/mg protein), whereas a 60% decrease was measured in hypoprolactinemia (55 +/- 5 vs. control 133 +/- 5 fmol/mg protein). Plasma T levels were increased in 4-day and 4-week hyperprolactinemic animals (4.3 +/- 0.4 and 3.9 +/- 0.4 ng/ml, respectively), but returned to normal levels in the 8-week group (3.0 +/- 0.5 vs. C: 2.3 +/- 0.2 ng/ml). No T modifications were observed in hypoprolactinemic animals. Two distinct populations of Leydig cells (I and II) were obtained by centrifugation of dispersed testicular cells on a 0-45% continuous Metrizamide gradient. Both possess LH/hCG binding sites. However, the T production from Leydig cells of population II increased in the presence of hCG, whereas that of cell population I which also contain immature germinal cells did not respond. The basal and stimulated T secretions from cell populations I and II obtained from CB-treated animals were similar to controls, whereas from 4 days to 8 weeks of hyperprolactinemia, basal and hCG induced T productions from cell population II decreased progressively. These data show that hyperprolactinemia causes, in a time-dependent manner, a trophic effect on the density of LH/hCG testicular receptors; reduces basal and hCG-stimulated T production from isolated Leydig cells type II; and results in an elevated plasma T concentration which decreases with time. The latter suggests a slower T catabolism and/or an impaired peripheral conversion of T into 5 alpha-dihydrotestosterone (DHT). Although hypoprolactinemia is associated with a marked reduction in testicular LH receptors, it does not affect T production.     

Neonatal castration of male and female rats affects luteinizing hormone responses to estrogen during adulthood

We examined the positive and negative feedback effects of estradiol (E2) on luteinizing hormone (LH) and prolactin (Prl) secretion in adult male and female rats which were gonadectomized within 24 h after birth (long-term castrates) and compared these responses to those elicited by E2 in short-term castrated (7 days) adult males and females. The high serum E2 did not reduce the elevated serum LH concentrations in long-term castrates until 4 days of treatment. Also, only after negative feedback was established were the positive feedback actions of E2 observed. In contrast, Prl surges were observed after 2 days of E2, and baseline Prl serum levels were elevated by Day 3 of E2 in long-term castrated male and female rats. Some long-term castrates lacked both LH and Prl surges, and E2 was ineffective in altering basal gonadotropin secretion in these animals. Short-term castrated males had elevated serum Prl levels but no Prl surges. Seemingly, when the hypothalamus is deprived of estrogen or androgen from birth to adulthood, an equal percentage of males and females become refractory to the positive feedback effects of estrogen during adulthood. Thus, it is difficult to separate castration effects from those which may be produced by the endogenous androgen secreted during the first 26 h of life.     

Further evidence that prolactin does not affect gonadotropin release at pituitary level

In a primary monolayer cell culture of the anterior pituitary from mature male rats the effects of exogenous rPrl (rPrl exog.) and endogenously secreted rPrl (rPrl endog.) on basal and LHRH stimulated LH secretion were investigated. In pilot studies basal Prl- and LH secretion as well as influence of various LHRH concentrations (10(-1)-10(+3) ng/ml) on Prl- and LH release were observed. The influence of exogenous rPrl was studied at various concentrations (50-500 ng/ml) and with preincubation periods of 2 hrs and 6 hrs before starting LHRH stimulation. The dopamine agonist bromocriptine and the dopamine antagonist sulpirid were preferentially used to prove physiologic function of the cell system presented. Basal LH secretion started after a delay of 3 hrs, whereas basal Prl secretion began immediately showing a linear rise for 9 hrs. LHRH stimulation resulted in a non-linear dose and time dependent LH secretion. LHRH showed no influence on endogenous Prl (rPrl endog.) secretion of the mammotroph cells. Exogenous Prl (rPrl exog.) did not affect spontaneous Prl release excluding ultra short loop inhibition in this cell system. Furthermore, exogenous Prl had no effect on either basal or LHRH stimulated LH secretion even after a preincubation period of up to 6 hrs and at concentrations generally observed for prolactin secreting tumors. Bromocriptine suppressed endogenous Prl release and did not affect LH secretion. Sulpirid had no influence on either Prl or LH secretion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of cranial cervical ganglionectomy and castration of male lambs. II. 24 hour plasma profiles of luteinizing hormone, testosterone and prolactin

Male lambs were utilized in an experiment designed to evaluate the effects of cranial cervical ganglionectomy (GX), castration and age on hormone secretion profiles. Blood plasma samples were collected at hourly intervals for 24 hours from 24 lambs aged 101 days and 20 lambs aged 277 days, then assayed for concentrations of luteinizing hormone (LH), testosterone and prolactin. At both ages pulsatile secretion of LH and testosterone was confirmed, but no circadian rhythm of LH testosterone or prolactin secretion was detected. Castration elevated LH levels significantly at both ages. GX and its interaction with castration had no effect on LH secretion at 101 days, but at 277 days these factors were significant, largely due to elevated levels being recorded from GX castrates. GX did not affect testosterone levels in entire animals at either age, while plasma from castrates contained no detectable testosterone. GX reduced prolactin concentrations at 101 days of age (summer) but elevated them at 277 days of age (winter). Castration and the interaction of castration with GX had no significant influence on plasma prolactin levels at either age. This study confirmed that the pineal gland of sheep is involved in the regulation of prolactin secretion, and probably influences LH secretion as well.     

