Changes in follicular endocrinology during final maturation of porcine oocytes

Thirty-one gilts were ovariectomized between 21 and 34 hr after the onset of estrus to compare changes in follicular endocrinology with stages of oocyte maturation. Oocytes were recovered from 6 to 8 mm follicles and classified by stage of meiosis. Remaining follicular fluid was assayed for steroids and dermatan sulfate. Amounts of prostaglandin F2 alpha (PGF2 alpha) and E2 (PGE2) were measured in intramural tissues. Coincident with germinal vesicle breakdown, the follicular content of all steroids except testosterone decreased (P less than .05). As oocytes approached metaphase II, the amount of progesterone within follicles increased (P less than .05), and estradiol continued to decrease (P less than .05). The pattern of dermatan sulfate content was biphasic and peaked at germinal vesicle breakdown and anaphase stages. Amounts of PGF2 alpha and PGE2 within intramural tissues increased (P less than .05) throughout oocyte maturation. Follicular atresia was evident during estrus; however, more (P less than .05) atretic follicles were recovered at germinal vesicle than metaphase II stages (20 vs 3%, respectively). Follicular development, within a gilt, was skewed (P less than .05) and classification of follicles by hormone content demonstrated that a majority were more mature than a minority of less mature follicles. These data suggest that follicular maturation and oocyte development are highly correlated in swine. Furthermore, partitioning the follicular variability by hour and stage of oocyte maturation allowed for more precise assessment of follicular endocrinology than previously reported.

Influence of estradiol on duration of anestrus and incidence of short estrous cycles in postpartum cows

The influence of exposure to exogenous estradiol on the interval from parturition to first ovulation, luteinizing hormone (LH) secretion and luteal function was examined in cows. Cows were assigned at parturition to one of three treatments. Cows received either a 3.0 (1-E; n = 30) or .75 cm (1/4-E; n = 28) implant containing 17 beta-estradiol or served as untreated control animals (C; n = 33). Implants were administered within 2 days following parturition and removed on day 40 postpartum (day 0 = day of parturition). Single blood samples were collected twice weekly and analyzed for progesterone to determine length of postpartum anestrus and duration of the initial increase in progesterone. Sequential blood samples were collected on day 35 +/- .1 postpartum (15 min intervals for 18 hrs) from 5 cows in each treatment and analyzed for LH. Concentrations of estradiol were higher (P less than .01) in the 1-E (5.3 +/- .24) than in C (3.9 +/- .23) or 1/4 E (3.9 +/- .25) cows on day 35 postpartum. The interval from parturition to the first estrous cycle of normal duration was similar for cows in the C and 1-E treatment (53 +/- 2.4 and 56 +/- 2.4 days, respectively). Cows in the 1/4-E treatment had a longer (P less than .05) interval (68 +/- 2.5 days). Secretion of LH was similar among treatments on day 35 postpartum. The first normal luteal phase after parturition was preceded by a transient rise in progesterone in 81, 64 and 85% of the cows in the C, 1-E and 1/4-E treatments, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Extension of short cycles in postpartum beef cows by intrauterine treatment with catecholestradiol

Eight multiparous beef cows were used to examine the effects of intrauterine infusion of catecholestradiol (4-hydroxylated estradiol) on development and function of the first corpus luteum after parturition. Calves were weaned on day 1 (day 0 = parturition) to initiate formation of a corpus luteum (CL) by approximately day 10 or 11. Before CL formation, on days 5 to 9, cows received twice daily infusions of catecholestradiol (4 micrograms; n = 4) or vehicle (n = 4) into the uterine horn opposite the previous pregnancy. Plasma progesterone during the first estrous cycle was elevated longer (P less than .001) and reached a higher (P less than .001) concentration in cows treated with catecholestradiol. The decline in progesterone was associated with an increase in plasma 13,14-dihydro, 15-keto-prostaglandin F2 alpha (PGFM) in all cows infused with catecholestradiol. In contrast, a rise in PGFM at the end of the first short cycle was detected in only one of four cows treated with vehicle. Furthermore, PGFM concentrations were linearly related (R2 = .870; P less than .001) to concentrations of progesterone. Estradiol-17 beta concentrations were not different during the infusion period, but after formation of the first CL, estradiol remained elevated (P less than .01) in cows that received vehicle. Results of this experiment suggest that exposure of postpartum beef cows to catecholestradiol extended luteal function in association with enhanced PGFM release.

