Plasma Testosterone in Bulls: Seasonal Variation

Levels of peripheral plasma testosterone and LH were studied in 4 bulls hourly during a 12 hr. sampling period at 5 times of the year. The average plasma testosterone levels were significantly lower in October (1.8 ng/ml, Ρ < 0.001) and December (2.5 ng/ml, Ρ < 0.05) than in February, June and August (3.5, 3.7 and 3.7 ng/ml respectively). LH showed a slight fluctuation during the day, with values ranging between 0.8 and 3.8 ng/ml, but underwent no significant seasonal variation. A significant increase in average plasma testosterone was observed 1 hr. after the LH peaks (P < 0.001).     

Neonatal hemiorchidectomy of bulls alters plasma growth hormone levels and advances onset of pubertal testosterone secretion

Bovine GH and testosterone profiles were determined in plasma collected at 20 min intervals during 3 hr bleeding periods on day 25 of life and every 15 days thereafter in six intact (I) Holstein bull calves and in six others which had been hemiorchidectomized (HO) at 10 days of age. In I bulls average plasma GH concentrations varied between 7.9 and 14.5 ng/ml (P greater than 0.05) until 130 days of age, after which the GH level gradually rose (P = 0.007) to a maximum of 19.4 ng/ml on day 205 of life. Episodic release of GH was apparent in 55 day-old and older I bulls and in HO bulls of all ages. Plasma GH concentrations in HO bulls were higher than in I bulls 15 and 30 days after surgery (P = 0.07), at which times the levels in HO bulls averaged 19.6 and 22.5 ng/ml and in I bulls 10.3 and 10.2 ng/ml, respectively. Plasma GH in HO bulls again exceeded that of I bulls at ages of 130-190 days (P = 0.04). Plasma testosterone was virtually nondetectable before 130 days of age in I bulls but thereafter exhibited the typical episodic pattern. In HO bulls, plasma testosterone concentrations began to rise 15 to 30 days before those in I bulls, resulting in an age X treatment interaction (P less than 0.0001). Furthermore, average testosterone levels were higher (P = 0.07) in HO than I bulls at 235 and 250 days of age.(ABSTRACT TRUNCATED AT 250 WORDS)     

Plasma testosterone and LH responses to LHRH in double-muscled bulls treated with trenbolone acetate and zeranol

Twenty-four double-muscled Belgian White Blue bulls were assigned, according to body weight, to two groups with and without treatment with anabolic agents. Implants containing 140 mg trenbolone and 36 mg zeranol (Forplix) were inserted sc on the upper face of the ear flap. Plasma concentrations of testosterone and luteinizing hormone (LH) were determined at d 0, 30 and 60 of the experimental period. Mean testosterone levels at d 0 for treated and controls were, respectively, 2.1 and 1.7 ng/ml during the 10-h sampling period. At d 30 and 60, testosterone levels were strongly depressed in implanted bulls (.2 ng/ml) as compared with 2.5 and 1.7 ng/ml in control bulls (P less than .001 at d 30 and P less than .01 at d 60). Average plasma LH concentrations were identical in the two groups at d 0 and 60 (1.1 and 1.5 ng/ml, respectively), but showed a slight decrease at d 30 in the treated group (P less than .10). The pulsatile character of LH and testosterone profiles was abolished by the Forplix treatment. Luteinizing hormone-releasing hormone (LHRH) injection at d 0 was followed in both groups by an immediate and sharp increase in plasma LH concentrations. The LH response reached a maximal value between 20 to 40 min postinjection and then declined rapidly. On the contrary, Forplix treatment strongly reduced LH and testosterone responses to LHRH stimulation in treated animals. Average daily gain and feed to gain ratios were 1.087 +/- .127 and 7.52 +/- .32 kg, respectively, for the control bulls and 1.335 +/- .092 and 6.24 +/- .24 kg for the Forplix-treated bulls, thus clearly showing a beneficial effect of Forplix treatment.     