Release of luteinizing hormone and growth hormone after recovery from maximal exercise

Pulsatile properties of luteinizing hormone (LH) and growth hormone (GH) release were evaluated in 19 eumenorrheic untrained females [mean age 31.1 +/- 1.1 yr, height 165.2 +/- 1.4 cm, weight 64.8 +/- 2.1 kg, peak oxygen uptake (Vo2) 41.6 +/- 1.4 (SE) ml.kg-1.min-1] during the early follicular phase of the menstrual cycle (days 3-4 after the onset of menses). Each subject was studied during two consecutive menstrual cycles under each of two conditions in random order: 1) no formal exercise for 72 h (C) and 2) 12-24 h after two maximal exercise bouts (peak Vo2/lactate threshold treadmill evaluation and a 3,200-m time-trial run or a maximal Vo2 inclined treadmill test) performed on consecutive days (EX). Blood sampling was performed every 10 min for 12 h. LH and GH pulsatile parameters were identified and characterized by the Cluster pulse detection algorithm. No significant differences were noted in the number of peaks, peak amplitude, interpeak interval, peak increment, or 12-h integrated concentrations between C and EX for LH or GH. We conclude that maximal exercise protocols typically used for exercise evaluation do not have an effect on the pulsatile characteristics of LH or GH release in untrained women during the early follicular phase of the menstrual cycle if 12-24 h of recovery are allowed before evaluation of the pulsatile secretion of gonadotropins or GH.     

Modulation of growth hormone, luteinizing hormone, and testosterone secretion by excitatory amino acids in boars

Ketamine hydrochloride, an n-methyl-d-aspartate (NMDA) receptor antagonist was used in an experiment that tested the hypothesis that fasting-induced increases in growth hormone (GH) secretion is mediated by excitatory amino acid (EAA) neurotransmission in boars. The effects of the drug on circulating concentrations of luteinizing hormone (LH) and testosterone were also evaluated. Blood was sampled at 15-min intervals for 8 h from 12 boars fitted with jugular vein catheters. At Hours 4 and 6, fasted boars (feed was withdrawn 48 h before the start of blood sampling) received i.m. injections of ketamine (19.9 mg/kg body weight; n=4) or .9% saline (n=4). Boars allowed feed on an ad libitum basis (n=4) received i.v. injections of n-methyl-d,l-aspartate (NMA; 2.5 mg/kg body weight), an NMDA receptor agonist, at Hours 4 and 6. Secretion of GH increased after NMA injections but was unaffected by treatment with ketamine or saline. Circulating concentrations of LH and testosterone were increased by injections of ketamine but were unaffected by injections of NMA or saline. Our results suggest that NMA is a potent GH secretagogue, but do not support the hypothesis that EAA neurotransmission drives the increased GH secretion displayed in fasted boars. Our finding that ketamine increased LH and testosterone release supports the notion that EAA have inhibitory effects on gonadotropin secretion in acutely fasted swine.     

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.     

Effects of food restriction and restoration on gonadotropin and growth hormone secretion in immature male rats

This experiment concerned the changing patterns in secretion of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and growth hormone (GH) under conditions of food restriction and subsequent catch-up growth. Weanling male rats were given either restricted (4 g food/day) or unrestricted access to food until 60 days of age. At this age, food-restricted rats weighed only 25% as much as rats fed ad libitum. Food restriction resulted in a dramatic decrease in the frequency of LH and GH pulses, and in the amplitude of GH pulses. It also slightly but significantly decreased mean blood levels of FSH (which was not secreted in a pulsatile manner in 60-day-old controls fed ad libitum). When restricted rats were given unrestricted access to food, frequency of LH and GH pulses and mean levels of FSH increased significantly and simultaneously within 2 days in half of the animals. Only an additional 8-10% of their body weight decrement was recovered at this time. After 10 days of food restoration, when restricted rats still weighed 50% less than controls, their secretory patterns of all three hormones were not significantly different from those of controls. Thus, recovery of gonadotropin and GH secretion was relatively rapid. Except for the quantitatively lesser impact of food restriction on FSH secretion, there was no evidence of any priorities in the secretion of the three hormones. Under conditions of rapid catch-up growth, the secretory patterns of LH, FSH, and GH appeared to develop simultaneously.     