Effect of estradiol on secretion of luteinizing hormone during the follicular phase of the bovine estrous cycle

Mean concentrations of luteinizing hormone (LH) increase during the follicular phase of the estrous cycle in cows. The working hypotheses in the present study were (1) that increasing concentrations of 17 beta-estradiol (E2) during the follicular phase of the estrous cycle cause an increase in mean concentration of LH by increasing amplitude of pulses of LH, and (2) that increasing E2 concentrations during this stage of the estrous cycle decrease frequency of pulses of LH in bovine females. Day of estrus was synchronized in seventeen mature cows. Treatments were initiated on Day 16 of the experimental estrous cycle (Day 0 = estrus). At Hour 0 (on Day 16), 4 cows were lutectomized. Lutectomy of these cows (EE; n = 4) allowed for endogenous secretion of E2. The remaining cows were ovariectomized at Hour 0 and were assigned to one of three E2 treatments: luteal phase E2 (LE, n = 5), increasing then decreasing E2 (DE, n = 5), and no E2 (NE, n = 3). Cows in the group that received LE were administered one E2 implant at Hour 0, which provided low circulating concentrations of E2 similar to those observed during the luteal phase of the estrous cycle. Cows in the group that received DE were administered one E2 implant at Hour 0, and additional implants were administered at 8-h intervals through Hour 40; then, two implants were removed at Hours 48 and 56, and one implant was removed at Hour 64.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of luteinizing hormone in postpubertal boars with large testes

The objective of the present study was to investigate endocrine control of LH in postpubertal boars with large testes. Eight boars with the highest estimated paired testis weights from a line selected for large testes and nine boars from a line selected at random were used. Blood samples were collected over a 13-h period at weekly intervals for 4 wk. Samples were collected at 12-min intervals for 12 h before and 1 h after exogenous LHRH. Boars were bled when they were intact during the initial week. The second and third blood collections were 7 and 14 d after castration. The fourth bleeding occurred 7 d after exogenous 17 beta-estradiol (E2) replacement. In intact boars, mean LH was similar between boars from the two groups, but amplitude of pulses of LH was lower in intact boars with large testes than in boars from the control line. Maximum concentration of LH after administration of LHRH was less in boars with large testes than in boars from the control group. Seven days after castration, characteristics of LH measured did not differ between males from the two groups. However, 14 d after castration, amplitude of pulses of LH and maximum concentrations of LH after LHRH were less in males from the group with large testes than in males from the control group. After E2 administration, amplitude of pulses of LH tended to be lower in males from the group with large testes than in males from the control group.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of season and lactation on postpartum fertility of Polypay, Dorset, St. Croix and Targhee ewes

The objective of this study was to compare rebreeding activities of spring- vs fall-lambing Polypay, Dorset, St. Croix and Targhee ewes that either suckled their lambs for 40 d or had lambs weaned at birth. Seasonal effects of male fertility were reduced by utilizing an excess number of fertile rams in the spring. Plasma concentrations of progesterone were monitored to assess days to the first normal ovulation, days to conception and estrous vs anestrous activity. Breed, season and lactation affected the rebreeding performance. Dorset ewes had similar conception rates between spring and fall but a shorter interval from lambing to first ovulation in the fall. Polypay and Targhee ewes were the opposite; they had higher conception rates in fall than in spring matings with no seasonal influence on postpartum interval. Postpartum ewes in the fall had higher conception rates, and fewer of these ewes became anestrous or had estrous cycles of abnormal duration than of those ewes lambing in the spring. Ewes that suckled for 40 d in the spring had delayed estrous activity, but when these ewes became estrual they had higher conception rates than ewes whose lambs were weaned at birth. Lactation had no inhibitory affect on the postpartum interval of fall lambing ewes. These data suggest that the response of different breeds to various components of postpartum fertility varies with season and management of the flock.

