Serum Gonadotropins After Gonadotropin-Releasing Hormone Injection in Bulls Subjected to Spacial Restriction

Response patterns of luteinizing hormone, follicle-stimulating hormone, and testosterone after injection of gonadotropin-releasing hormone were investigated in bulls grouped by weight (250 to 459 kg body weight) and confined five per pen in 9.2 or 6.4 m² space per bull in two replicates. Blood samples were collected for 1 h prior to injection of 100 μg gonadotropin releasing hormone and 5 h after injection at 15-min intervals. Overall mean luteinizing hormone concentrations were not affected by spacial restriction or replicate. Interaction of treatment by time revealed that luteinizing hormone response curves were not similar. Restricted bulls had a higher response of luteinizing hormone to gonadrotropin-releasing hormone. Follicle-stimulating hormone increased in all groups within 15 min and peaked at 219.4 ng/ml at 45 min. Both gonadotropin responses returned to preinjection concentrations by 4 h. Testosterone was affected by treatment, replicate, and time of sampling. Testosterone was higher in restricted bulls and higher in replicate 2. Mean testosterone peak following gonadotropin-releasing hormone was 3.86 ng/ml and occurred between 105 and 120 min which was approximately 90 min after the gonadotropin peaks. It appears that hormone responses to gonadotropin-releasing hormone were not depressed by spacial restriction, and additional spacial restriction of young bulls could be used commercially.     

Effect of adrenocorticotropic hormone and associated hormonal responses on semen quality and sperm output of bulls

The objective was to determine if semen quality and output could be affected by pharmacological doses of adrenocorticotropin. Three Holstein bulls, one 7 yr old and two yearlings, were treated with 200 IU adrenocorticotropic hormone every 8 h for 6 days. Effects of treatment on semen traits and peripheral concentrations of glucocorticoids, testosterone, and androstenedione were measured. Viability of spermatozoa (percentage motility and percentage intact acrosomes), ejaculate volume, sperm concentration, and weekly sperm output were unaffected by adrenocorticotropin treatment up to 8 wk posttreatment. The proportion of spermatozoa with cytoplasmic droplets and head abnormalities was elevated slightly from .8 +/- .1 and 4.2 +/- .3% to 1.3 +/- .2 and 5.8 +/- .4% during treatment. Total glucocorticoids increased from 11.9 +/- 2.7 ng/ml before treatment to 73.5 +/- 4.1 ng/ml during treatment. Testosterone decreased in the yearling bulls from 5.5 +/- .9 ng/ml plasma before treatment to .5 +/- .5 ng/ml afterward; the decrease began 8 h following the initial adrenocorticotropin injection and persisted until 24 h following last injection. The mature bull had normal testosterone concentrations for the first 4 days of injection, and decreased concentrations for the last 2 days of injection. Semen viability, concentration, and sperm output are unaffected by a pharmocological administration of adrenocorticotropin and subsequent marked increase of glucocorticoids and decrease of testosterone. Only a small increase of semen content of immature sperm or sperm with abnormal heads may be associated with these marked endocrine changes.     

Testosterone concentrations in plasma of dairy cows with ovarian cysts.

Testosterone in plasma was measured by radioimmunoassay in cows with ovarian cysts and in cows during the estrous cycle. Concentrations of testosterone in plasma (60.8 +/- 2.7 pg/ml; mean +/- standard error) in cows with ovarian cysts were variable but within the range in cows during the estrous cycle. Mean concentrations of testosterone in plasma increased from 46.3 pg/ml at estrus to 85.1 pg/ml on day 13 of the estrous cycle. In cows with ovarian cysts, concentrations of testosterone in plasma declined within 24 h following prostaglandin F2 alpha and tended to increase by 9 days following gonadotropin releasing hormone. The role of testosterone in estrual behavior and inhibiting resumption of ovarian cycles is likely minimal.     

Growth hormone response to growth hormone-releasing factor in selected and control Holstein heifer calves

Holstein heifer calves from selected and control lines were treated with growth hormone-releasing factor to determine whether the magnitude of the pituitary growth hormone response is associated with genetic potential for milk yield. Cows in the selected line were bred to highest Predicted Difference milk sires available each year, while control line cows were bred to bulls that were average for 1964 milk production. Eight calves from each line of similar age (206 to 210 days) and weight (205 kg average) were injected intravenously with .1 microgram/kg growth hormone-releasing factor. Dams' first lactation 305-day mature equivalent records were 10,192 +/- 495 kg for selected calves and 6,908 +/- 376 kg for control calves. Before treatment, GH tended to be higher in selected than control calves (mean +/- SE: selected = 7.5 +/- 1.8, control = 4.6 +/- .4 ng/ml). After injection, peak GH were reached within 20 min and were similar in selected and control calves, averaging 66 +/- 16 and 73 +/- 13 ng/ml, respectively. Results indicate that the magnitude of the growth hormone response to growth hormone-releasing factor is not related to future performance of dairy calves.     