Suppression of luteinizing hormone and testosterone secretion in bulls following adrenocorticotropin hormone treatment

The present investigation was conducted to evaluate the inhibitory effects of adrenal corticosteroids on testosterone production by the bull testis. Administration of a single i.v. dose of adrenocorticotropic hormone (ACTH; 80 IU) resulted in a corticosteroid peak which lasted approximately 6 h. During this 6 h period, no episodic increases in secretion of LH or testosterone were initiated and basal concentrations of testosterone were suppressed (P less than 0.05) below control values. Episodic secretion of LH and testosterone resumed 6--7 h after ACTH when concentrations of serum corticosteroids had returned to basal levels. These results suggest that ACTH-induced increases in serum corticosteroids suppress the episodic secretion of LH, resulting in a suppression of testosterone secretion by the bull testis.     

Effects of morphine dependence,withdrawal and tolerance on prolactin and growth hormone secretion in the rat

The effects of morphine dependence and withdrawal on prolactin (Prl) and growth hormone (GH) secretion were examined in the rat. Morphine dependence, induced by morphine pellet implantation, had no effect on nonstress concentrations of plasma Prl or GH, but it potentiated the response of Prl secretion to the stress associated with blood collection + injection of saline. Naloxone-induced withdrawal had no demonstrable effect on the changes in Prl and GH secretion produced by stress. In addition, signs of tolerance to both the Prl- and GH-stimulating effects of morphine injection were observed in morphine-dependent rats.     

Effect of vitamin A deficiency upon gonadotropin response to gonadotropin-releasing hormone

There is a monotypic change in basal serum gonadotropin levels following retinol treatment of chronically vitamin A-deficient (VAD) male rats. The present study was undertaken to investigate the hypothesis that the specific increase in serum follicle-stimulating hormone (FSH) represents a change in gonadotrope responsiveness to gonadotropin-releasing hormone (GnRH). To this end, a test dose of GnRH was given to VAD rats pre-, 5 days post-, and 10 days postreplacement of vitamin A (PVA). In VAD rats, basal serum FSH and luteinizing hormone (LH) levels were higher than those of controls. Increased LH/testosterone ratios, both in basal levels and in the secretory response to GnRH, suggested Leydig cell hyporesponsiveness in VAD animals. Both the FSH and LH responses to GnRH were maximal at 1 h, declining thereafter. Although the absolute increments in FSH and LH 1 h after GnRH in VAD rats were greater than in controls, the percent increase in FSH tended to be lower in VAD rats and to increase after vitamin A replacement. The specific enhancement of FSH release PVA became evident only when assessing total secretion of FSH and LH after GnRH. Luteinizing hormone response to GnRH increased PVA, but not significantly, while FSH secretion after GnRH increased both 5 and 10 days PVA, times during which basal FSH levels were also increasing. These changes in FSH secretion could not be attributed either to increases in endogenous GnRH or to changes in testosterone or estradiol levels. Basal serum androgen binding protein levels, elevated in VAD animals, did not respond to the acute increases in FSH after GnRH and remained high PVA, suggesting no acute change in Sertoli cell function. Thus, the PVA increase in FSH secretion unmasks a partial inhibition of the gonadotrope present in the retinol-deficient, retinoic acid-fed male rat.     

Effects of cranial cervical ganglionectomy and castration of male lambs. III. Hormonal responses following administration to gonadotrophin releasing hormone (GnRH)

Entire and castrate male lambs, which were cranial cervical ganglionectomized (GX) or untreated, were utilized in a study of responses to intravenous GnRH; 24 animals were treated at both 101 and 277 days of age. GX caused a reduction in basal LH concentrations of both wethers and rams at the first sampling, but increased pre-injection levels of this hormone in 277 day old wethers. Basal LH levels of castrates were substantially higher than those of entires, but GX had no significant influence on pretreatment testosterone secretion in rams. GnRH treatment elevated plasma LH levels in all animals, while in entires increases in testosterone concentrations also occurred. Castration significantly increased peak LH levels together with total LH output. At neither age were the LH or testosterone reponses influenced significantly by GX, nor was the interaction of castration and GX significant for LH response data. The major effect of age at GnRH treatment was that markedly higher testosterone responses were recorded from the older rams.     