Survival of small and large littermate blastocysts in swine after synchronous and asynchronous transfer procedures

Thirty-two crossbred sows were assigned to synchronous and asynchronous embryo transfer procedures to determine if, within a litter, small blastocysts were as viable as large blastocysts. Synchronous embryo transfers were established when donors and recipients displayed the onset of estrus (Day 0) within 6 h of each other. Asynchronous transfers were established when recipients displayed the onset of estrus 18 to 24 h after that of donors. An equal number (four or five) of the smallest and largest diameter blastocysts, from a Day 7 donor, were transferred to separate uterine horns of a Day 7 (synchronous) or a Day 6 (asynchronous) recipient. Each recipient's uterine horns were ligated at the external bifurcation to prevent transuterine embryonic migration. The percentage of blastocysts surviving was determined 300 h (12.5 d) after donors exhibited estrus. Small as well as large Day 7 blastocysts survived following asynchronos transfer to a Day 6 recipient. However, fewer (P<0.01) small blastocysts survived synchronous transfer than large blastocysts. These data suggested that small blastocysts were lost due to asynchrony with the uterine environment; however, when transferred to a less advanced environment, small blastocysts were equally viable as large blastocysts.

Ca2+ not cyclic AMP mediates the fluid secretory response to isoproterenol in the rat mandibular salivary gland: whole-cell patch-clamp studies

We have performed whole-cell patch-clamp studies on dispersed secretory cells of the rat mandibular gland to determine how beta-adrenergic stimulation causes fluid secretion. When the pipette contained a high K+ solution, the resting membrane potential averaged -33 mV +/- 1.1 (SEM, n = 34) and the clamped cell showed strong outward rectification. We monitored K+ and Cl- currents for periods of 15 min by recording the currents needed to clamp the cell potential at 0 and -80 mV, respectively. Isoproterenol (1-2 mumol/l) caused increases in the clamp current at 0 mV (the K+ current) and at -80 mV (the Cl- current) in about 80% of cases, although the responses were variable in size and time-course; the responses were indistinguishable from those induced by acetylcholine or the Ca2+ ionophore, A23187. The alpha-adrenergic antagonist, phentolamine (1-2 mumol/l), had no effect on the response, but the beta-adrenergic antagonist, propranolol (10 mumol/l), blocked it completely. The isoproterenol response could not be mimicked by application to either surface of the cell membrane, of cyclic AMP (100 mumol/l), forskolin (1 or 20 mumol/l) or cholera toxin (2.5 micrograms/ml). However, increasing the Ca2+-chelating capacity of the pipette solution by raising its EGTA concentration from the customary 0.5 to 20 mmol/l, blocked the response to isoproterenol, suggesting that beta-adrenergic agonists activate Cl- and K+ channels by raising cytosolic Ca2+. Since neomycin, which blocks phospholipase C, blocked the action of isoproterenol without impairing the cell responsiveness to A23187, it appears that isoproterenol, like muscarinic agonists, increased cytosolic Ca2+ via the phosphatidylinositol cycle.

Expression of the gene for preproatriopeptin in the central nervous system of the rat

Atriopeptin (AP) is a peptide hormone synthesized and secreted by the atria of the heart that participates in the regulation of fluid and electrolyte balance. AP-like materials have been detected immunologically in neurons in the central nervous system of the rat. In this study, we have used a solution hybridization-nuclease protection assay to determine whether the brain of the rat contains RNA coding preproatriopeptin, the atrial biosynthetic precursor of AP, and to study the regional distribution of preproatriopeptin mRNA in the brain. We have found that the brain contains mRNA identical to the atrial messenger RNA for preproatriopeptin. AP mRNA is differentially distributed in the brain; the highest concentration was found in the hypothalamus, followed by the cortex and septum, hippocampus, midbrain, spinal cord, olfactory bulb, striatum, and pons and medulla. Very low levels were found in the cerebellum, while no detectable AP mRNA was observed in retina, anterior pituitary, or rat liver. The presence of AP mRNA in the brain demonstrates that neurons have the capacity to synthesize preproatriopeptin identical to that produced in the heart. Bioactive peptides produced from this precursor may be endogenous central neuromodulators as well as a circulating hormone.