Changes of luteinizing hormone and progesterone for dairy cows after gonadotropin-releasing hormone at first postpartum breeding

Gonadotropin-releasing hormone administered at breeding enhances fertility of dairy cows, so a study was designed to evaluate the mechanism for enhanced fertility following administration of gonadotropin-releasing hormone at first postpartum breeding. Twenty-four cows were assigned randomly to one of two treatments, 100 micrograms of gonadotropin-releasing hormone intramuscular or saline vehicle intramuscular at insemination. Blood samples for luteinizing hormone assay were taken at 2-h intervals prior to breeding and .5-h intervals for 3 h after insemination. Composite morning milk samples for progesterone assay were collected for 30 days after insemination or until next estrus. Cows given gonadotropin-releasing hormone had higher luteinizing hormone concentrations in blood serum following treatment than cows given saline, 13.2 versus 3.0 ng/ml. There was no relationship between luteinizing hormone and subsequent conception. Progesterone for cows that became pregnant was higher throughout sampling days. Mean progesterone concentrations were 4.6 versus 2.2 ng/ml in pregnant and nonpregnant cows during the first 4 days after insemination. Cows treated with gonadotropin-releasing hormone that conceived had higher progesterone than other cows, and that was evident at the first 4 days postbreeding.     

Effects of selection for milk yield on growth hormone response to growth hormone releasing factor in growing Holstein calves

Bull and heifer calves (n = 81) from genetic lines of Holstein cows that differed by more than 4000 kg milk/305-d lactation were used to determine effects of selection for milk yield on growth hormone (GH) response to a GH releasing factor (GRF) analog. Calves received GRF (4 microg/100 kg BW) on 10, 56, 140, 196, 252, and 364 +/- 3 d of age. Jugular blood samples (n = 15) were obtained from -30 to 120 min relative to GRF administration. Area under the GH response curve (0 to 60 min, AUC60) was quantified after subtracting mean prechallenge GH concentrations. Data were analyzed for effects of line, age, gender, and their interactions with PROC MIXED of SAS for repeated measures and incorporated the spatial power law for unequally spaced data with age as the repeated effect. Means were considered different when P < 0.05. Prechallenge GH concentrations did not differ between lines, were greater in bulls than heifers (4.6 vs. 3.7 ng/ml), and decreased with age. The AUC60 decreased with age but did not differ between lines. Heifers responded more to GRF than bulls (1550 vs. 1336 ng x min/ml). Peak GH concentration decreased with age and was less in bulls than heifers (54.7 vs. 62.1 ng/ml) but did not differ between lines. Although plasma GH has been identified as an inheritable trait, we conclude the GH variables measured in this study were not useful in predicting genetic merit of calves from these substantially different lines of cows.     

Response of bovine serum prolactin and growth hormone to duodenal, abomasal, and oral administration of thyrotropin-releasing hormone.

Thyrotropin-releasing hormone was injected into the duodenum of two 500-kg steers, placed into the abomasum of two prepubertal bulls, and fed to four bull calves (1 to 3 wk of age) to test the effect on concentrations of prolactin and growth hormone in blood serum. Before 20 and 200 mg of thyrotropin-releasing hormone were injected into the duodenum, prolactin in serum averaged 7.5 and 9.4 ng/ml and increased to 52.5 and 129.6 ng/ml at 45 and 35 min after treatment. Average growth hormone concentration of serum was increased also, but the response was more variable than prolactin. Peak concentrations of prolactin and growth hormone in blood serum were 5 to 10 times greater after treatment with thyrotropin-releasing hormone (40 mg/100 kg body weight into abomasum) than before treatment. Within 30 min after oral administration of thyrotropin-releasing hormone (0, .5, 1, and 2 mg/kg body weight) growth hormone concentration of serum was 30, 306, 356, and 317% greater than pretreatment. Prolactin concentration of serum, however, was increased in only one calf.     

