Effect of variable suckling intensity and estrogen administration upon serum luteinizing hormone in Brahman cows

Ten mature Brahman cows were randomly allotted within calving intervals to either a suckled (S) or nonsuckled (NS) treatment group. All cows received a 20 mg intramuscular injection of estradiol-17beta (E2), suspended in 2 ml of corn oil, to determine the effect of suckling on the estrogen induced LH surge. Starting on day 21 postpartum the S cows were suckled at six hour intervals for 24 hours, at which time they were challenged with a 20 mg E2 injection. The suckling regimen was continued for 48 hours postinjection. The NS cows were separated from their calves on day 21 postpartum and received no suckling stimulus for 72 hours. At 24 hours after calf separation, the NS cows were challenged with a 20 mg E2 injection. Blood samples were removed at two hour intervals beginning 10 hours post E2 injection until 36 hours postinjection, at which time blood samples were removed at four hour intervals until 48 hours postinjection. Blood samples were processed to yield serum and assayed for luteinizing hormone (LH) via radioimmunoassay. The injection of a 20 mg dose of E2 induced an LH surge in all cows. The NS cows were found to exhibit a longer (P<.05) duration of the estrogen induced LH surge than the S cows, 15.6 +/- .98 and 12.4 +/- .75 hours, respectively. The timing parameters (time to start of LH surge, time to peak LH value and time to end of surge) and LH concentration parameters (LH concentration at start of LH surge, peak value of LH surge and LH concentration at end of LH surge) were not different between suckling regimens. No blockage of the LH response to estrogen challenge was found on day 22 postpartum. Suckling did depress the duration of the LH surge indicating some blockage due to suckling stimuli.     

Serum luteinizing hormone levels in cattle under various reproductive states

Serum lutinizing hormone (LH) levels in cattle during various reproductive states were measured by radioimmunoassay. A sharp LH peak observed at estrus (22.72 ± 5.68 ng/ml) was about 26 times higher than at other stages of the cycle (0.87 ± 0.06 ng/ml). LH levels during the first 90 days of pregnancy (0.75 ± 0.15 ng/ml) were similar to those of the estrous cycle, except during estrus, while those during the second (0.22 ± 0.07 ng/ml) and third trimesters of pregnancy (0.22 ± 0.08 ng/ml) were significantly lower. Higher levels than those of the cycling cows, except during estrus, were seen in ovariectomized cows (2.21 ± 0.56 ng/ml). Levels of LH in cows with cystic follicles (2.00 ± 0.49 ng/ml) were higher than the levels in the cycle. LH levels in bulls (1.29 ± 0.39 ng/ml) were comparable to that of estrous cows. Serum LH of calves increased with age from 1.00 ± 0.32 ng/ml (less than 30 days of age), to 2.30 ± 0.83 ng/ml (181 to 210 days of age), and the level after 151 days was significantly higher than that of the cyclic cows, except during estrus.     

Effect of monensin and suckling on the GnRH induced luteinizing hormone surge and the effect of monensin on the postpartum interval in Brangus cows

Twenty-seven fall calving Brangus cows were randomly allotted to one of four treatment groups: nonsuckled monensin (NSM), suckled monensin (SM), nonsuckled control (NSC), and suckled control (SC). Cows were group fed 1.82 kg/hd/day concentrate and Coastal bermuda grass hay $\text{ad}$$\text{libitum}$. Monensin cows received 200 mg monensin/hd/day in the concentrate. At 0800 hr on day 21 postcalving, the calves were separated from the cows. Suckled monensin and SC cows were allowed to suckle their calves for 30 min at 6-hr intervals. Nonsuckled monensin and NSC cows were not suckled. Calves were given free access to the cows after 1400 hr on day 22 postpartum. At 0800 hr on day 22 postpartum, a blood sample was collected. A 100 μg GnRH challenge was administered IM at 0801 hr. Blood samples were collected at 15-min intervals for 6 hr postinjection. Changes in body weight and body condition from day 21 postpartum to the day of first estrus were not different (P>0.10) by dietary treatment. Monensin cows consumed 10.7% less hay than did the control cows. Serum luteinizing hormone (LH) following GnRH was greater (P<0.005) in suckled than nonsuckled cows. Control cows released more (P<0.005) LH in response to GnRH than did the monensin cows. The postpartum interval (to first estrus) for the monensin cows (92.4±14.7 days) was shorter (P<0.025) than the controls (138.5±9.5 days). A greater proportion (P<0.005) of the monensin cows (8 of 14) exhibited estrus by 90 days postpartum compared to the control cows (0 of 13). Monensin and suckling appear to exert independent and agonistic influences on pituitary function in the postpartum beef cow.     

