H. Allen Tucker

Relationship of Ambient Temperature to Serum Prolactin in Heifers

Summary Four heifers were exposed to 10° and 27° for 5 days after preconditioning to 21° in a controlled environment chamber. Serum prolactin decreased from 13 to 4 ng/ml during the 4-hr interval when ambient temperature was reduced from 21° to 10°. Prolactin increased from 8 to 22 ng/ml during the 3-hr period when ambient temperature was increased from 21° to 27°. During 5 days of chronic exposure to 10°, serum prolactin was 38% lower (P < 0.05) than during a control period at 21°; and while at 27°, heifers had twice the concentration of serum prolactin (P < 0.10) as those at 21°. Injection of 10 μg of thyrotropin releasing hormone (TRH) caused serum prolactin to increase within 5 min from 20 to 140 ng/ml in heifers at 27° and from 8 to 70 ng/ml in heifers at 10°. We conclude that ambient temperature influences basal and TRH-stimulated concentrations of serum prolactin in heifers.

Effects of Ambient Temperature and Relative Humidity on Serum Prolactin and Growth Hormone in Heifers

: Twelve heifers were exposed to 21 degrees ambient temperature for 10 days, and then subjected to 4.5, 21, or 32 degrees for 9 days in controlled environmental chambers. Serum prolactin (PRL) decreased linearly (P less than 0.01; 0.6 ng/ml/degrees) as the temperature was reduced during the first day from 21 to 4.5 degrees; serum PRL increased linearly (P less than 0.05; 1.17 ng/ml/degrees) as the temperature was increased from 21 to 32 degrees. Between Days 2 and 9 serum PRL averaged 2.6, 13.0, and 27.7 ng/ml (P less than 0.05) at 4.5, 21, and 32 degrees, respectively. Injection of thyrotropin-releasing hormone (TRH) caused serum PRL to increase within 5 min from 20.4 to 109.8 ng/ml at 32 degrees, at 21 degrees serum PRL increased from 15.7 to 62.8 ng/ml, whereas at 4.5 degrees serum PRL did not respond to TRH. Serum growth hormone (GH) averaged 4.0, 6.3, and 9.4 ng/ml at 4.5, 21, and 32 degrees, respectively, but these means were not different (P greater than 0.10). TRH released GH at all temperatures tested, but the quantity released was unaffected by ambient temperature. Relative humidities of 50 and 90% did not significantly alter (P greater than 0.05) serum PRL or GH. We conclude that ambient temperature affects basal and TRH-stimulated concentrations of serum PRL but not GH in heifers.Summary Twelve heifers were exposed to 21° ambient temperature for 10 days, and then subjected to 4.5, 21, or 32° for 9 days in controlled environmental chambers. Serum prolactin (PRL) decreased linearly (P < 0.01; 0.6 ng/ml/°) as the temperature was reduced during the first day from 21 to 4.5°; serum PRL increased linearly (P < 0.05; 1.17 ng/ml/°) as the temperature was increased from 21 to 32°. Between Days 2 and 9 serum PRL averaged 2.6, 13.0, and 27.7 ng/ml (P < 0.05) at 4.5, 21, and 32°, respectively. Injection of thyrotropin-releasing hormone (TRH) caused serum PRL to increase within 5 min from 20.4 to 109.8 ng/ ml at 32°; at 21° serum PRL increased from 15.7 to 62.8 ng/ml, whereas at 4.5° serum PRL did not respond to TRH. Serum growth hormone (GH) averaged 4.0, 6.3, and 9.4 ng/ml at 4.5, 21, and 32°, respectively, but these means were not different (P > 0.10). TRH released GH at all temperatures tested, but the quantity released was unaffected by ambient temperature. Relative humidities of 50 and 90% did not significantly alter (P > 0.05) serum PRL or GH. We conclude that ambient temperature affects basal and TRH-stimulated concentrations of serum PRL but not GH in heifers. The technical assistance of Eileen Bostwick, Larry Brock, and David Kreider is most appreciated.