Luteinizing hormone response to pulsatile luteinizing hormone-releasing hormone in prepubertal heifers

The effects of 12 hourly 5-micrograms injections of luteinizing hormone-releasing hormone on luteinizing hormone release, were examined in 18 prepubertal Holstein heifers at 4, 7, or 10 mo of age. During a 6-h pretreatment period, mean serum luteinizing hormone concentrations and mean number of endogenous luteinizing hormone episodes per hour were not influenced by age. The 12-h treatment regimen induced a pulsatile release of luteinizing hormone in all heifers. The magnitude, pattern, and total amount of luteinizing hormone released were not influenced by age. However, in the 4 and 10-mo-old age groups, magnitude of luteinizing hormone response to the 3rd hourly injection of luteinizing hormone-releasing hormone was greater than the response to the second injection. Magnitudes of luteinizing hormone responses decreased with time after the 4th hourly injection through the 12th injection and patterns of decline appeared similar among the three age groups. The pituitary of the prepubertal dairy heifer is able to respond to an hourly pulsatile administration of luteinizing hormone-releasing hormone and this treatment regimen appears to produce a self-priming effect on luteinizing hormone release.     

Adrenal responsiveness in pre- and postpartum dairy cows.

Adrenal responsiveness was evaluated by injecting 10 multiparous dairy cows with 200 IU adrenocorticotropin between -13 and -2 days prepartum (I) and postpartum between 24 and 40 h (II) and 21 and 24 days (III). Concentrations of glucocorticoids following injection were influenced by day of injection, temperature, and minimum percent relative humidity but not by breed, breed X injection day interaction, or age of cow. Likewise differences in regressions for adrenal response and mean response (ng/ml) for the three injections were nil. Mean concentrations at peak (45, 60, and 120 min postinjection samples) adjusted for preinjection concentrations also did not differ for the three periods of injection. Mean concentrations of glucocorticoids in plasma for daily samples between -13 and -2 days prepartum were 5.3 +/- .4 (n = 61), reached a peak of 14.8 +/- .3 ng/ml the day of calving, and remained high for 2 days postpartum. Estradiol increased through prepartum sampling from 23.3 to 339.6 +/- 94.1 pg/ml the day of calving, then declined abruptly. Progestins began to decline about -5 days prepartum from mean concentration of 4.09 +/- .62 (n = 25) and attained low concentrations (.30 +/- .06 ng/ml) 2 days postpartum. Although there was a surge of glucocorticoids at parturition, this was not associated with a modification in adrenal responsiveness or with prepartum concentrations of other steroid hormones of plasma. Adrenal potential in prepartum and postpartum dairy cows appears well maintained.     

Effect of Carrier and Administration on Luteinizing Hormone Release By Gonadotropin Releasing Hormone

Ovariectomized ewes in groups of four were administered either saline (I) or 50 μg of gonadotropin releasing hormone (II to VII). Methods of administration and carriers were: II, intramuscular saline; III, intravenous saline; IV, subcutaneous saline; V, subcutaneous gelatin; VI, subcutaneous carboxymethylcellulose; and VII, subcutaneous gelatin capsule. Only subcutaneous administration of gonadotropin releasing hormone in the gelatin capsule (VII) prolonged the interval from treatment to luteinizing hormone peak; however, there were fewer ewes with peak luteinizing hormone concentations equal to or greater than 100 ng/ml in VII than II, IV, V, and VI. Ewes in V, VI, and VII had luteinizing hormone surges of longer duration than II and III. Ewes in VII had the longest luteinizing hormone surge, which was 7.5 h, even though luteinizing hormone concentrations did not return to pretreatment concentrations by 8 h posttreatment in three of the four ewes.     

Reproductive hormones associated with normal and abnormal changes in ovarian follicles in postpartum dairy cows.

Concentrations of reproductive hormones and ovarian changes were monitored every 4 days during the postpartum period in 35 dairy cows. The cows were classified as either control (22 cows) or cystic (13 cows) based on per rectum examination of the ovaries. Control cows were detected with a corpus luteum after the first postpartum ovulation while in cystic cows, ovarian follicles of at least 2.5 cm in diameter persisted in the absence of a corpus luteum for at least 14 days. The mean postpartum interval to the first follicle 1.5 to 2.0 cm in diameter was about 16 days for both groups. Ovulation was detected earlier postpartum for cows in the control group (18.4 +/- 1.7 days) than for cows in the cystic group (36.3 +/- 2.7 days). Luteinizing hormone in plasma increased during the early postpartum period (days 1 to 10) for both groups, but mean concentrations of luteinizing hormone, progesterone, and estradiol-17 beta in plasma were not different between groups then or on days -8, -4, or 0 (day of the first postpartum follicle 1.5 to 2.0 cm in diameter). During this period (days -8 to 0), estradiol-17 beta and luteinizing hormone in plasma were correlated positively, but regression coefficients differed between groups. These results suggest a partial hypothalamic and/or pituitary failure in releasing pituitary luteinizing hormone as a cause for postpartum ovarian cysts.     