Insulin-like growth factors and IGF-binding proteins in bovine seminal plasma.

Insulin-like growth factor-I (IGF-I) is an important factor for germ cell development and maturation of spermatozoa. Actions of IGFs are modulated by IGF-binding proteins (IGFBPs) that may, depending on their concentration and site of expression, inhibit or enhance effects of IGF-I. We characterized IGFs and IGFBPs in seminal plasma from bulls routinely used for artificial insemination (AI) and from bulls producing poor-quality semen (low mass and individual motility of spermatozoa). IGFs were measured by specific radioimmunoassay in 22 samples of seminal plasma from nine different AI bulls with high (> 76.8%), average (72.8-73.4%), or low (< 69.5%) nonreturn rate (NRR). IGF-I and IGF-II levels were 144 +/- 9 ng/ml (mean +/- SE; range, 79-238 ng/ml) and 144 +/- 10 ng/ml (range, 55-221 ng/ml), respectively, and did not correlate with NRRs. IGF-I concentrations in seminal plasma from bulls producing poor-quality semen (n = 10) were significantly (P < 0.05) greater (194 +/- 26 ng/ml; range, 94-370 ng/ml), whereas IGF-II levels were significantly (P < 0.05) lower (93 +/- 17 ng/ml; range, 38-183 ng/ml) than in AI bulls. Ligand blot analysis of seminal plasma for IGFBPs revealed the presence of a 38-/45-kDa doublet band and a 30-kDa IGFBP. These IGFBPs were identified as IGFBP-3 and IGFBP-5, respectively, by immunoprecipitation using specific antibodies. In addition, a low amount of IGFBP-4 was detected in bovine seminal plasma by immunoprecipitation. There was a marked difference in the activity of IGFBPs between individual bulls, with a relatively small within-bull variance. The differences in IGFBP activities did not correlate with the fertilization capacity of the bulls in vivo or in vitro nor with immunoreactive IGF-I and IGF-II levels in seminal plasma. Our results demonstrate the presence of IGFBPs in bovine seminal plasma. In contrast to human seminal plasma, high activity of IGFBP-3 was detected in seminal plasma of some bulls, suggesting species-specific regulation of IGFBP activity by proteases.     

Animal performance, plasma hormones and metabolites in Holstein and Belgian Blue growing-fattening bulls

Six Holstein (light-muscled type) and six Belgian Blue bulls (double-muscled type) were fed a finishing diet. Average daily gain was 1.36 kg for the Holstein bulls vs 1.24 kg for the Belgian Blue bulls (P less than .05). Holstein bulls consumed more feed (2.3 vs 1.8 kg/100 kg body weight, P less than .001) than the Belgian Blue bulls. The dressing percentage (55.4 vs 65.8%, P less than .001) and the proportion of muscle (56.1 vs 71.3%, P less than .001) in the carcass were less, whereas the proportions of adipose tissue (28.3 vs 15.4%, P less than .001) and bone (15.7 vs 13.4%, P less than .05) were higher in the Holstein bulls. Plasma creatinine determined in samples obtained once a week was lower (11.0 vs 20.3 mg/liter, P less than .001) in the Holstein bulls. In contrast, Holstein bulls tended to produce more triiodothyronine (2.3 vs 1.8 nM, P less than .10), tetraiodothyronine (71.9 vs 54.7 nM, P less than .10) and insulin-like growth factor I (IGF-I; 340 vs 205 ng/ml, P less than .20) than the Belgian Blue bulls. Growth hormone, insulin, IGF-I and testosterone were measured at 20-min intervals during two 24-h periods. In wk 6, Holstein bulls tended to produce more growth hormone than the Belgian Blues, as indicated by higher total peak area (3,185 vs 2,431 ng), peak amplitude (34.1 vs 22.6 ng/ml, P less than .10) and baseline (4.6 vs 3.3 ng/ml, P less than .20). In wk 27, the trends were opposite.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of unilateral castration and unilateral cryptorchidism on gonadotropin and testosterone response to gonadotropin releasing hormone in the bull