Luteinizing hormone and testosterone secretory profiles of boars: Effects of stage of sexual maturation

Two short term studies of LH and testosterone secretory profiles were carried out to evaluate the effects of stage of sexual maturity on the patterns of secretion of these hormones in Large White x Landrace boars. Four pubertal and three post-pubertal boars were subjected to plasma sampling every twenty minutes for 24 hours. During puberty, plasma profiles of LH varied in a manner indicative of a highly pulsatile mode of secretion. Likewise, large fluctuations in plasma testosterone levels were noted at this age, but they were not as frequent as those of LH. In contrast, plasma LH and testosterone profiles of post-pubertal boars showed fewer and smaller fluctuations in hormone concentrations. The overall mean levels of LH and testosterone were 0.82 and 1.04 ng/ml in pubertal boars, and 0.39 and 0.81 ng/ml in post-pubertal animals. At neither age was there any evidence of diurnal variations in plasma hormone concentrations.     

Age-dependent effect of Freund's adjuvant on 24-hour rhythms in plasma prolactin, growth hormone, thyrotropin, insulin, follicle-stimulating hormone, luteinizing hormone and testosterone in rats

The effect of Freund's adjuvant administration on 24-hour changes of plasma prolactin, growth hormone (GH), thyrotropin (TSH), insulin, follicle-stimulating hormone (FSH), luteinizing hormone (LH) and testosterone were studied in young (2 months) and aged (18 months) male Wistar rats. Rats were injected s.c. with Freund's adjuvant or adjuvant's vehicle and, 18 days later, they were killed at 6 different time intervals throughout a 24-hour cycle to measure circulating hormone levels by specific RIAs. Young rats receiving adjuvant's vehicle exhibited significant time-of-day-dependent variations in plasma TSH, LH and testosterone, with maximal levels at 1300 h, 0100 h and 1700 h, respectively. Prolactin and insulin levels, analyzed globally in a factorial ANOVA, showed significant time-of-day changes with maximal levels at 1300 - 1700 h and 2100 h, respectively. The daily rhythms in plasma LH and testosterone found in young rats were not longer observed in Freund's adjuvant-injected rats, while as far as TSH, a second peak was observed at 0100 h after Freund's adjuvant administration. Twenty-four hour rhythms in circulating TSH, LH and testosterone were blunted in old rats receiving either Freund's adjuvant or its vehicle. Aged rats exhibited significantly higher circulating levels of prolactin, and lower levels of GH, TSH, FSH and testosterone. The results indicate that secretion of prolactin, GH, TSH, FSH and testosterone are age-dependent, as are the responses of TSH, LH and testosterone to Freund's adjuvant administration.     

Insulin-like growth factor-I feedback regulation of growth hormone and luteinizing hormone secretion in the pig: evidence for a pituitary site of action

Three experiments (EXP) were conducted to determine the role of insulin-like growth factor-I (IGF-I) in the control of growth hormone (GH) and LH secretion. In EXP I, prepuberal gilts, 65 ± 6 kg body weight and 140 days of age received intracerebroventricular (ICV) injections of saline (n = 4), 25 μg (n = 4) or 75 μg (n = 4) IGF-I and jugular blood samples were collected. In EXP II, anterior pituitary cells in culture collected from 150-day-old prepuberal gilts (n = 6) were challenged with 0.1, 10 or 1000 nM [Ala15]-h growth hormone-releasing hormone-(1-29)NH2 (GHRH), or 0.01, 0.1, 1, 10, 30 nM IGF-I individually or in combinations with 1000 nM GHRH. Secreted GH was measured at 4 and 24 h after treatment. In EXP III, anterior pituitary cells in culture collected from 150-day-old barrows (n = 5) were challenged with 10, 100 or 1000 nM gonadotropin-releasing hormone (GnRH) or 0.01, 0.1, 1, 10, 30 nM IGF-I individually or in combinations with 100 nM GnRH. Secreted LH was measured at 4 h after treatment. In EXP I, serum GH and LH concentrations were unaffected by ICV IGF-I treatment. In EXP II, relative to control all doses of GHRH increased (P < 0.01) GH secretion. Only 1, 10, 30 nM IGF-I enhanced (P < 0.02) basal GH secretion at 4 h, whereas by 24 h all doses except for 30 nM IGF-I suppressed (P < 0.02) basal GH secretion compared to control wells. All doses of IGF-I in combination with 1000 nM GHRH increased (P < 0.04) the GH response to GHRH compared to GHRH alone at 4 h, whereas by 24 h all doses of IGF-I suppressed (P < 0.04) the GH response to GHRH. In EXP III, all doses of IGF-I increased (P < 0.01) basal LH levels while the LH response to GnRH was unaffected by IGF-I (P > 0.1). In conclusion, under these experimental conditions the results suggest that the pituitary is the putative site for IGF-I modulation of GH and LH secretion. Further examination of the role of IGF-I on GH and LH secretion is needed to understand the inhibitory and stimulatory action of IGF-I on GH and LH secretion.