Seasonality in cattle

Abstract Although cattle are basically not seasonal breeders, several physiological changes occur in response to two major climatic variables of season: ambient temperature and photoperiod. Ambient temperatures above 27°C lengthen the estrous cycle, decrease duration and intensity of estrus, decrease fertility and increase embryonic mortality. Some of these effects may be mediated via decreased secretion of luteinizing hormone and increased secretion of progesterone. Heat stress also suppresses growth rate, milk production and feed intake. In comparison with cattle exposed to less than 12 h of light per day, 14 to 16 h of light has relatively little effect on secretion of gonadotropins and fertility; however, growth rates, feed intake and milk yield are stimulated. Of all hormones measured in cattle, prolactin is the most responsive to changes in seasons, ambient temperature and photoperiod. Prolactin is greatest in summer when ambient temperatures are highest and photoperiods are longest.

Influence of Number of Suckling Young on Nucleic Acid Content of Lactating Rat Mammary Gland.

Summary The influence of varying numbers of suckling young on the number of cells (DNA) and protein synthetic activity (RNA and RNA/DNA) was determined in abdominalinguinal mammary glands of rats lactating 21 days. The data indicated that mammary gland development and secretory activity increased as the number of suckling young per mother rat increased.

Estimates of parenchymal, stromal, and lymph node deoxyribonucleic acid in mammary glands of C3H/Crg1/2 mice☆☆☆

Abstract The DNA content of intact and mammary gland (MG)-free segments of #4 pads of virgin and lactating C3H/Crg 1 2 female mice was determined. The DNA of inguinal MG lymph nodes was also estimated. The DNA of the MG-free-pad did not change between the virgin and lactational states; MG-DNA in the intact virgin and lactating fat pads was 23% and 89% of the total DNA, respectively. In lactating mice, MG-DNA increased 27-fold over that present in virgin mice. Inguinal lymph node DNA was 8.5 times greater than the MG-DNA of virgins and about 31% as high as the MG-DNA of the lactating gland.

Estrogen and androgen elicit growth hormone release via dissimilar patterns of hypothalamic neuropeptide secretion.

The dimorphic pattern of growth hormone (GH) secretion and somatic growth in male and female mammals is attributable to the gonadal steroids. Whether these hormones mediate their effects solely on hypothalamic neurons, on somatotropes or on both to evoke the gender-specific GH secretory patterns has not been fully elucidated. The purpose of this study was to determine the effects of 17beta-estradiol, testosterone and its metabolites on release of GH, GH-releasing hormone (GHRH) and somatostatin (SRIF) from bovine anterior pituitary cells and hypothalamic slices in an in vitro perifusion system. Physiological concentrations of testosterone and estradiol perifused directly to anterior pituitary cells did not affect GH releases; whereas, dihydrotestosterone and 5alpha-androstane-3alpha, 17beta-diol increased GH. Perifusion of testosterone at a pulsatile rate, and its metabolites and estradiol at a constant rate to hypothalamic slices in series with anterior pituitary cells increased GH release. The androgenic hormones increased GHRH and SRIF release from hypothalamus; whereas, estradiol increased GHRH but decreased SRIF release. Our data show that estradiol and the androgens generated distinctly different patterns of GHRH and SRIF release, which in turn established gender-specific GH patterns.

Effect of light therapy on salivary melatonin in seasonal affective disorder

To investigate the role of a light-induced advance in the timing of the melatonin rhythm in seasonal affective disorder, 11 depressed patients underwent 2 weeks of light therapy with full spectrum or cool white light. Evening saliva samples were collected before and after each week of treatment and assayed for melatonin to determine the time of onset of nocturnal secretion. Both treatments reduced depression scores, advanced the timing of the melatonin rhythm, and increased melatonin concentrations. Time of onset of the nocturnal increase in melatonin did not differ between clinical responders and nonresponders, suggesting that a phase advance in the onset of nocturnal melatonin secretion is not sufficient to induce clinical remission in seasonal affective disorder.

Influence of melatonin on mammary gland growth: in vivo and in vitro studies.

