E. M. Convey

Bovine Serum Prolactin, Growth Hormone, Cortisol and Milk Yield after Ergocryptine

Ten lactating Holstein cows were randomly assigned to receive, on 2 consecutive days, either 80 mg ergocryptine (Sandoz-Basel) in 50% ethanol, or 50% ethanol alone. On the day before treatment, serum

Gonadotropin releasing hormone induced luteinizing hormone release after prostaglandin F2alpha in heifers.

Summary Twenty Holstein heifers in diestrus were given 30 mg of PGF2α Tham salt (im). Thereafter each of five heifers received no further treatment or 40 μg of GnRH (im) at 10, 30, or 50 hr after PGF2α. Serum progesterone decreased to less than 1 ng/ml at 24 hr after PGF2α and remained low throughout the remainder of the sampling period. Following PGF2α, estradiol concentrations increased gradually in serum but GnRH given at 30 hr after PGF2α abruptly decreased estradiol concentration. The magnitude of LH release induced by GnRH given at 30 or 50 hr after PGF2α was greater than that at 10 hr. All heifers exhibited behavioral estrus except those in group GnRH-50. Spontaneous LH peaks, which usually occur after PGF2α were observed in 5/5 (control), 4/5 (GnRH-10), 0/5 (GnRH-30), and 2/5 (GnRH-50) heifers. We conclude that pituitary responsiveness to GnRH increases with time after PGF2α and GnRH given at 30 hr completely inhibits and at 50 hr partially inhibits endogenous LH release.

Bovine Serum LH, GH, and Prolactin During Late Pregnancy, Parturition and Early Lactation

Summary Jugular blood was collected from 34 Holstein heifers from 26 days before parturition until 26 days postpartum or until first estrus. Serum prolactin varied between 80 and 110 ng/ml until Day 2 prepartum when it increased markedly to peak of 285 ng/ml at 1 day prepartum. Prolactin decreased linearly (p < 0.01) from the peak until 2 days after calving, stabilized at about 90 ng/ml until Day 9, and gradually declined until day 26 (36 ng/ml). Postpartum (Day 5-26) prolactin was lower (p < 0.05) than prepartum (Day 26-5) prolactin. Growth hormone followed a pattern similar to prolactin but peaked 24 hr later, at parturition, and decreased from the peak to concentrations characteristic of prepartum values by Day 4 postpartum. Serum growth hormone during late pregnancy (Day 26-5) was lower (p < 0.05) than during early lactation (Day 5-26). Serum LH was consistently low (0.4-0.8 ng/ml) until Day 9 postpartum when it increased to about 1.5 ng/ml until first estrus.

Prolactin and Growth Hormone Circadian Periodicity in Lactating Cows

Summary Prolactin and growth hormone in serum of 10 lactating cows was measured by radioimmunoassay at hourly intervals on 4 days. Prolactin in lactating cows was released in erratic pulses throughout the day. Serum prolactin concentration followed a 24-hr periodicity (p < .01), with the highest value occurring at 4:00 p.m. (58 ng/ml) and lowest values being recorded between 4 and 10 a.m. (28 ng/ml). However, circadian changes accounted for only 8% of the total variation in prolactin values, differences within cows accounted for 45% and differences among cows accounted for 47%. Serum growth hormone concentration was more stable than prolactin and did not exhibit circadian periodicity.

Release of Bovine Luteinizing Hormone by Purified Porcine and Synthetic Gonadotropin Releasing Hormone

Summary LH concentration in serum of heifers averaged 0.5 ng/ml and increased to peaks of 1.9, 7.3 and 11.1 ng/ml after a single iv injection of 5, 20 or 80 μg GnRH, respectively. Similarly, LH concentration in serum from mature bulls averaged 1.1 ng/ml before GnRH and increased to peaks of 9.2, 19.3 and 39.1 ng/ml after 10, 40 and 160 μg GnRH, respectively. The interval from GnRH to the LH peak and magnitude of the peak were positively correlated. Exposure of pituitary explants, in constant superfusion, to 1 or 4 ng GnRH increased LH release from a pre-treatment base line of 0.25 ng/mg/min to 0.94 and 1.50 ng/mg/min after 20 min, respectively. LH release did not return to base line by 40 min after exposure to GnRH. A comparison of purified porcine and synthetic GnRH revealed parallel increases in media LH concentration with time after GnRH.

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.

Blood LH after PGF2α in diestrous and ovariectomized cattle

Abstract Two experiments were conducted to determine whether the increased serum LH which occurs within 12 hr after a luteolytic dose of PGF 2α is dependent upon changes in progesterone or estradiol secretion. In the first experiment, exogenous progesterone abolished the increase in serum LH caused by a subcutaneous injection of 25 mg PGF 2α in diestrous heifers, but not in ovariectomized heifers. In the second experiment, progesterone pessaries were removed at 6 hr after a subcutaneous injection of 25 mg PGF 2α . LH remained at pre-PGF 2α values while the pessaries were in place, but began to increase within 1 hr after they were removed. Blood estradiol also remained at pre-PGF 2α values until the pessaries were removed, and began to increase at 2 hr after pessary removal. We conclude that the increase in serum LH within 12 hr after PGF 2α treatment in diestrous cattle is dependent upon withdrawal of progesterone; it is not due to increased serum estradiol.

