Plasma LH concentrations after single or double injections of synthetic LH-RH in dairy cows.

Milked Friesian dairy cows were given 1 or 2 i.v. injections of synthetic LH-RH. The second of 2 injections of LH-RH given 1.5 h apart induced a significantly greater release of LH than did the first. The response to this injection was also significantly greater than that caused by injections given 6 h apart. There was no significant difference between the LH responses to the two injections given 6 h apart. The total LH release produced by the two injections of 100 microgram synthetic LH-RH given 1.5 h apart was significantly greater than that produced by single injection of 200 microgram synthetic LH-RH. It is concluded that there is a short-term increase in the responsiveness of the bovine pituitary gland to LH-RH after an initial LH-RH stimulus.     

Response to synthetic gonadoliberin (GnRH) in anestrous cows

Fifty-three anestrous cows of Hariana crossed with Holstein, Friesian, Brown Swiss or Jersey half-breed crosses were treated with Buserelin acetate aqueous solution 0.0042 mg/ml, administered as a single intramuscular (i.m.) dose of 21 ng. Twelve (22.64%) animals exhibited complete follicular development, ovulation and behavioural expression of oestrus. Twenty-eight (52.83%) cows showed only partial follicular development with no ovulation or oestrous manifestations. Thirteen (24.53%) animals did not respond. Of eight control animals, three (37.50%) showed partial follicular development and no ovulation or symptoms of oestrus while five (62.50%) did not respond.In GnRH-treated cows showing follicular development, oestrous manifestation, and ovulation, nine animals responded between 4 to 10 d after administration. Three animals took from 14 to 155 d. The mean time from treatment to response for this group was 29.17 d. In animals showing partial follicular development and no oestrous behaviour, the interval was 3 to 11 d in 28 animals. In the control group, three animals showed partial follicular development on 94 d average. The administration of GnRH in anestrous cows promoted the development of follicles to a certain stage as evident from the clinical observations, but the follicles failed to grow beyond a particular stage. The reasons for failure of follicular development beyond a particular stage in GnRH-treated animals are not clear. It appeared that the response to GnRH may be expected to peak by the eleventh day and a second dose may then be tried for a favourable response.     

Response of adult male rats to LH-RH after neonatal immunization with antiserum to LH-RH

Male rats were passively immunized at Day 5 of age with LH-RH antiserum. Their response to LH-RH at 100 days of age was examined. Serum FSH and LH concentrations increased more than in control rats, although basal plasma levels were similar. The pituitary content of LH, but not FSH, was significantly increased, and hypothalamic content of LH-RH was similar in both groups. The testes and seminal vesicles were smaller in experimental animals than in controls. It is suggested that transient blockade of LH-RH secretion during the neonatal period produces abnormalities in pituitary-testicular functon in the adult rat.     

Influence of pre- and post-partum nutrition on LH secretion in suckled postpartum beef heifers

The influence of nutrition during the last trimester of pregnancy and the early postpartum period on postpartum LH secretion was evaluated in two-year-old Hereford and Simmental heifers maintained on a high or low plane of nutrition (experiment 1) or in Hereford heifers fed a high or low energy (150% vs 100% NRC) ration (experiment 2). Amount of LH released with 10 mg estradiol benzoate (IM) at 14 and 28 days postpartum in experiment 1 or at 14,32,50 and 74 days postpartum in experiment 2 was less (P<.01) for heifers fed the low vs high plane or energy ration and less (P<.05) at 14 days postpartum than at subsequent postpartum periods. The interval from estradiol benzoate injection to the LH peak concentration was longer (P<.05) in Simmental than Hereford heifers, longer (P<.05) in heifers fed the low rather than high energy ration, and longer (P<.01) at 14 days postpartum than at subsequent postpartum periods. The amount of LH released was inversely related to the time required for initiation of the release (r = -.62). Tonic LH secretion was higher (P<.05) in heifers fed the high energy ration and was correlated with average daily gain (r = .75), but was unaffected (P.05) by days postapartum or breed of cattle. Results indicate that increased dietary energy intake increases LH secretion and shortens the anovulatory period in suckled postpartum beef heifers.     