Variation in conception rates following synchronization of estrus with melengestrol acetate and prostaglandin F2 alpha

Beef cows and heifers (n = 263) at three locations that were exhibiting estrous cycle either were fed .5 mg/d melengestrol acetate (MGA) for 7 d and administered prostaglandin F2 alpha (25 mg, i.m.) on the last day of MGA feeding or were untreated. State of the estrous cycle at the beginning of the experiment was determined based on estrous detection and (or) progesterone concentrations in pretreatment blood samples. Estrous was checked twice daily for 30 d posttreatment. Animals were artificially inseminated approximately 12 h after detection of estrus. A synchronized estrus (less than 7 d posttreatment) was detected in 72% of the treated animals. More animals in the treated group became pregnant during the first 7 d of breeding, but their conception rate was lower than that of animals in the control group (P less than .05). Conception rate (36%) was reduced among treated animals when MGA feeding began late (d 14 to 20) in the estrous cycle. Conversely, the conception rate (66%) of treated animals fed MGA beginning earlier in the cycle was not different from that of control animals (73%; treatment x stage of cycle; P less than .05).

Feedback of 17 beta-estradiol on secretion of luteinizing hormone during different seasons of the year

The working hypothesis was that the amount of increase in secretion of luteinizing hormone (LH) that results from positive feedback of 17 beta-estradiol (E2) is dependent on season of the year in mature bovine females. Seven beef cows, ovariectomized approximately 2 mo before the initiation of the experiment, were used in the initial year (1983) of the study. Three of the ovariectomized cows (OVX-E2) received an sc E2 implant, which provided low circulating levels of E2. The remaining four cows (OVX) were not implanted. Blood samples were collected serially (at 10-min intervals for 6 h) at each spring and fall equinox and at each summer and winter solstice. This protocol was replicated with a different group of cows in 1985 (OVX-E2, n = 4; OVX, n = 6). Concentration of LH in blood serum was quantified in all samples. Concentration of E2 in blood serum was measured in pools of samples from each serial blood collection. Concentrations of E2 were higher (P less than .05) in the implanted cows. Mean concentration of LH and amplitude of pulses of LH were higher (P less than .05) at each season of the year in cows that were ovariectomized and implanted with E2 than in cows that were ovariectomized and did not receive E2. An effect of season of the year on mean concentration of LH was detected (P less than .01). No influence of season or E2 was detected for frequency of pulses of LH. There was no significant treatment X season interaction.(ABSTRACT TRUNCATED AT 250 WORDS)

Endocrine mechanisms of puberty in heifers. Role of hypothalamo-pituitary estradiol receptors in the negative feedback of estradiol on luteinizing hormone secretion

The hypothesis tested was that the decline in negative feedback of estradiol on secretion of luteinizing hormone (LH) that occurs as puberty approaches in heifers results from a decline in the number of receptors for estradiol in the hypothalamus and/or pituitary. In addition, associated changes in receptors for luteinizing hormone-releasing hormone (LHRH) in the pituitary, ovarian follicle development, and uterine growth were characterized. Fifty prepubertal heifers, 234 to 264 days of age, were used. Six heifers of median body weight were designated controls, and sequential blood samples were collected at 20-min intervals for 24 h every 2 wk from 249 days of age through puberty and analyzed for concentrations of LH. Frequency of LH pulses/24 h was regressed on number of days prepuberty to develop a prediction equation for puberty. Thirty of the remaining 44 heifers were killed at 253, 302, and 351 days of age (n = 10/group), and tissues for described analyses were collected. Three to 5 days before tissue collection, sequential blood samples were obtained from these heifers, as described for control heifers to determine frequency of release of LH. With this information, number of days prepuberty at the time of tissue collection was estimated from the prediction equation developed with data from control heifers. The average age at puberty in control heifers was 366 days. The average age at puberty of heifers that were not killed or included in the control group (n = 14) was 360 days. Receptor and morphological data were related to the estimated onset of puberty. Cytosolic concentration of receptors for estradiol (fmoles receptor/mg cytosolic protein) in the anterior hypothalamus, medial basal hypothalamus, and anterior pituitary declined (p less than 0.05) as puberty approached. No change in concentration of receptors for estradiol was observed in the stalk median eminence or preoptic area. The concentration of receptors for LHRH in the anterior pituitary did not change as puberty approached. Uterine weight increased rapidly during the 50 days preceding puberty. The number of small, medium, or large follicles and the wet, pressed, or dry weight of the ovaries did not change as puberty approached. Follicles with a diameter greater than 12 mm were found only in the 3 heifers estimated to be closest to puberty at the time of tissue collection. The hypothesis that the decline in estradiol feedback on secretion of LH during the prepubertal period in heifers may result from a decline in the concentration of binding sites for estradiol at the hypothalamus and/or pituitary is supported by this study.