The effects of unilateral castration (UC) and induced unilateral cryptorchidism (UCR) on basal plasma luteinizing hormone (LH), follicle stimulating hormone (FSH) and testosterone, and on the responses of these hormones to gonadotropin releasing hormone (GnRH), were investigated in bulls altered at 3, 6 or 9 months of age. Blood plasma was collected before and after GnRH (200 micrograms) stimulation approximately 1 year following gonadal manipulation. Neither mean baseline concentrations nor GnRH-induced increases in plasma testosterone were altered (P greater than .1) by hemicastration or UCR (P greater than .1). Both mean baseline LH and GnRH-induced LH release were greater (P less than .05) in bulls altered at 3 months of age than in bulls altered at 9 months of age. UC increased (P less than .05) plasma LH response to GnRH over that observed in intact bulls, but not above that in UCR bulls. UCR had no detectable effect on either baseline concentrations or GnRH-stimulated LH release. FSH was increased (P less than .05) in hemicastrates, while UCR had a variable effect on peripheral FSH: FSH was reduced (P less than .05) in UCR animals altered at 3 months of age but increased (P less than .05) in UCR bulls altered at both 6 and 9 months of age when compared to FSH in intact bulls. The results indicate that, compared with intact bulls, UC bulls release increased amounts of both gonadotropins but similar amounts of testosterone in response to GnRH stimulation. UCR had a variable effect on FSH release and did not alter either LH or testosterone.     

Serum kisspeptin: New possible biomarker for sexual behaviour and sperm concentration in buffalo bulls

The study was designed to decipher the inter-relationship between peripheral hormones (kisspeptin and testosterone), sexual behaviour and seminal variables of Murrah buffalo bulls (n = 134). In this study, we recorded that 13%, 37%, 40%, 6% and 4% Murrah buffalo bulls had reaction time of <30, 31–60, 61–180, 181–300 and >300 s, respectively. Further, it was observed that 4%, 85% and 10% buffalo bulls were sexually aggressive, active and dull, respectively, during semen collection. The courtship behaviour was not found to be desirable for the bulls used for the semen collection. Mean of ejaculate volume, sperm concentration and mass motility (0–5 scale) were 3.57 ml, 977.11 million/ml, 2.7, respectively. Correlation studies revealed that the reaction time was positively correlated with courtship behaviour and body weight, and negatively correlated with sexual aggressiveness and sperm concentration. Serum kisspeptin in buffalo bulls, measured for the first time, was found to 3.8 ± 0.7 ng/ml. Serum kisspeptin and testosterone level are negatively correlated to each other and kisspeptin level influenced the sexual behaviour (reaction time, sexual aggressiveness and penile erection) of study bulls. Serum kisspeptin was higher in the buffalo bulls with higher sperm concentration indicating its role in spermatogenesis. In conclusion, for the first time basic information related to sexual behaviour of Murrah buffalo bulls in large population along with its inter-relationship with peripheral hormones (kisspeptin and testosterone) has been documented.     

Prepubertal changes in plasma FSH and inhibin in Holstein bull calves: responses to castration and(or) estradiol