Effect of estrogen administration on endogenous and luteinizing hormone-releasing-hormone-induced luteinizing hormone secretion and follicular development in the lactating sow

The objectives of this study were to investigate whether estradiol treatment during lactation modifies 1) the patterns of endogenous LH, FSH, and prolactin (PRL) release; 2) the sensitivity of the pituitary to exogenous injections of LHRH; and 3) the responsiveness of the ovarian follicles to gonadotropin. Plasma LH, FSH, and PRL were determined in samples taken repeatedly from 18 sows on Days 24-27 of lactation. Ovaries were then recovered, and follicular development was assessed by measuring the follicular diameter (FFD) and follicular fluid estradiol-17 beta concentration (FFE) of the ten largest follicles dissected from each ovary. Sows were randomly allocated to one of four treatments: 1) Group C (4 sows) received no treatment; 2) Group LHRH (5 sows) received 800 ng of LHRH every 2 h throughout the sampling period; 3) Group E2 (4 sows) received subcutaneous implants containing estradiol-17 beta 24 h after start of sampling; 4) Group LHRH + E2 (5 sows) were administered a combination of LHRH and estradiol-17 beta implants. Between-animal variability for plasma LH, FSH, and PRL was considerable. LH concentration and LH pulse frequency increased (p less than 0.05) after LHRH treatment in the LHRH and LHRH + E2 groups; however, an acute inhibition of LH secretion was observed in the latter group immediately after estradiol implant application. In the absence of LHRH treatment, estradiol caused chronic inhibition of LH secretion. Follicular development was greater in the LHRH and LHRH + E2 groups compared to the C and E2 groups (p less than 0.05 for both FFD and FFE).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of suckling and ovariectomy on the control of luteinizing hormone secretion during the postpartum period in beef cows

Twenty-two mature pluriparous beef cows were randomly assigned to one of six treatments in a 2 X 3 factorial experiment in order to study the role of suckling and ovarian factors on control of the tonic and episodic release of luteinizing hormone (LH). Twelve cows remained intact (INT) and 10 were ovariectomized (OVX) within 4 days following the day of parturition (Day 0). The suckling intensities were nonsuckled (0), suckled once daily for 30 min (1) and suckled ad libitum by two calves (2). Blood samples were collected at 15-min intervals for 6 h weekly, from Days 6 to 76 postpartum. The postpartum intervals to initiation of ovarian luteal function were 31 +/- 3, 41 +/- 4 and 67 +/- 1 days (means +/- SEM) for INT cows with 0, 1 and 2 suckling intensities, respectively. Mean LH concentrations and frequency of LH pulses increased as time of ovulation approached in INT cows. In OVX animals, both mean LH concentrations and frequency of LH pulses increased as time postovariectomy progressed. No differences were detected in mean LH concentrations or frequency of LH pulses between the two suckled OVX groups. Mean LH in the OVX-0 cows was greater on Days 13, 20 and 27 postpartum when compared to the respective days in suckled OVX cows. Frequency of LH pulses tended to be lower (P less than 0.10) in both suckled OVX groups when compared with OVX-0 cows from Day 6 to Day 55 postpartum. It is postulated that suckling and ovarian factors act together during the postpartum period to suppress LH levels and frequency of LH pulses in beef cows.     

Serum hormone levels following partial hepatectomy in the rat.