Suckling behavior of calves with dams varying in milk production

Two experiments were performed to test the hypothesis that suckling behavior of calves with similar growth potential varies depending on cows' level of estimated milk production and stage of lactation. Eleven mature cows, which varied in estimated 205-d milk production (996 to 2354 kg/205 d), nursing heifer calves of similar growth potential were used in Exp. 1. Suckling behavior of calves was observed for two 24-h periods at three stages of lactation (average of 52, 104 and 167 d postpartum). Suckling frequency (suckling bouts/24 h) declined as milk production increased at 52 d of lactation (-.00382 bouts/kg milk) but was unrelated to milk production at later stages. Duration of suckling (minutes/suckling bout) increased with estimated level of milk production at all stages of lactation (means = .001556 min/kg milk). Total time suckling tended to increase as estimated level of milk production decreased at 52 d of lactation, but this component of suckling behavior was unaffected by milk level at later stages. Suckling frequency declined from 8.6 bouts/24 h at 52 d of lactation to 4.5 bouts/24 h at 167 d of lactation when averaged across all cows. Total minutes nursed/24 h declined in a similar manner (64 min/24 h to 44 min/24 h) between 52 and 167 d of lactation. Duration of each suckling bout did not change with stage. In the second experiment the relationship of suckling behavior to estimated milk production was evaluated at four early stages (average of 17, 38, 59 and 80 d postpartum) of lactation using 20 mature cows.(ABSTRACT TRUNCATED AT 250 WORDS)

Ventricular atriopeptin. Unmasking of messenger RNA and peptide synthesis by hypertrophy or dexamethasone

Left ventricular hypertrophy or treatment with dexamethasone caused a 2.5-fold to threefold increase in both immunoreactive atriopeptin (AP) and AP messenger RNA (mRNA), primarily in left ventricular tissue. The combined treatments increased immunoreactive AP and AP mRNA more than either treatment alone. In the animals in which cardiac hypertrophy had been produced by abdominal aortic constriction, there was a decrease in atrial levels of AP and an increase in plasma levels of immunoreactive AP. The increase in left ventricular immunoreactive AP was confirmed by immunohistochemical staining of tissue from hypertrophied and/or dexamethasone-treated rats. The mRNA accumulated in the left ventricle was identical to atrial AP mRNA, as judged by transcriptional start site and by size on Northern blots. Because the mass of ventricular tissue is substantially greater than that of atrial tissue, the induced mRNA levels may represent a total abundance approaching one third of the total AP mRNA in the atria. High performance liquid chromatographic purification of ventricular extracts primarily demonstrated the presence of the high molecular precursor and small amounts of C-terminal peptide AP. Induction of ventricular AP (mRNA and peptide) may represent regression of the tissue to an earlier developmental form. These data provide a unique example of regulation of AP biosynthesis in nonatrial tissue.

Developmental changes in the rat atriopeptin hormonal system

We undertook a study of fetal synthesis, storage, and release of atriopeptin (AP). Plasma levels of both atriopeptin immunoreactivity (APir) and the NH2-terminal fragment of the prohormone immunoreactivity (NTFir) were very high in the fetus (4 and 20 times the maternal plasma, respectively). However, the atrial content of the AP was low, but surprisingly, ventricular content of AP was quite high (relative to the adult) in the fetus and fell postnatally. Atrial AP messenger RNA (mRNA) increased with postnatal age, whereas ventricular mRNA was extremely high in the fetus and fell rapidly after birth. High fetal plasma peptide levels may derive from the mother since infusion of exogenous atriopeptin 24 into the mother resulted in parallel increases in fetal and maternal peptide levels. Fetal plasma APir and NTFir levels partially reflect the markedly reduced total renal metabolic capacity compared with that of the adult. Plasma levels fell progressively after birth; whereas neonatal atrial content rose substantially. Plasma AP and NTF were simultaneously elevated in both the maternal and fetal circulation after vasopressin injection of the mother. The fetus can also respond to exogenous stimuli (vasopressin or indomethacin--presumably via ductal closure) and promptly release substantial amounts of peptide into its circulation. Thus, it appears that the AP hormonal system is functional during fetal life and responds avidly to increases in intracardiac pressure as does the mature animal.