Photoperiodic alteration of postpartum reproductive function in suckled cows

Three experiments were conducted using a total of 41 cows to determine if photoperiod modulates the establishment of postpartum estrous cycles and conception. Cows calving in the autumn and winter were exposed to either 18 hr light/day (18L:6D) or natural photoperiods. In Exp. 1, cows receiving 18L:6D had shorter (P<0.025) intervals from calving to estrus (61 +/- 3.8 days) than cows not receiving supplemental light (154 +/- 23.9 days). The same was true for primiparous cows in Exp. 2 (76 +/- 5.5 days vs 153 +/- 38.0 days; P<0.06) but not for the multiparous cows in Exp. 2 (56 +/- 5.2 days vs 40 +/- 7.4 days) or for all cows in Exp. 3 (60 +/- 10.1 days vs 70 +/- 13.5 days). Because conception rate was higher for cows exposed to 18L:6D for the multiparous cows in Exp. 2 and all cows in Exp. 3, interval to conception was significantly shorter for animals exposed to 18L:6D in all experiments. Generally, interval from calving to uterine involution was also reduced by exposure to 18L:6D. No effects of photoperiod were observed on body weight changes, serum levels of luteinizing hormone (LH), follicle stimulating hormone (FSH) or estradiol or on estradiol-induced release of gonadotropins. It was concluded that in certain situations day length can hasten estrus and conception in postpartum cows. The effect of photoperiod interacts with other conditions, one of which is parity. The endocrine basis for these effects are not known.     

Hormone responses to low-dose GnRH treatment in post-partum beef cows

Acyclic beef cows received 1.0, 2.5 or 5.0 micrograms GnRH/2 h for 48 h as 24 X 2 h repeated i.v. injections or by continuous i.v. infusion. Preovulatory-type LH surges were detected in 9/18 injected and 8/15 infused cows and occurred 30.6 +/- 5.1 h and 3.3 +/- 0.7 h after the start of treatment respectively. Cows receiving the lowest infusion dose did not exhibit gonadotrophin surges. The LH response to individual injections increased with dose but the proportion of injected cows showing preovulatory-type surges at each dose level did not change. A total of 20 cows (10 injected and 10 infused) showed evidence of luteal activity within 7 days of the end of GnRH treatment, although this was transitory in most animals. Cows which exhibited preovulatory-type LH surges in response to treatment had significantly higher plasma oestradiol-17 beta concentrations and lower FSH concentrations before treatment than those which did not. The results suggest that the LH response to GnRH treatment is dependent on follicular status in the immediate pretreatment period.     

Hormone responses to exogenous GnRH pulses in post-partum dairy cows

Nine Friesian dairy cows were treated with 2.5 micrograms GnRH i.v. at 2-h intervals for 48 h commencing between Days 3 and 8 post partum. Hormone concentrations were measured in jugular venous plasma. An episodic pattern of LH release was induced in all animals and there was no significant change in amplitude during treatment. However, cows treated between Days 7 and 8 ('late') showed higher LH episode peaks than did those treated between Days 3 and 6 ('early'). Plasma FSH concentrations showed a less clear episodic pattern in response to GnRH injection. The mean height of FSH responses to GnRH tended to be higher in the 'early' group than in the 'late' group, as did mean FSH concentrations during the pretreatment sampling period. Although clear episodic changes were not observed, GnRH treatment induced a rapid sustained rise in plasma oestradiol-17 beta concentrations, indicating the responsiveness of ovarian follicles to gonadotrophin stimulation early in the post-partum period. There was no difference in oestradiol-17 beta concentrations between the 'early' and 'late' groups during the treatment period. Only one cow exhibited preovulatory-type LH, FSH and oestradiol-17 beta surges during the 96-h post-treatment sampling period. It is concluded that: (1) responsiveness to GnRH pulses increases significantly and FSH responsiveness tends to decrease with time post partum, (2) ovarian follicles are able to secrete oestradiol-17 beta in response to GnRH-induced LH and FSH release during the early post-partum period and there is no time-dependent change in responsiveness; and (3) the lack of preovulatory surges, except in one cow, may reflect a temporary defect in the positive-feedback mechanism by which high concentrations of oestradiol-17 beta induce preovulatory gonadotrophin release.     