The objectives of two studies were to determine 1) whether plasma concentrations of inhibin (INH) changed with age in prepubertal bulls and whether these changes were related to changes in FSH, testosterone or testis length; and 2) whether castration and(or) estradiol implants affected plasma concentrations of INH and FSH. In Exp. 1, plasma INH remained constant from 4 until 8 wk of age then increased from 120 pM to 202 pM between 8 and 12 wk. Thereafter, INH decreased to 90 pM by 36 wk. Between 4 and 10 wk, plasma FSH increased from .32 to .43 ng/ml, apparently increasing before the initial rise in plasma INH. Between 10 and 12 wk, FSH declined from .43 to .33 ng/ml. After 12 wk, FSH increased as INH decreased. Initial increases in testis length and concentrations of plasma testosterone occurred at 14 wk coincident with the second rise in FSH. In Exp. 2, bull calves were either left intact, castrated, castrated and implanted with estradiol, or left intact and implanted with estradiol at 7.5 wk of age. Castration decreased concentrations of INH and increased concentrations of FSH. Castrated calves implanted with estradiol had decreased concentrations of both INH and FSH. Intact bulls implanted with estradiol had decreased concentrations of FSH relative to intact unimplanted bulls; however, concentrations of INH did not display the age-related changes observed in intact, unimplanted bulls. In summary, age-related changes in plasma INH and FSH occur in bulls. Furthermore, plasma concentrations of INH and FSH increased before changes in gonadal size were detected. The bovine testis may be a major source of circulating INH because castration decreased concentrations of plasma INH.     

不同季节肉用种公牛精液品质与生理常值、血清及精清生化指标相关关系研究

以利木赞和夏洛来种公牛为试验牛，研究不同季节对其精液品质、冻精产量、生理常值、血清及精清生化指标的影响。结果表明：（1）与其它季节相比，夏季肉用种公牛的原精活力、冻精活力、活精于百分数、顶体完整率均显著降低，而精于畸形率显著升高（P〈0．05）。精子密度以秋季最低。（2）冻精产量夏季处于最低水平，平均每月仅为7388支。（3）夏季肉用种公牛的血清LH、睾酮、T3、皮质醇的含量分别为48．12、4．15、1．11、4．89ng／L，显著低于春、秋、冬季（P〈0．01）。（4）夏季肉用种公牛血清钠、钾、钙、镁均处于最低水平，显著低于其它季节（P〈0．01）。（5）夏季肉用种公牛精清睾酮水平明显下降，显著低于春季、秋季和冬季。（6）精清钠、钾、钙夏季含量最低，分别为83．12mmol／L、18，77mmol／L和4．63mmol／L；精清镁含量由春季至秋季呈显著下降趋势；精清磷含量夏季最高，为4．65mmol／L，显著高于春季、秋季和冬季。（7）肉用种公牛精清ALP夏季含量为1784．3U／L，显著高于春、秋两季，但低于冬季。（8）精清睾酮与精液品质各指标存在显著相关；血清生化指标与精清生化指标存在显著相关；直肠温度、呼吸频率与精液品质及血清生化指标存在显著相关性，与精清生化指标相关性不大。     

Function of contralateral testis after artificial unilateral cryptorchidism in dogs

The effects of a cryptorchid testis on the contralateral testis were investigated after artificially producing unilateral cryptorchidism in 8 beagle dogs. Bilateral testicular biopsy and collection of spermatic vein blood and peripheral vein blood were performed at the time of the operation to produce the cryptorchidism and 52 weeks later. The testicular tissue was used for histological examination by light microscopy and measurement of the testicular transferrin (Tf) concentration by enzyme immunoassay. Plasma testosterone (T), estradiol-17 beta (E2), and luteinizing hormone (LH) levels were measured by radioimmunoassay. Semen was collected weekly and its quality was examined. No spermatogenesis was observed in the cryptorchid testes at 52 weeks after the operation, and the number of germ cells in the contralateral testes had decreased but the number of Sertoli cells did not change. The Tf concentration in both testes had also decreased. The mean total number of sperm between 48 and 52 weeks after the operation (194 x 10(6)) was less than half the number before the operation (510 x 10(6)). Mean spermatic vein plasma T levels (51 ng/ml) in the cryptorchid testes 52 weeks after the cryptorchid operation were significantly lower than before the operation (91 ng/ml; P < 0.05). By contrast, spermatic vein plasma E2 levels (80 pg/ml) were significantly higher than the values before the operation (51 pg/ml P < 0.05). The peripheral plasma LH levels decreased. These findings indicate that a large quantity of E2 secreted by the cryptorchid testis inhibits the endocrine and spermatogenic functions of the contralateral testis in the dog. In particular, it is assumed that dysfunction of the contralateral testis is associated with Sertoli cell dysfunction suggested by the low Tf concentration.     