Effects of 17 beta-estradiol and diets varying in energy on secretion of luteinizing hormone in beef heifers

An experiment was conducted to test the hypothesis that 17 beta-estradiol (E2) would not suppress secretion of luteinizing hormone (LH) in heifers fed a diet limited in energy during the period before the onset of nutritionally induced anestrus. Sixteen of 20 heifers that had been exhibiting normal estrous cycles (20 mo of age, 409 +/- 6 kg body weight) were ovariectomized, and half of them were assigned at random to receive an E2 implant. The ovariectomized heifers were assigned at random to receive diets that contained low (L; 5.8 Mcal X animal-1 X d-1, n = 8) or high levels of energy (H; 20.0 Mcal X animal-1 X d-1, n = 8) for 100 d. The other four heifers remained intact and were fed the L-diet. The intact heifers were utilized to determine the status of reproductive function in animals fed the L-diet. Heifers lost body weight rapidly after initiation of feeding the L-diet. Heifers fed the L-diet then stabilized at a lighter weight until the latter part of the experiment. One of the four intact heifers fed the L-diet became anestrus near the end of the study. Mean concentrations of LH in blood serum increased linearly (P less than .05) in ovariectomized heifers fed the L- and H-diet. Mean concentration of LH in heifers fed the H-diet that were implanted with E2 was similar to ovariectomized heifers fed the H-diet that received no E2. Mean LH in serum of ovariectomized heifers implanted with E2 fed the L-diet was suppressed and remained low throughout the study. Frequency of pulses of LH in ovariectomized heifers fed the L-diet was less (P less than .01) than that in ovariectomized heifers fed the H-diet. Estradiol decreased the number of pulses of LH in heifers fed the L-diet. We conclude that dietary energy restriction in beef heifers has a direct action on the hypothalamo-pituitary axis to lower the number of pulses of LH in the absence of ovarian steroids. However, ovarian E2 appears to suppress further secretion of LH in heifers fed limited levels of dietary energy before the onset of nutritional anestrus occurs, therefore, our working hypothesis is rejected.

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)

Influence of dietary-induced weight changes on serum luteinizing hormone, estrogen and progesterone in the bovine female

The working hypothesis in the present study was that changes in concentrations and secretory patterns of luteinizing hormone (LH), 17 beta estradiol (E2), and progesterone in sexually mature beef heifers fed diets deficient in energy are related to changes in body weight of the animals. Another important component of the study was to determine if concentrations and secretion patterns of the reproductive hormones changed over time as feeding of the experimental diets continued. Twelve Red Angus X Hereford heifers (20 mo of age; 355 +/- 7 kg) were assigned randomly to receive a low- (L, n = 7) or high- (H, n = 5) energy diet for 100 days (Day 0 = day of initiation of dietary treatment). All heifers were exhibiting estrous cycles at regular intervals when the experiment was initiated and continued to exhibit estrous cycles at regular intervals throughout the study. Stage of the estrous cycle was synchronized in all 12 heifers by administration of prostaglandin F2 alpha (PGF2 alpha) on two occasions (Days 45 and 75) during the experiment. Serial blood samples (taken at 12-min intervals for 4 h) were collected at 0, 12, 24, 36, 48, and 60 h after the PGF2 alpha injections (Days 45-47 and 75-77) to determine patterns of LH secretion during the follicular phase of the estrous cycle. In addition, serial blood samples (taken at 20-min intervals for 18 h) to monitor LH secretion during the luteal phase of the estrous cycle, in which the stage of the cycle was standardized between heifers, were obtained (Days 59 and 89).(ABSTRACT TRUNCATED AT 250 WORDS)

Endocrine changes during restoration of estrous cycles following induction of anestrus by restricted nutrient intake in beef heifers