Effect of supplemental beta-carotene on incidence and responsiveness of ovarian cysts to hormone treatment

Sixty-two multiparous and 35 primiparous Holstein cows were assigned randomly at 10 days postpartum to receive a ration with or without 300 mg beta-carotene/cow per day. Multiparous and primiparous cows were grouped separately and group-fed complete rations once daily. Incidence of ovarian cysts (26% by rectal palpation) was not affected by beta-carotene fed. Multiparous cows had greater incidence (39%) of ovarian cysts than primiparous cows (11%). Fifty-seven percent of cysts were classified follicular by rectal palpation. Progesterone concentration of milk also was used for diagnosis of type of cyst. Cows with ovarian cysts and with progesterone concentrations in milk less than 1 ng/ml were classified follicular, and those having concentration greater than 1 ng/ml were classified luteal. As determined by milk progesterone, rectal palpation was more accurate for diagnosis of luteal cysts than for diagnosis of follicular cysts. Progesterone concentrations of milk for animals with luteal and follicular cysts were 10.66 +/- 1.29 and .37 +/- .07 ng/ml. beta-Carotene did not affect response or days to respond to treatment with human chorionic gonadotropin or gonadotropin-releasing hormone. Supplemental beta-carotene was not beneficial for reducing incidence of ovarian cysts in cows receiving an adequate supply of beta-carotene in their diet.     

Transitory changes in plasma progestins, estradiol, and luteinizing hormone approaching ovulation in the bovine.

Progestins, estradiol, and luteinizing hormone were measured by radioimmunoassay in bovine plasma samples collected from indwelling jugular catheters daily on days minus 6 to minus 4, every 6 h on days minus 3 and minus 2, and every 2 h from day minus 1 to ovulation. Least squares analyses characterized time trends within animals and interrelationships between these hormones. Plasma progestins decreased from 5.7 ng/ml at day minus 6 to .07 ng at the peak of luteinizing hormone. No preovulatory peripheral increase in plasma progestins was detected. Estradiol increased from 2 pg/ml at day minus 4 to 6 pg at minus 12 h and then increased abruptly to 7.4 pg at the peak of luteinizing hormone. This matter increase in estradiol was synchronous with the acute preovulatory surge of luteinizing hormone. Association was positive between estradiol and luteinizing hormone, but negative between progestins and estradiol. Estradiol decreased 50% by 5 h following the peak of luteinizing hormone with a return to base (2 pg) at 14 h. An increase of .1 ng luteinizing hormone/ml plasma per day was linear from day minus 6 to 8 h prior to the luteinizing hormone peak. Luteinizing hormone increased to a peak of 13.5 ng/ml and remained elevated for 10.5 h. These results are consistent with the hypothesis that an increase at proestrus in estradiol rather than progestins triggers the preovulatory surge of luteinizing hormone in the bovine.     

Growth hormone response to growth hormone releasing hormone in calves that differ in genetic merit for milk yield

Holstein heifer, steer, and bull calves from control (CL) and select (SL) lines of cows that differed by more than 4000 kg of milk during a 305-d lactation (SL > CL) were used to determine growth hormone (GH) response to 5 doses of GH releasing hormone (GHRH) and how this response was affected by gender, period (age), and genetic merit for milk yield. Doses (0, 2.5, 5, 10, and 20 microg/100 kg of BW) of a GHRH analog were assigned randomly to each heifer (4 CL, 4 SL), steer (4 CL, 4 SL), and bull (3 CL, 3 SL) calf and administered on consecutive days at approximately 3, 6, and 10 mo of age (periods; P1, P2, and P3). Jugular blood samples (n = 15) collected between -30 and 240 min relative to GHRH administration were used to quantify area under the GH response curve (AUC) after subtracting mean prechallenge GH concentrations. Estimates of maximum response (Rmax) and sensitivity (ED50) to GHRH were obtained from the hyperbolic dose response curves (AUC vs. dose). Data were analyzed for effects of dose, line, period, gender, and their interactions with period as the repeated effect. Prechallenge GH concentrations were not affected by genetic line, gender, or period. The AUC was not affected by line, but decreased with period and increased with GHRH dose. The Rmax did not differ between lines or among genders, but decreased with period. The ED50 did not differ between lines or among periods, but heifers were more sensitive to GHRH than steers or bulls. Although GH response to GHRH has been identified as a potential indicator of genetic merit, it did not differ between these substantially different genetic lines.     