Effect of prolactin on the LH response to synthetic LH-RH in ovariectomized ewes.

When ovariectomized ewes were treated with LH-RH, all 3 receiving prolactin infusion and 4 out of 5 receiving an infusion of NaCl solution responded.     

Differential responses of cyclic GMP-dependent and cyclic AMP-dependent protein kinases to synthetic peptide inhibitors.

The peptide Arg-Lys-Arg-Ala-Arg-Lys-Glu was synthesized and tested as an inhibitor of cyclic GMP-dependent protein kinase. This synthetic peptide is a non-phosphorylatable analogue of a substrate peptide corresponding to a phosphorylation site (serine-32) in histone H2B. The peptide was a competitive inhibitor of cyclic GMP-dependent protein kinase with respect to synthetic peptide substrates, with a Ki value of 86 microM. However, it did not inhibit phosphorylation of intact histones by cyclic GMP-dependent protein kinase under any conditions tested. Arg-Lys-Arg-Ala-Arg-Lys-Glu competitively inhibited the phosphorylation of either peptides or histones by the catalytic subunit of cyclic AMP-dependent protein kinase, with similar Ki values (550 microM) for both of these substrates. The peptide Leu-Arg-Arg-Ala-Ala-Leu-Gly, which was previously reported to be a selective inhibitor of both peptide and histone phosphorylation by cyclic AMP-dependent protein kinase, was a poor inhibitor of cyclic GMP-dependent protein kinase acting on peptide substrates (Ki = 800 microM), but did not inhibit phosphorylation of histones by cyclic GMP-dependent protein kinase. The selectivity of these synthetic peptide inhibitors toward either cyclic GMP-dependent or cyclic AMP-dependent protein kinases is probably based on differences in the determinants of substrate specificity recognized by these two enzymes. It is concluded that histones interact differently with cyclic GMP-dependent protein kinase from the way they do with the catalytic subunit of cyclic AMP-dependent protein kinase.     

Prolactin levels and the LH-response to synthetic LH-RH in the lactating sow

To determine the role of prolactin in the suppression of ovarian activity during lactation in the sow experiments were performed to investigate a possible inhibitory action of prolactin at the pituitary level. Therefore the LH-response to an intravenous injection of 25 μg synthetic LH-RH was measured in the 1st, 2nd and 3rd week of lactation. The compound was injected under high and low concentrations of prolactin in the peripheral blood, the latter achieved by removal of the piglets 6 h before administration of LH-RH. The results showed no difference in the effect of LH-RH injected at high or low prolactin levels. However, although the mean prolactin concentrations in the 1st, 2nd and 3rd week of lactation were similar, the results clearly demonstrated an increase in LH-response as lactation proceeds.The low responsiveness of the pituitary shortly post partum was also observed when the preparturient rise of prolactin was suppressed by treatment with bromoergocryptine. Injections of LH-RH in the last week of gestation given before and after the physiological increase of PRL, which occurred about 48 h before delivery, all showed low LH-response.It is obvious from the presented data that the LH-response to an intravenous injection of 25 μg LH-RH is in no way correlated with the prolactin levels at the time of treatment.     