Luteinizing Hormone Release by Gonadotropin Releasing Hormone Before and After Castration in Bulls

Summary Six 9-mo-old Holstein bulls were used to compare the magnitude of LH release in response to gonadotropin releasing hormone administered prior to and after castration and following testosterone replacement. In addition, effects of castration and testosterone replacement on patterns of episodic LH release were investigated. GnRH caused release of LH before and after castration, but the magnitude of LH increase in response to GnRH increased 2.5-fold at 14 days postcastration relative to precastration changes. Testosterone replacement did not reduce the magnitude of LH response to GnRH to precastration levels. LH concentration in serum collected at hourly intervals prior to castration averaged 1.1 ng/ml, only 17% of the comparable value at 21 days after castration (6.7 ng/ml). The number of episodic increases in serum LH concentration during a 24-hr period prior to castration averaged 3.7 and increased to 6.5 at 21 days postcastration. Testosterone replacement restored neither the average number nor the magnitude of LH increase to levels characteristic of the precastration period. We conclude that LH release in response to GnRH is increased after castration and the increase is not reversed by testosterone. In addition, LH is released episodically in bulls, and peaks of LH normally are closely followed by increased testosterone in serum. Castration increases the frequency but not the magnitude of LH increase and testosterone does not restore frequency to precastration levels.

Estradiol-17β stimulates basal and thyrotropin releasing hormone induced prolactin secretion by bovine pituitary cells in primary culture

Abstract A series of experiments were conducted which demonstrate that estradiol-17β directly affects bovine pituitary cells in primary culture causing an increase in basal and thyrotropin releasing hormone (TRH)-induced prolactin secretion. Prolactin release by pituitary cells incubated with TRH at concentrations of 0.001, 0.01, 0.1 and 1 ng/ml increased linearly with increasing log concentrations. Exposure of pituitary cells to 5, 50 or 500 ng/ml estradiol for 4 h did not affect basal or TRH-induced prolactin release. However, when the period of exposure to estradiol was prolonged to 6, 12, or 24 h, 0.5, 5 or 50 ng estradiol/ml medium caused pituitary cells to release more prolactin and there was more total prolactin in the system (medium +cell content) than for comparable controls. These increases were linearly related to increasing log concentrations of estradiol used. To determine the chronic effect of estradiol on prolactin secretion, pituitary cells were incubated with estradiol-17β for 11 days during which medium was collected at 24 h intervals beginning on day 3. On day 3, prolactin accumulation in medium of control cultures averaged 2.5 ng/ml, and decreased gradually reaching relatively low levels by day 11 (100 ng/ml). Although prolactin secretion decreased during the culture period, stimulatory effects of estradiol were evident throughout. In addition, these cells still released prolactin in response to TRH (1 ng/ml) on day 11 and magnitude of TRH-induced prolactin release increased with increasing concentrations of estradiol-17β. We conclude that estradiol will increase basal and TRH-induced prolactin release by bovine lactotrophs. These results are consistent with the view that the increase in estradiol that occurs at the end of pregnancy in cattle, may participate in the prolactin surge that occurs at parturition in this species.

Modulation of growth hormone-releasing factor-induced release of growth hormone from bovine pituitary cells☆

Growth hormone (GH)-releasing factor (GRF) at concentrations of 10(-12) through 10(-7) M for 6 hr linearly increased GH release (b1 = 10.4 +/- .3) from bovine anterior pituitary cells in culture. Maximum release of GH (262% above controls) occurred at 10(-7) M GRF. In contrast, GH release-inhibiting factor (SRIF) at 10(-12) through 10(-5) M had no effect on basal concentrations of GH. In a second experiment, as the proportion of SRIF relative to GRF increased, SRIF suppression of GRF-induced GH release from anterior pituitary cells increased. In a third experiment, anterior pituitary cells cultured in media containing fetal calf serum (FCS) were treated with cortisol (0 or 10 ng/ml media) for 24 hr before exposure to 10(-13) through 10(-7) M GRF. GRF linearly increased GH secretion (b1 = 7.4 +/- .3) and cortisol augmented this response (b1 = 10.5 +/- .6). However, when cells were cultured in media containing dextran-charcoal treated FCS, cortisol did not alter GRF-induced GH release. Our results demonstrate that GH response of bovine anterior pituitary cells to GRF was modulated negatively by SRIF. However, augmentation of GRF-induced GH release by cortisol was evident only when cells were cultured in media supplemented with untreated FCS.