Effect of synthetic human pancreatic growth hormone-releasing factors on plasma growth hormone concentrations in lactating cows

Two fragments of human pancreatic growth hormone-releasing factor, hpGRF-(1-24)-NH2 and hpGRF(1-29)-NH2, were administered to lactating dairy cows in separate experiments. Each experiment involved a 4 X 4 Latin square with treatments being intravenous administration of saline (controls) or three doses of releasing factor (500, 1000, and 2000 micrograms). Administration of the 24-amino acid fragment increased plasma growth hormone concentration from baseline 3 ng/ml to peak 14 ng/ml within 10 min. Growth hormone concentrations returned to baseline by 60 min post treatment. The 29-amino acid fragment resulted in greater responses in both peak concentration (30 ng/ml) and persistency (greater than 240 min). Relationships between dose of releasing factor and response in growth hormone release (as measured by area under the response curve) were not consistent in most cows. Responses of concentrations of growth hormone in blood plasma that result from intravenous injection of up to 2000 micrograms of growth hormone-releasing factor (1-29) would not be sufficient to increase milk yield of cows.     

Seasonal changes in testicular size and serum LH,prolactin and testosterone concentrations in male polar bears (Ursus maritimus)

Little is known about the reproductive endocrinology of the male polar bear, Ursus maritimus, except that serum testosterone concentrations are high in April and May during the mating season and are low from August to November during the non-mating season. The objective of this study was to describe the relationship between seasonal changes in testicular size and serum concentrations of testosterone, LH and prolactin. Blood samples and testicular measurements were obtained from free-ranging male polar bears in Canada in April (n = 5) and May (n = 15) near Resolute Bay, Northwest Territories and near Churchill, Manitoba in July (n = 15) and October (n = 22). Testis size was greater in May (39.4 +/- 3.5 cm(2)) than in October (27.3 +/- 2.0 cm(2)) (P = 0.002). Serum testosterone concentrations were approximately three-fold higher in April (5.8 +/- 0.8 ng ml(-1)) than in May (1.7 +/- 0.5 ng ml(-1)), July (0.6 +/- 0.2 ng ml(-1)) and October (1.1 +/- 0.2 ng ml(-1)). Similarly, serum LH concentrations were high in April (0.14 +/- 0.04 ng ml(-1)) and low in May (0.09 +/- 0.01 ng ml(-1)), July (0.10 +/- 0.02 ng ml(-1)) and October (0.08 +/- 0.00 ng ml(-1)). Serum prolactin concentrations were high in April (1.9 +/- 0.3 ng ml(-1)), highest in May (2.5 +/- 0.2 ng ml(-1)), lower in July (1.3 +/- 0.1 ng ml(-1)) and lowest in October (0.8 +/- 0.07 ng ml(-1)). The present study demonstrates a positive relationship between testicular size and serum concentrations of LH, prolactin and testosterone in the male polar bear and confirms the previously reported seasonal changes in serum testosterone concentrations. Data from the present study provide important baseline and comparative endocrine information that can be used to aid captive breeding programmes in zoos and to further ecological-behavioural studies of polar bears.     

Growth hormone-releasing factor stimulates milk production and sustains growth hormone release in Holstein cows

Serum growth hormone was determined in lactating cows following repeated intravenous injections of growth hormone-releasing factor. A given dose was injected every 4 h for 24 h in a 4 (cow) X 4 (d) Latin square. Growth hormone increased similarly above controls after 10, 20, or 40 micrograms releasing factor/100 kg body weight. In another experiment the effects on lactational performance and growth hormone responses of cows to repeated injections of releasing factor for 10 d were determined in a 2 (cow) X 2 (period) Latin square crossover. Administration of 20 micrograms releasing factor/100 kg body weight to 16 Holstein cows (lactating 4.5 to 7.5 mo) every 4-h for 10 d increased milk yield from 25.4 to 27.7 kg/d and increased total fat, protein, and lactose 11% above controls. Releasing factor did not affect milk composition or feed intake. Peak growth hormone response to releasing factor was similar between d 1 (19.9 ng/ml) and 10 (24.4 ng/ml). Exogenous growth hormone-releasing factor administered to lactating Holstein cows at the doses tested: 1) increases growth hormone consistently, although the response is not dose dependent, 2) is galactopoietic, 3) causes an apparent increase in feed to milk conversion, and 4) increases growth hormone to at least the same magnitude on d 10 as on d 1.     