Induction of ovulation in postpartum suckled beef cows: a review

Prolonged postpartum acyclicity in suckled beef cows reduces the calf crop, and causes economic loss to beef cattle producers. Once anterior pituitary LH stores have been replenished between Days 15 and 30 post partum in suckled beef cows, methods to initiate cyclicity include non-hormonal methods such as weaning of calves (either complete, temporary or partial), or exposure to bulls, and hormonal methods such as administration of GnRH (either single injection, intermittent injections, or continuous infusion), gonadotropins (eCG, FSH, hCG), and steroids (estrogens, anti-estrogens, and progestogens). Weaning is costly, reduces growth rate of weaned calves, and short cycles are common after weaning-induced ovulation. Exposure of cows to bulls is not practical and its effect is not predictable. Repeated injections of GnRH, or a single injection of hCG are not always effective; ovulation is always followed by a short cycle, and usually a return to acyclicity. Estrogens and anti-estrogens do not consistently shorten postpartum anestrus. Exogenous progestogens include intravaginal devices, such as controlled-internal drug release (CIDR) or progesterone-releasing intravaginal device (PRID), norgestomet implants, and the feed-additive melengestrol acetate (MGA). Administration of exogenous progestogens is more practical than, and offers more advantages over, other treatments to shorten postpartum acyclicity in suckled beef cows. Mimicking the short cycle after Week 3 post partum, by maintaining circulating progesterone at subluteal concentrations or circulating progestin at intermediate concentrations, extends the life-span and allows terminal maturation of the postpartum dominant follicle as in cyclic cows, by initiating endogenous GnRH and LH pulses. This is followed by an LH surge, ovulation and normal cycles. The benefit from using exogenous progestogens after Week 3 post partum in suckled beef cows is that ovulation is induced, cyclicity is initiated, the resulting CL has a normal life-span and function, and there is no need to change management, such as weaning of calves. We present a model for the induction of ovulation and initiation of cyclicity using exogenous progestogens after Week 3 post partum in suckled beef cows.     

LH release and luteal function in post-partum acyclic ewes after the pulsatile administration of LH-RH

The administration of LH-RH in a pulsatile regimen (100 ng i.v./h for 48 h) to acyclic ewes 26-30 days post partum increased plasma LH concentrations, and both the frequency and amplitude of plasma LH pulses. In 12/14 ewes these increases were followed by plasma LH surges similar to the preovulatory surges observed in 10 control cyclic ewes. Subsequent luteal function in the post-partum ewes was deficient. Plasma progesterone was detected in 7/12 post-partum ewes showing plasma LH surges. The concentrations were lower (1.3 +/- 0.2 ng/ml) and detected for shorter periods (3-10 days) than in cyclic ewes (2.4 +/- 0.2 ng/ml, 12/15 days). In the post-partum ewes the increases in plasma LH concentrations before the LH surge were higher but of shorter duration than in the cyclic ewes. The inadequate luteal function in the post-partum ewes could therefore have been due to inappropriate LH stimulation of the ovary before the LH surge.     

Pituitary responsiveness to synthetic LH-RH and pituitary LH content at various reproductive stages in the sheep.

In sheep the basal concentration of LH in jugular vein plasma was significantly higher during the first 50 days of gestation in late pregnancy or at parturition. The pituitary response to a single i.v. injection of 200 microng synthetic LH-RH was determined at different stages of gestation and compared with that of anoestrous and cyclic sheep. Pituitary response to LH-RH decreased progressively with advancing gestation: by 56 days after mating the response had declined to 35% and by parturition to 14% of the value in anoestrous sheep. The pituitary response to LH-RH increased after parturition and the pattern of recovery differed in non-lactating and lactating sheep. By 63 days postpartum the response to LH-RH in non-lactating and lactating animals had returned to values similar to those in sheep during anoestrus and sheep during the luteal phase of the oestrous cycle. A decrease in pituitary responsiveness during pregnancy was associated with a decrease in pituitary content of LH. The quantity of LH released in response to a standard injection of LH-RH was linearly related to pituitary LH content.     

Radioimmunoassay of FSH in the plasma of post-partum dairy cows

A method for measurement of bovine plasma FSH has been established with an interassay coefficient of variation of 15.5% over the workable range of the assay. Compared to values at the end of pregnancy (42-122 ng/ml), FSH concentrations were greater (P less than 0.01) between 0 and 20 days post partum in 3 cows.     