The working hypotheses in this experiment were: that ovarian estradiol would inhibit luteinizing hormone (LH) secretion in heifers that were anestrus as a result of restricted dietary energy intake and the responsiveness of LH secretion to estradiol negative feedback would decrease during the period when restoration of estrous cycles occurred following feeding of diets adequate in energy. Fifteen heifers weighing 341 +/- 12 (mean +/- SE) kg were fed a diet containing 50% of the energy required for maintenance until 40 to 50 d following cessation of estrous cycles. Heifers were assigned to intact control (C, n = 5), ovariectomized (OVX, n = 5) or ovariectomized-estradiol-17 beta-implanted (OVX + E2, n = 5) treatments. Heifers were subsequently provided a high-energy (HE) diet until termination of the study. Progesterone concentrations indicating cessation of corpus luteum function were detected after heifers had lost 71 +/- 8 kg body weight over 186 +/- 28 d. Control heifers re-initiated estrous cycles as indicated by increased progesterone concentrations in serum at 49 +/- 9 d after initiation of feeding the HE diet (360 +/- 18 kg body weight). Initiation of pulsatile LH secretion was observed in heifers by d 12 following OVX. Estradiol suppressed LH secretion in OVX + E2 heifers during the period of nutritional anestrus in C heifers. Suppressive effects of E2 on LH secretion continued in OVX heifers after C heifers had initiated corpus luteum function. Therefore, the working hypothesis that LH secretion is inhibited by E2 in the nutritionally anestrous heifer is accepted but responsiveness to estradiol does not subside with re-initiation of estrous cycles, thus this working hypothesis is rejected.

Regulation of pulsatile LH secretion by ovarian steroids in the heifer

Two experiments were conducted to evaluate relationships among luteinizing hormone (LH), estradiol-17 beta (E2) and progesterone secretion during the preovulatory period in the heifer after prostaglandin F2 alpha (PGF2 alpha)-induced regression of the corpus luteum. A second objective was to elucidate the effects of E2 in regulating LH secretion. In Exp. 1, LH, E2 and progesterone concentrations were determined in serial samples collected during the preovulatory period after PGF2 alpha-induced luteal regression in five Red Angus X Hereford heifers. Progesterone declined to 1 ng/ml by 12 h after the second injection of PGF2 alpha. Frequency of LH pulses increased linearly (P less than .01), whereas no change in amplitude of LH pulses was detected before the preovulatory LH surge. This resulted in a linear increase (P less than .01) in mean LH concentrations. Estradiol also increased in a linear manner (P less than .01), and the rise in E2 was parallel to the increase in mean LH concentrations. In Exp. 2, 12 Angus X Hereford heifers were ovariectomized and administered either 13.5- or 27-cm silastic implants containing E2 at ovariectomy. Four heifers served as nonimplanted controls. Thirty-one days after ovariectomy all heifers were bled at 12-min intervals for 6 h. Frequency of LH pulses declined linearly (P less than .03) while mean LH (P less than .09) and pulse amplitude (P less than .01) increased linearly as E2 dose increased. These results indicate that a reduction in progesterone increases the frequency of LH pulses during the follicular phase of the estrous cycle in cattle.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of restriction of dietary energy intake during the prepubertal period on secretion of luteinizing hormone and responsiveness of the pituitary to luteinizing hormone-releasing hormone in heifers

The working hypothesis that a low plane of nutrition during the prepubertal period delays puberty in heifers by retarding the prepubertal increase in secretion of luteinizing hormone (LH) was investigated. Secretion of LH and the responsiveness of the pituitary to LH-releasing hormone (LHRH) were compared in heifers fed a growing diet (which allowed spontaneous occurrence of puberty; n = 12; control) or an energy deficient diet (which delayed puberty; n = 11; delayed) during the prepubertal period. The dietary treatments were initiated when the heifers were 299 +/- 14 (mean +/- SD) d of age (d 0 of the experiment) and continued until d 175 of the experiment (474 +/- 14 d of age). Weight gains were .79 +/- .05 (mean +/- SE) and .21 +/- .03 kg X head-1 X d-1 for control and delayed heifers, respectively. Puberty occurred on d 120 +/- 14 of the experiment (428 +/- 13 d of age) in control heifers, whereas none of the delayed heifers attained puberty during the feeding period. Serum concentration of LH and the frequency of LH pulses increased rapidly during the 175-d feeding period in control heifers. In delayed heifers, serum LH concentration increased less rapidly and no increase in pulse frequency was detected during the experimental period. Amplitude of LH pulses tended to be higher in control than delayed heifers. Responsiveness of LH secretion to LHRH was lower in delayed than control heifers. It is speculated that failure of secretion of LH to increase is the causative factor for delayed puberty when dietary energy is limited during the prepubertal period in heifers.