Follicular development after administration of adrenocorticotropin to nonlactating Holstein cows

The effect of chronic administration of adrenocorticotropin on ovarian follicular development was studied. Twelve nonlactating Holstein cows received either 100 IU adrenocorticotropin (n = 6) or saline (n = 6) at 12-h intervals, commencing d 16 and continuing until d 23 of an induced estrous cycle (estrus = d 0). Cows were slaughtered on d 24, ovaries collected, and number of visible antral follicles recorded. Estradiol-17 beta, androstenedione, and testosterone in follicular fluid, and luteinizing hormone and follicle-stimulating hormone receptors in follicular tissue of the largest follicles were determined. Largest follicles were classified as ovulatory or nonovulatory based on the estrogen to androgen ratio. One cow treated with adrenocorticotropin, but none treated with saline, had ovulated by slaughter. The numbers of small, medium, and large antral follicles were 0, 1, and 5 for cows treated with adrenocorticotropin and 0, 1, and 6 for cows treated with saline. Follicular diameter (15.0 +/- 1.0 versus 14.0 +/- 2.0 mm) and follicular fluid volume (2.9 +/- .8 versus 2.2 +/- .5 ml) of the largest follicle in cows treated with adrenocorticotropin or saline were not different. No differences were found in the number of luteinizing hormone and follicle stimulating hormone receptors nor in the proportion of ovulatory versus nonovulatory follicles between treatments. We conclude that adrenocorticotropin administered at 100 IU every 12 h during the follicular phase does not significantly alter follicular development in the nonlactating dairy cow.     

Comparison of total and free cortisol in bovine serum and milk or colostrum

Total (free plus protein-bound) and free cortisol concentrations in paired samples of serum and skim milk or colostrum were determined by specific radioimmunoassay after extraction or ultrafiltration. For 11 cows in established lactation mean total cortisol concentrations +/- SE in serum and milk were 4.5 +/- .7 ng/ml and .35 +/- .06 ng/ml, respectively. Concentrations of free cortisol in serum and milk were about .3 ng/ml, which is at the lower limit of the present assay. Following 200 IU Synacthen injection to 5 cows, total cortisol in serum and milk rose to 67 +/- 6.0 ng/ml and 11.4 +/- 1.5 ng/ml by 8 h postinjection, while free concentrations increased to 4.3 +/- .7 ng/ml and 6.5 +/- .8 ng/ml in serum and milk, respectively. Eight additional cows were sampled at 1 to 2 d after parturition, and total cortisol in serum and colostrum was 16.6 +/- 1.6 ng/ml and 4.4 +/- 1.3 ng/ml, and free cortisol was 2.4 +/- .4 ng/ml and 1.8 +/- .5 ng/ml, respectively. Free cortisol concentrations in colostrum or in milk from cows in established lactation, before or after adrenal stimulation, are directly related to free cortisol concentrations in the blood, while the bound cortisol in milk is more related to these two stages of lactation rather than the bound hormone in the blood. Measurement of free cortisol in milk should allow the monitoring of changes in plasma free cortisol in studies of stress in dairy cows.     

Continuous elevation of blood growth hormone concentrations by beeswax implant

We examined constancy of release of purified ovine growth hormone from an implant containing soybean oil and beeswax. Implants contained an amount of growth hormone that was sufficient to increase concentrations in blood plasma by 20 and 40 ng/ml and to maintain those concentrations over 1 wk. Growth hormone in plasma increased to approximately 65 ng/ml in lambs receiving low dose implants the 1st day after implantation, returned to 31 ng/ml on day 2, and remained near this concentration for the remainder of the week. Pulse release of growth hormone was not similiar in the high dose lambs where growth hormone concentration in plasma averaged 45 ng/ml 1 day after implantation, then gradually increased to 60 ng/ml on day 6. Unimplanted control lambs had mean growth hormone concentrations of 2.9 to 3.9 ng/ml throughout the 6-day observation. This approach should interest investigators studying the chronic influence of purified or synthetic growth hormone on dairy cows, beef steers, or lambs.