Abstract Our objective was to determine whether melatonin influenced mammary growth in response to mammogenic hormones. Prepubertal female BALB/c mice were injected for 9 days with 1 μg of 17β-estradiol and 1 mg of progesterone or 17β-estradiol/ progesterone plus 50, 100, or 200 μg of melatonin. Area of the parenchyma and total DNA content of the second thoracic gland were similar between controls and melatonin-injected mice. However, μg of DNA/100 mg of mammary tissue were lower in animals treated with 17β-estradiol/progesterone plus 200 μg of melatonin than in controls. Triglyceride content of mammary glands from animals treated with 100 or 200 μg of melatonin/day increased relative to controls. In an in vitro experiment, thoracic mammary glands of 21-day-old mice were cultured for 6 days in a mammogenic milieu of hormones (17β-estradiol/progesterone, aldosterone, bovine prolactin, growth hormone, and insulin) with 0 (control), 10-6, 10-9, or 10-12 M melatonin. Relative to controls, 10-12 M melatonin increased and 10-6 M melatonin decreased mammary DNA and uptake of [methyl-3H]thymidine. We conclude that high doses of melatonin reduce mammary development in normal mice and that some of this effect may be mediated directly at the mammary tissue.

Milking, thyrotropin-releasing hormone and prostaglandin induced release of prolactin and growth hormone in cows.

Summary Within 30 min of starting continuous iv infusion of 333 μg thyrotropin-releasing hormone (TRH)/hr into 12 cows, serum prolactin (PRL) increased more than 10-fold and growth hormone (GH) increased 2.6- to 4-fold above basal concentrations. Constant infusions of 30 mg/hr of prostaglandin F2α (PGF2α) increased serum PRL and GH to maxima within 30-40 min which were 64- and 5-fold greater than basal concentrations. Despite continuous infusion of TRH or PGF2α, PRL declined throughout the 6- to 13-hr infusion interval although it remained well above preinfusion or saline-infusion control values. Serum GH declined more rapidly than PRL in the face of TRH or PGF2α infusions, reaching basal concentrations in one experiment within 1-3 hr. Application of milking stimuli during the 5th hr of TRH infusion caused an additional increase of 23 ng/ml of PRL above the TRH-stimulated concentrations. Similarly, intravenous injection of 5 mg PGF2α during the 5th or 12th hr of TRH infusion increased serum PRL an additional 582-682 ng/ml and further increased serum GH 9-91 ng/ml. When 200 μg TRH was injected during the 5th hr of a PGF2α infusion, serum PRL increased another 267 ng/ml and GH increased an additional 61 ng/ml. Administration of 10 doses of TRH of 200 μg each in 2 hr did not increase PRL or GH in the serum above that observed when 2.2 mg TRH was infused over a 6-hr period. Collectively, the data suggest that a ceiling exists in cows for secretion of PRL and GH, but this ceiling may be overcome with application of a second heterologous stimulus.

Growth Hormone Response of Bull Calves to Growth Hormone-Releasing Factor

Abstract Three experiments were conducted to determine serum growth hormone (GH) response of bull calves (N = 4; 83 kg body wt) to iv injections and infusions of human pancreatic GH-releasing factor 1-40-OH (hpGRF). Peak GH responses to 0, 2.5, 10, and 40 μg hpGRF/100 kg body wt were 7 ± 3, 8 ± 3, 18 ± 7, and 107 ± 55 (mean peak height ± SEM) ng/ml serum, respectively. Only the response to the 40-μg dose was greater (P < 0.05) than the 0-μg dose. Concentrations of prolactin in serum were not affected by hpGRF treatment. In calves injected with hpGRF (20 μg/100 kg body wt) at 6-hr intervals for 48 hr, GH increased from a mean preinjection value of 3.1 ng/ml serum to a mean peak response value of 70 ng/ml serum. Differences in peak GH response between times of injection existed within individual calves (e.g., 10.5 ng/ml vs 184.5 ng/ml serum). Concentrations of GH in calves infused continuously with either 0 or 200 μg hpGRF/hr for 6 hr averaged 7.4 ± 3 and 36.5 ± 11 ng/ml serum, respectively (P < 0.05). Concentrations of GH oscillated markedly in hpGRF-infused calves, but oscillations were asynchronous among calves. We conclude that GH response of bull calves to hpGRF is dose dependent and that repeated injections or continuous infusions of hpGRF elicit GH release, although magnitude of response varies considerably. We hypothesize that differences in GH response to hpGRF within and among calves, and pulsatile secretion in the face of hpGRF infusion may be related to the degree of synchrony among exogenous hpGRF and endogenous GRF and somatostatin.