Serum levels of insulin, glucagon, growth hormone (somatotrophin) and thyroxine (TT₄) were measured by radioimmunoassay following both sham operation and 70% partial hepatectomy in the rat to evaluate changes in hormone levels during liver regeneration. An eleven fold increase in glucagon was observed (from 112 ± 10 pg/ml to 1500 ± 200 pg/ml) 6 hours following partial hepatectomy but not sham operation. In contrast, insulin levels remained unchanged compared to sham controls for up to 72 hr while growth hormone fell to low levels, 6 to 48 hr after partial hepatectomy. Both total thyroxine and free thyroxine levels also fell 24–72 hours after hepatectomy. These studies suggest that growth hormone, thyroxine and insulin are not primary stimulants of hepatic regeneration although the data suggests that glucagon may modify this growth process.     

Serum luteinizing hormone follicle-stimulating hormone and prolactin in neonatal-androgenized rats.

Serum levels of LH, FSH, Prolactin and Testosterone of 90 days old male rats androgenized soon after birth were determined by specific radioimmunoassay and were compared to untreated rats. LH and FSH levels were also determined in 90 days old female rats neo-natally treated with testosterone and compared with normal diestrus rats. Androgenization of male rats significantly increased serum FSH and Prolactin levels without producing changes in plasma LH and testosterone concentrations. Similar increase in the FSH levels were found in androgenized female rats although plasma FSH concentrations were lower than in the male groups. These results obtained in male rats give an additional evidence that androgens acting in the first days of life are responsible of the higher levels of FSH and Prolactin that characterize the male or tonic pattern of gonadotrophin secretion.     

Effect of intermittent injections of gonadotrophin releasing hormone at various frequencies on the release of luteinizing hormone and ovulation in dairy cows

Gonadotrophin releasing hormone (GnRH, 5 μg every 4 h) was administered to six dairy cows between days 5 and 10 post-partum and the release of luteinizing hormone (LH) and the onset of ovulation were determined. LH was measured using a specific radioimmunoassay and the occurrence of ovulation was assessed from changes in the concentration of progesterone in milk. Treatment with GnRH resulted in a median time of first ovulation of 17.0 days after calving. This was less (P < 0.05) than that observed for control cows (21.5 days, n = 7). Determinations of plasma LH concentrations over an 8-h period on days 6 and 10 post-partum indicated that there was a tendency for GnRH-treated cows to have higher levels of LH on these days. The 5 μg dose of GnRH did not repeatably induce a release of LH between days 6 and 10. Endogenous pulsatile release of LH did, however, increase in frequency from 3.18 pulses per 8 h on day 6 to 5.18 pulses per 8 h on day 14 post-partum (P < 0.01).In a second experiment groups of 20 cows were treated with either 5 μg GnRH every 4 h or 15 μg GnRH every 12 h from days 5 to 10 post-partum. Seventeen untreated cows served as controls. The median times to first ovulation were 27.0 days for the control cows, 22.5 days for those cows treated with 5 μg GnRH every 4 h and 17.0 days for cows treated with 15 μg every 12 h. The latter treatment significantly advanced the time of first ovulation (P < 0.05) relative to controls. This difference had, however, disappeared by the time of the second and third ovulations. Primiparous cows ovulated later (P < 0.01) than the pluriparous cows in the group treated with 5 μg GnRH every 4 h. This was a major reason for the lack of effect of this treatment. Some treated cows were blood sampled at frequent intervals on day 8 to evaluate the LH responses to GnRH injections. The administration of 5 μg GnRH on day 8 did not elicit a pulse of LH which could be distinguished from endogenous pulsatile secretion at this time. The dose of 15 μg on this day did, however, elicit a more defined pulse on some, but not all, occasions.The injection of a small dose of GnRH twice a day from day 5 to day 10 after calving, therefore, advanced the time of first ovulation in dairy cows by 10 days.     