Effects of copulation on timing of the LH surge following synchronization of estrus in the bovine

Forty-four crossbred postpubertal bovine females were used to study how mating with a bull affected estradiol-17beta (E(2)) secretion and timing of the preovulatory LH surge. Estrous cycles were synchronized with two injections of prostaglandin-F(2alpha) (PGF(2alpha)) 11 d apart. Females were either isolated from males (NE) or exposed to epididectomized bulls (BE) after the second PGF(2alpha) injection. Females exposed to bulls were allowed to mate once and then were separated from the bull. Blood samples were collected at 2-h intervals from the second PGF(2alpha) injection until 12-h post injection to monitor progesterone (P(4)) and luteinizing hormone (LH) concentrations and at hourly intervals from 12 h to 60 h post-injection to monitor LH secretion and timing of the preovulatory LH surge. Samples were also collected at 4-h intervals until 60 h post-injection to monitor estrogen (E(2)) secretion. LH surges were detected in 16 and 14 of 22 females from the BE and NE groups, respectively, during the 60-h period after PGF(2alpha) injection Mean P(4) concentrations and time of P(4) decline to <1 ng/ml were not different between the two treatment groups (P>0.30). Mean E(2) concentration during the 60-h sampling period was different (P<0.003) between BE and NE groups, and a significant treatment effect (P<0.002) occurred 48 h, 52 h and 60 h after the second PGF(2alpha) injection. However, mean LH concentration before the LH surge, duration of the LH surge and peak LH concentration during the surge were not different between the BE and NE groups (P>0.40). Mean time for the second PGF(2alpha) injection to the beginning of the LH surge was 51.6 +/- 1.5 h (X +/- S E) for the females not exposed to bulls and 48.5 +/- 1.4 h for females exposed to bulls (P>0.14). In this study, the presence of and/or mating by a bull did not affect LH secretion or timing of the preovulatory LH surge after PGF(2alpha) administration.

Influence of exposure to bulls on resumption of estrous cycles following parturition in beef cows

The effect of bull exposure on the resumption of estrous activity following parturition was studied in an experiment using mature Hereford and Hereford X Angus beef cows. In the spring of 1981 and 1982, cows were assigned by breed and calving date to one of two treatment groups. Cows were exposed to bulls either from 3 to 85 d postpartum (BE; n = 45, 1981; n = 35, 1982) or from 53 to 85 d postpartum (NE; n = 39, 1981, n = 36, 1982). Blood samples were collected from all cows once weekly from calving until 85 d postpartum to determine progesterone concentrations. The first increase in progesterone, which indicated onset of estrous cycles occurred at 43 +/- 2 vs 63 +/- 2 d (P less than .01) in 1981 and at 39 +/- 2 vs 61 +/- 3 d (P less than .01) postpartum in 1982 in BE cows and NE cows, respectively. Early postpartum exposure of cows to bulls reduced the postpartum anestrous interval.

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.

Endocrine mechanisms of puberty in heifers: estradiol negative feedback regulation of luteinizing hormone secretion

The hypothesis that luteinizing hormone (LH) secretion in prepubertal females is responsive to estradiol negative feedback and that decreased feedback occurs as puberty approaches was tested in heifers. In the first experiment, seven heifers were maintained prepubertal by dietary energy restriction until 508 days of age (Day 0). All heifers were placed on a high-energy diet on Day 0 at which time they received no additional treatment (CONT), were ovariectomized (OVX) or were ovariectomized and subcutaneously implanted with estradiol-17 beta (OVX-E2). This feeding regimen was used to synchronize reproductive state in all heifers. A second experiment was performed with 16 prepubertal heifers using the same treatments at 266 days (Day 0) of age (CONT, OVX and OVX-E2) but no dietary intake manipulation. In both experiments, LH secretion increased rapidly following ovariectomy in OVX heifers. In the initial experiment, LH secretion was maintained at a low level in OVX-E2 heifers until a synchronous rapid increase was noted coincidental with puberty in the CONT heifer. In the second experiment, LH secretion increased gradually in OVX-E2 heifers and attained castrate levels coincidental with puberty in CONT heifers. A gradual increase in LH secretion occurred as puberty approached in CONT heifers. These results indicate that: a) LH secretion in prepubertal heifers is responsive to estradiol negative feedback; and b) estradiol negative feedback decreases during the prepubertal period in beef heifers.