Basal levels and GnRH-induced responses of peripheral testosterone and estrogen in Holstein bulls with poor semen quality

The present study investigated the basal levels and GnRH-induced responses of peripheral testosterone and estrogen in Holstein bulls with poor semen quality. On the basis of semen parameters, bulls (n=5) having poor semen quality were selected as experimental bulls, and good semen quality bulls (n=4) were used as control bulls. Both groups were treated intramuscularly once with GnRH (250 μg of fertirelin acetate). Blood samples were collected at -1 day (d), -30 min and 0 h (treatment) followed by every 30 min for 5 h and 1, 3 and 5 d post-GnRH treatment (PGT), and LH, testosterone and estradiol-17β (E(2)) concentrations were measured. The pretreatment concentrations were used as basal levels. The percentage increments based on the 0-h levels were calculated per bull for each sampling time until 5 h PGT, and differences were compared between the experimental and control groups. The PGT concentrations of testosterone and basal and PGT concentrations of E(2) were significantly lower in the experimental group. The testosterone increment in the experimental group was delayed and significantly lower from 1 to 5 h PGT than those in the control group. It can be suggested that bulls with poor semen quality have delayed and lower GnRH-induced testosterone response and may also have lower estrogen levels.     

Pharmacokinetics of phenylbutazone in mature Holstein bulls: steady-state kinetics after multiple oral dosing

Six mature Holstein bulls were given an 8-day course of phenylbutazone (PBZ) orally (loading dose, 12 mg of PBZ/kg of body weight and 7 maintenance doses of 6 mg of PBZ/kg, q 24 h). Plasma concentration-vs-time data were analyzed, using nonlinear regression modeling. The harmonic mean +/- pseudo-SD of the biologic half-life of PBZ was 61.8 +/- 12.8 hours. The arithmetic mean +/- SEM of the total body clearance and apparent volume of distribution were 0.0021 +/- 0.0001 L/h/kg and 0.201 +/- 0.009 L/kg, respectively. The predicted mean minimal plasma concentration of PBZ with this dosage regimen was 75.06 +/- 4.05 micrograms/ml. The predicted minimal plasma drug concentration was compared with the observed minimal plasma drug concentration in another group of bulls treated with PBZ for at least 60 days. Sixteen mature Holstein bulls were given approximately 6 mg of PBZ/kg, PO, daily for various musculoskeletal disorders. The mean observed minimal plasma concentration of PBZ in the 16 bulls was 76.10 +/- 2.04 micrograms/ml, whereas the mean predicted minimal plasma concentration was 74.69 +/- 3.10 micrograms/ml. Dosages of 4 to 6 mg of PBZ/kg, q 24 h, or 10 to 14 mg of PBZ/kg, q 48 h, provided therapeutic plasma concentrations of PBZ with minimal steady-state concentrations between 50 and 70 micrograms/ml.     

Peripheral plasma levels of corticosteroids in normal beagles and greyhounds measured by a rapid competitive protein binding technique

A rapid competitive protein binding method for the measurement of peripheral plasma levels of corticosteroids in dogs was evaluated and applied. The rapid method was found to give a minor overestimation of the Cortisol levels as compared to the method including purification of the plasma extract on Sephadex LH-20 columns. In Beagles and Greyhounds mean peripheral plasma levels of corticosteroids of 25.4 and 18.0 ng/ml were found respectively. No diurnal variation of peripheral plasma levels of corticosteroids was obtained. After the intravenous administration of dexamethasone peripheral plasma levels of corticosteroids were found to decrease to about 20 % of the pretreatment levels. When ACTH was administered i.m. a nearly 10 fold increase of the corticosteroid levels was found 60 to 90 min. after the injections. The rapid method described and evaluated should be of value for diagnosis of adrenal syndrome in the dog.     