Receptors for luteinizing hormone and follicle-stimulating hormone in largest follicles of postpartum beef cows

Follicles collected from cows destined to enter relatively normal or short luteal phases if induced to ovulate were compared for numbers of receptors for luteinizing hormone (LH) in granulosal and thecal cells and for follicle-stimulating hormone (FSH) in granulosal cells. Eleven suckled beef cows received ear implants of 6 mg norgestomet for 9 days (expected normal luteal phase) and 11 served as controls (expected short luteal phase). At 48 h after implants were removed (average 34 days postpartum), the ovary containing the largest follicle was identified by transrectal ultrasound and removed. The largest follicle was dissected free of surrounding ovarian stroma and frozen in liquid nitrogen. Thecal and granulosal cells were isolated, and numbers of receptors for LH and FSH in granulosal cells and for LH in thecal cells were quantified. Concentrations of estradiol were measured in follicular fluid. Both granulosal and thecal cells from norgestomet-treated cows had greater numbers of receptors for LH than did those from control cows (p less than 0.01). Numbers of receptors for FSH in granulosal cells did not differ between treated and control cows. Follicles from norgestomet-treated cows were heavier (p less than 0.01) than follicles from control cows, mostly due to greater amounts of follicular fluid (p less than 0.01). Concentrations of estradiol were higher in follicular fluid from the treated cows (p less than 0.05). It is suggested that increases in numbers of follicular receptors for LH and secretion of estradiol are integral components of a sequence of events by which norgestomet prepares follicles to become fully functional corpora lutea.     

Changes in prolactin levels caused by luteinizing hormone releasing hormone

The acute effects of luteinizing hormone releasing hormone (LHRH) on the release of prolactin (PRL) were investigated in 12 normal cycling women and 42 women with various menstrual disorders. LHRH (100 micrograms) was bolusly injected intramuscularly and PRL levels were measured immediately before the injection and at 30 minutes and 60 minutes after the injection. LHRH elicited an increase of more than 25% in PRL levels in 15 cases (27.8%) at both 30 minutes and 60 minutes after the injection, whereas PRL levels were decreased by more than 25% in 7 cases (13.0%). The PRL response to LHRH seemed to be related to basal PRL levels. Especially when the PRL concentration was 20 ng/ml or more, LHRH decreased PRL levels in 7 cases out of 16. On the other hand, LHRH increased PRL levels in the majority of cases with a PRL concentration less than 20 ng/ml. In conclusion, the LHRH injection occasionally alters PRL levels in either a positive or negative manner, depending upon the basal PRL levels.     

Influence of season and estradiol on secretion of luteinizing hormone in ovariectomized cows

The hypotheses that secretion of luteinizing hormone (LH) varies with season and that estradiol may modulate the seasonal fluctuation in secretion of LH in cows were investigated. Seven mature cows were ovariectomized approximately 30 days before initiation of the experiment. Three of the ovariectomized cows (OVX-E2) were administered a subcutaneous estradiol implant that provided low circulating levels of 17 beta-estradiol. The remaining 4 cows (OVX) were not implanted. From December 21, 1982, to September 20, 1984, blood samples were collected sequentially (at 10-min intervals for 6 h) at each summer and winter solstice, and each spring and autumn equinox. In addition, from March 17, 1983, to March 17, 1984, sequential samples were collected midway between each solstice and equinox. Concentration of LH was measured in all samples, and concentration of estradiol was measured in pools of samples. An annual cycle in mean serum concentration of LH and amplitude of LH pulses was detected in both groups of cows. The seasonal pattern did not differ in the two treatment groups. Serum concentration of LH and amplitude of LH pulses were highest around the spring equinox, decreased gradually to the autumn equinox, and then increased and peaked again during the following spring equinox. Frequency of LH pulses and concentration of estradiol in serum did not vary with season. Circulating concentrations of LH and amplitude of pulses tended to be higher in OVX-E2 than OVX cows throughout the experimental period. Frequency of pulses of LH was lower in OVX-E2 than OVX cows throughout the experiment. Concentrations of estradiol were higher in the implanted cows.(ABSTRACT TRUNCATED AT 250 WORDS)