Fertility in the Afrikaner cow. 2. Ovarian recovery and conception in suckled and non-suckled cows post partum

The post-partum reproductive performance of suckled and non-suckled Afrikaner cows was compared in order to assess the importance of suckling in the resumption of post-partum ovarian function in this breed. Non-suckled cows were weaned at 3 days post partum whilst calves in the control group had continuous access to their dams. Commencement of ovarian activity was significantly (P < 0.01) earlier in non-suckled cows at 18.1 ± 5.2 (mean ± SD) days post partum than in suckled cows 53.2 ± 19.7 days. Conception rates were significantly higher (P = 0.01) in non-suckled cows (80%) than in suckled cows (50%). Mean intervals from parturition to conception were 45.8 ± 15.8 (mean ± SD) days in non-suckled cows as compared with 70.9 ± 17.9 days in suckled cows. The conception rate associated with each ovulation appears to be highest between 50 and 70 days post partum regardless of the degree of suckling stimulus. Four classes of ovarian activity were recorded from the progesterone profiles compiled. The incidence of short cycles was unaffected by the degree of suckling stimulus. Weaning may (P = 0.05) reduce the proportion of cows which exhibit regular cyclic ovarian activity without conceiving. Coitus during oestrus may (0.01 < P > 0.05) be more easily observed in non-suckled cows than in their suckled contemporaries. Suckling altered the diurnal distribution of coitus, with non-suckled cows showing the highest frequency for this component of oestrus before 09.00 h and after 15.00 h, whilst in suckled cows coitus appeared to be more evenly distributed throughout the day.     

Uterine oxytocin receptors in cyclic and pregnant cows.

Binding of [3H]oxytocin to uterine subcellular preparations ('oxytocin receptor concentrations') was measured in uterine tissue of heifers and multiparous dairy cows at various stages of the oestrous cycle and during early pregnancy. A method for the assay of ovine uterine oxytocin receptors was optimized for use on bovine tissue. Oxytocin receptor concentrations were increased in cyclic animals around the period of luteolysis and oestrus, rising on Day 15 in endometrium and on Day 17 in myometrium while pregnant animals showed no comparable rise. Receptor concentrations then declined on Day 3 after oestrus in myometrium and on Day 5 in endometrium. Some cyclic animals did not show the expected rise in receptors in the late luteal phase; these animals had abnormally high progesterone concentrations for this stage of the cycle. In animals slaughtered on Day 18 after oestrus and/or insemination which had low oxytocin receptor levels, plasma progesterone concentrations were consistently high; while all animals showing the late luteal phase elevation in receptor values had low progesterone concentrations. Oxytocin receptor and progesterone concentrations were negatively correlated (P less than 0.05). These data support the hypothesis that oxytocin receptor level is a key factor in the process of luteolysis in cattle and that in pregnancy there is suppression of uterine oxytocin receptor at the expected time of luteolysis. We suggest that uterine oxytocin receptor levels are partly controlled by circulating steroid hormones and are suppressed during early pregnancy.     

Hormonal induction of ovulation and artificial insemination in suckled beef cows under nutritional stress

The objective was to develop a program for inducing estrus (followed by insemination) of suckled beef cows under nutritional stress (poor body condition). A total of 123 cows, from 60 to 75 days postpartum, were classified according to their body condition score (BCS; range from 1 to 5, in increments of 0.5) and allocated into two groups. On Day 0 (without regard to stage of the estrous cycle), cows (n = 59) in the hormone induction (HI) treatment group were given an intravaginal device (IVD) containing 250 mg of medroxiprogesterone acetate (MAP) and an i.m. injection of 2.5 mg estradiol benzoate (EB). On Day 6, these cows were given 500 IU eCG i.m. and calves were weaned for 96 h. The IVD were removed on Day 7. Cows detected in estrus by 45 h after IVD removal were inseminated 12 h after standing estrus; cows not in estrus by 45 h after IVD removal received an i.m. injection of 100 microg gonadorelin (GnRH) and were inseminated 16-18 h later. In the control group (C), cows (n = 64) only had their calves weaned at Day 6 (for 96 h), with estrus detection and AI from Days 6 to 11. Overall, the BCS ranged from 2.0 to 3.0. In the treatment group, estrus and pregnancy rates in cows with BCS 2.0 (20 and 30%, respectively) was lower (P < 0.05) than those with BCS 3.0 (50 and 66.6%, respectively), but did not differ (P > 0.05) from BCS 2.5 (23.3 and 47.6%). In C group, only 2 of 66 cows were detected in estrus and bred (neither was pregnant). In conclusion, the program for induction of ovulation using MAP, EB, eCG and GnRH increased the pregnancy rate in beef cows in poor body condition, enabling AI to be done in a 63-h interval.