The sequential appearance of sperm abnormalities after scrotal insulation or dexamethasone treatment in bulls.

Scrotal insulation and dexamethasone treatment were used as a model to compare the effect of testicular heating and stress on spermatogenesis. Insulation was applied to the scrotum of eight bulls (insulated) for a period of four days, eight bulls were treated daily for seven days with 20 mg dexamethasone injected intramuscularly, and four bulls were untreated controls. Semen from four bulls in each group was collected and evaluated over a six-week period after treatment. Blood samples for testosterone analysis were taken hourly for eight hours at the beginning and the end of the six-week period from the control bulls and before and after treatment from the four insulated and four dexamethasone-treated bulls that were not used for semen collection. At the end of the last blood sampling period, the four bulls in each group were castrated for the collection of testicular tissue for the determination of testosterone concentrations. Basal, peak episodic, and mean serum testosterone concentrations among control bulls, pre and postinsulated bulls, and pretreatment samples of dexamethasone-treated bulls were not different (p > 0.05); however, bulls that had received dexamethasone treatments had significantly lower basal, peak episodic, and mean testosterone concentrations (p < 0.05). Tissue concentrations of testosterone in control, insulated, and dexamethasone-treated bulls were not significantly different but tended to be lower in dexamethasone-treated bulls (p > 0.13). The spermiograms of the control bulls varied insignificantly over the six-week sampling period; however, there was a marked increase in sperm defects in insulated and dexamethasone-treated bulls. The types of sperm defects and the temporal relationships of rises and declines of sperm defects were quite similar for both treatments. All bulls recovered to approximately pretreatment levels of sperm defects by six weeks after the initiation of treatment. Results indicate that two of the most common types of insults to spermatogenesis in bulls, heat and stress, result in similar spermiograms.     

Effect of graduated vitamin D3 doses and different ration composition on the 25OH-vitamin D3 plasma concentration and fattening and slaughter production of bulls]

In a individual feeding experiment (348 days) 24 fattening bulls were given either a ration high in roughage (2.8 kg concentrate mixture, wheat straw ad libitum: group I) or high in concentrate (5.6 kg concentrate mixture, wheat straw ad libitum group II) supplemented with various levels of vitamin D3 (0, 250, 500, 1,000, 2,000, 4,000 and 8,000 IU per 100 kg body weight (bw) and day) and minerals as required. After 58, 101, 134, 172, 205, 277 and 340 days 25-OH-D3 plasma concentration was estimated. Fattening and slaughtering parameters were measured. The 25OH-D3 plasma concentration was significantly influenced by vitamin D3 supply, kind of ration and day of taking samples. 25-OH-D3 plasma concentration decreased below 5 ng per ml when vitamin D3 supply was less than or equal to 250 IU per 100 kg bw and day. The initial plasma levels were maintained when 500 IU vitamin D3 per 100 kg bw and day was given (6.6 ng per ml). Administration of greater than or equal to 1,000 IU per 100 kg bw and day increased 25OH-D3 plasma level (greater than 10 ng/ml). Plasma 25OH-D3 concentration was significant higher when bulls consumed diets rich in concentrate (10.6 and 18.2 ng/ml for I and II after 340th day). Differences in content of cell walls and crude fat of rations may be responsible for results. Daily weight gain of bulls amounted to 712 and 945 g when fed diets I or II. Dry matter intake and live weight gain were not significantly influenced by different vitamin D3 supply. Clinical symptoms of rachitis did not appear.     

Insulin-like growth factor-I concentrations in plasma of intact and castrated male and female cattle at four ages

The influence of age, sex and castration on plasma concentrations of insulin-like growth factor-I (IGF-I) and other metabolic hormones related to growth was studied in cattle. Plasma was sampled from bulls, steers, heifers, and ovariectomized heifers at 20-min intervals for 12 hr at 5, 8, 12, and 15 mo of age. Plasma samples from each animal taken during each 12-hr period were composited for analysis of IGF-I, testosterone, total estrogens, thyroxine, triiodothyronine, insulin, and glucose. The mean plasma IGF-I concentration in all cattle increased from 61.6 to 158.6 ng/ml as the animals aged (p less than .01). Over all ages, bulls had greater concentrations of IGF-I than steers, heifers, or ovariectomized heifers (P less than .01). Bulls also had higher concentrations of testosterone (P less than .01) and total estrogens (P less than .01). Triiodothyronine concentration was greater in ovariectomized heifers than in bulls (P less than .01) or steers (P less than .05). Females had higher concentrations of thyroxine than males (P less than .01). Concentrations of triiodothyronine in the cattle were greater (P less than .01) during the winter and early spring as compared with the summer. Concentrations of insulin and glucose were not influenced by sex or castration; however, insulin increased in all cattle with age (P less than .01). The mean increase in IGF-I concentration with age within each of the four groups was associated with an increase in concentration of plasma insulin but the differences due to sex were not related to differences in insulin.(ABSTRACT TRUNCATED AT 250 WORDS)     

Secretion of dopamine and norepinephrine in hypophyseal portal blood and prolactin in peripheral blood of Holstein cattle

The objective was to test the hypothesis that dopamine regulates prolactin (PRL) secretion by determining acute changes in catecholamine concentrations in hypophyseal portal blood of cattle, and their relation to peripheral blood concentration of PRL in hypophyseal stalk-transected (HST) and sham-operated controls (SOC). Holstein heifers (606 +/- 21 kg BW; mean +/- SE) were subjected to neurosurgery for 8 h to collect hypophyseal portal blood with a stainless steel cannula designed with a cuff placed under the pituitary stalk and peripheral blood via a jugular vein catheter. PRL plasma concentration was measured by radioimmunoassay, and dopamine and norepinephrine in portal plasma by radioenzymatic assay. During anesthesia before HST or SOC, PRL plasma concentration ranged from 20-40 ng/ml throughout 255 min. PRL abruptly increased and remained above 90 ng/ml after HST compared with a steady decrease to <20 ng/ml in SOC heifers throughout 440 min. Within 5 min after severing the hypophyseal stalk, dopamine in portal blood (>8 ng/ml) was significantly increased (P < 0.05) compared with peripheral blood (<2 ng/ml). Norepinephrine concentration in portal blood was significantly greater (P < 0.05) than in peripheral blood during the first 60 min. The sustained high PRL level in peripheral plasma after severing the hypophyseal stalk stimulated hypothalamic dopamine secretion from hypophyseal portal vessels during the prolonged period of blood collection. Norepinephrine concentration in these cattle was greater in hypophyseal portal than in peripheral blood, implicating both an important hypothalamic source of the catecholamine as well as an adrenal gland contribution during anesthesia.     

Differential responses in anterior pituitary luteinizing hormone (LH) content and LHβ- and α-subunit MRNA, and plasma concentrations of LH and testosterone, in bulls treated with the LH-releasing hormone agonist deslorelin

Anterior pituitary gland contents of LH and LHß- and α-subunit mRNAs, and circulating concentrations of LH and testosterone, were determined in bulls treated with the LH-releasing hormone (LHRH) agonist deslorelin. Brahman (Bos indicus) bulls (14-month-old) were allocated to two groups and received the following: Control (n = 5), no treatment; Deslorelin (n = 4), four deslorelin implants (approximately 200 μg total deslorelin/day) for 36 d. Plasma concentrations of LH were higher in bulls treated with deslorelin on Day 1, had returned to typical levels by Day 8, and did not differ for control bulls and bulls treated with deslorelin from Day 8 to Day 29. Pituitary content of LH on Day 36 was reduced (P < 0.001) in bulls treated with deslorelin (33 ± 4 ng/mg) compared with control bulls (553 ± 142 ng/mg). Relative pituitary content of LHß-subunit mRNA was also reduced on Day 36 in bulls treated with deslorelin (Control, 0.65 ± 0.10; Deslorelin, 0.22 ± 0.04; P = 0.003). However, α-subunit mRNA relative content did not differ (Control, 0.73 ± 0.15; Deslorelin, 1.06 ± 0.12; P > 0.05). Plasma concentrations of testosterone were increased over the period of the experiment in the bulls treated with deslorelin compared with control bulls. This is the first demonstration of reduced pituitary content of LHß-subunit mRNA and LH, and unaltered content of α-subunit mRNA, in bulls treated with LHRH agonist. This was associated with apparently typical plasma concentrations of LH and elevated plasma testosterone. The anterior pituitary in bulls treated with LHRH agonist, therefore, undergoes classical desensitization and downregulation, but plasma LH and testosterone are not suppressed.     

Circulating testosterone and inhibin levels at different ages in the male beluga (Delphinapterus leucas)

This study is the first report on circulating testosterone and inhibin levels in a species of whales, the beluga. Circulating testosterone and immunoreactive (ir-) inhibin levels in two captive male belugas ("Nack", originally from Canada and "Duke", from the Okhotsk Sea) were measured every month for 9 years between 1995 and 2003. Assuming that clearly increased testosterone levels in the circulation indicates that the belugas had reached sexual maturity, at the ages of 10 ("Nack") and 11 years old ("Duke"). Their testosterone levels before the significant increase (pre-pubertal) were 0.42 ± 0.07 ng/ml (n=18) and 0.35 ± 0.10 ng/ml (n=18) and, those of after the increase (maturity) were 1.65 ± 0.14 ng/m l (n=74) and 2.06 ± 0.14 ng/ml (n=74). Circulating ir-inhibin levels before sexual maturity were 0.78 ± 0.04 ng/ml (n=18) and 0.64 ± 0.04 ng/ml (n=15) and, after sexual maturity were 0.52 ± 0.02 ng/ml (n=56) and 0.43 ± 0.02 ng/ml (n=67). Seasonal changes were observed in the testosterone levels after sexual maturity and the levels increased during March and April in Canadian origin "Nack", and peaked in February in Okhotsk origin "Duke". Circulating ir-inhibin level gradually decreased as they aged. A negative correlation between the circulating testosterone and ir-inhibin was observed. No seasonal changes were observed in the ir-inhibin levels after sexual maturity. These data will surely correspond to clarification of endocrinology and the successful reproduction of the beluga.     

Pituitary-testicular responses of estradiol-17 beta-implanted bull calves to continuous versus pulsatile infusion of luteinizing hormone releasing hormone

The luteinizing hormone (LH), follicle-stimulating hormone (FSH) and testosterone response of bull calves implanted with estradiol-17 beta to continuous and pulsatile infusion of luteinizing hormone releasing hormone (LHRH) has been examined. Estradiol-17 beta reduced serum LH and FSH concentrations and suppressed testosterone secretion and testicular growth when compared with sham-implanted bulls. Pulsatile iv infusion of LHRH [500 ng every 2 h (6 micrograms/d)] for a 4-wk period to estradiol-17 beta-implanted bulls resulted in elevated mean serum LH and testosterone concentrations that were characterized by discrete secretory episodes. Mean serum FSH was also increased by LHRH pulse infusion, but LHRH-coupled secretory episodes were not apparent. Continuous infusion of LHRH (6 micrograms/d) did not increase the low serum gonadotropin levels observed in estradiol-17 beta-implanted calves. Testicular growth was normal in LHRH pulse-infused calves, but was markedly curtailed in continuously infused calves. These results suggest that estradiol-17 beta inhibits testicular development by blocking gonadotropin release at the level of the hypothalamus because pulsatile administration of LHRH can override the inhibitory effect by increasing LH and FSH secretion.