Relationship among insulin-like growth factor-I,blood metabolites and postpartum ovarian function in dairy cows

The relationship among nutritional status, systemic insulin-like growth factor-I (IGF-I) and ovarian function early postpartum were investigated. A total of 27 Holstein-Friesian cows, 10 that cycled normally within 20 days postpartum, 5 diagnosed with follicular cysts, 8 with persistent corpus luteum (CL) after the first ovulation postpartum and 4 with inactive ovaries were used for the study. Blood samples were collected 1-3 times per week, for 60 days pre- and postpartum, for IGF-I, progesterone, estradiol, free fatty acids (FFA), blood urea nitrogen (BUN), and aspartate aminotransferase (AST) determination. Inactive ovary and cystic cows had a higher body condition score before calving and lost more condition than normal or persistent CL cows. Immediately postpartum, IGF-I levels were higher and rose sharply in cows that cycled normally than in cystic, inactive ovary or persistent CL cows. At calving and early postpartum, FFA was higher in inactive ovary and cystic than in normal and persistent CL cows. There was a significant strong positive relationship between IGF-I and BUN, and strong negative relationships between IGF-I and FFA and AST in all groups. There was a positive relationship between serum IGF-I and estradiol in normal cystic and inactive ovary cows. This study found that overconditioned cows during the dry period or at calving, lost more body condition postpartum. These cows also had a deeper and longer period of negative energy balance (NEB), poor liver function and low circulating IGF-I concentrations early postpartum. Such cows were likely to have poor reproductive function as seen in development of cystic ovaries, persistent CL and inactive ovary. Changes in serum IGF-I early postpartum may help predict both nutritional and reproductive status in dairy cattle.     

Relationships between insulin-like growth factor-I,milk yield,body condition score,and postpartum luteal activity in high-producing dairy cows

The relations between body condition score (BCS), milk yield, serum insulin-like growth factor-I (IGF-I) profile, and luteal activity were investigated in postpartum dairy cows. Seventy-one healthy high-producing multiparous Holstein cows were subjected to transrectal ultrasound scanning twice weekly from the first to the eighth week postpartum. Blood samples were collected twice weekly to measure serum progesterone (P4) and every 2 weeks to detect serum IGF-I concentrations. BCS was monitored weekly after calving. Cows with serum P4 concentrations ≥1 ng/ml on at least two consecutive samplings were considered to have commenced luteal activity. Commencement of luteal activity (C-LA) was observed earlier than 45 days postpartum in 71.8% of cows while 28.2% showed C-LA later than 45 days. Prolonged luteal phase was the most common abnormal pattern of luteal activity observed. Cows with a C-LA earlier than 45 days postpartum had higher (P ≤ 0.05) mean serum concentrations of IGF-I than those with later C-LA. In addition, cows which showed C-LA earlier than 45 days postpartum had more optimal productive indices including shorter calving to conception interval and calving to first service interval (P ≤ 0.05), and fewer services per conception (P = 0.07). C-LA was significantly later in cows that lost more than 0.5 BCS units within 3 weeks postpartum than in those that lost less than 0.5 units BCS during the same interval (P = 0.02). We conclude that high-producing dairy cows with higher postpartum serum IGF-I concentrations have earlier commencement and normal luteal activity, and better reproductive performance. Severity and duration of BCS loss adversely affect commencement of luteal activity.     

Genetic parameter estimates for serum insulin-like growth factor-I concentration and carcass traits in Angus beef cattle

Divergent selection for serum insulin-like growth factor-I (IGF-I) concentration began at the Eastern Ohio Resource Development Center (EORDC) in 1989 using 100 spring-calving (50 high line and 50 low line) and 100 fall-calving (50 high line and 50 low line) purebred Angus cows. Following weaning, bull and heifer calves were fed in drylot for a 140-d postweaning period. At the conclusion of the postweaning test, bulls not selected for breeding were slaughtered and carcass data were collected at a commercial abbatoir. At the time of this analysis, IGF-I measurements were available for 1,283 bull and heifer calves, and carcass data were available for 452 bulls. A set of multiple-trait, derivative-free, restricted maximum likelihood (MTDFREML) computer programs were used for data analysis. Estimates of direct heritability for IGF-I concentration at d 28, 42, and 56 of the postweaning period, and for mean IGF-I concentration were .32, .59, .31, and .42, respectively. Direct heritabilities for carcass traits ranged from .27 to 1.0, .26 to 1.0, and .23 to 1.0 when the age-, fat-, and weight-constant end points, respectively, were used, with marbling score having the smallest heritability and longissimus muscle area having the highest heritability in each case. Maternal heritability and the proportion of phenotypic variance due to permanent environmental effect of dam generally were < or = .21 for IGF-I concentrations and for carcass traits other than longissimus muscle area. Additive genetic correlations of IGF-I concentrations with backfat thickness, longissimus muscle area, hot carcass weight, marbling score, quality grade, and yield grade averaged -.26, .19, -.04, -.53, -.45, and -.27, respectively, when carcass data were adjusted to an age-constant end point. Bulls with lower IGF-I concentrations had higher marbling scores and quality grades, but also had higher backfat thickness and yield grades regardless of the slaughter end point. Serum IGF-I concentration may be a useful selection criterion when efforts are directed toward improvement of marbling scores and quality grades of beef cattle.     

Enclomiphene does not alter the postpartum interval of suckled beef cows.

The objective of this study was to determine whether an antiestrogen (enclomiphene) would shorten the interval to first estrus and conception in postpartum beef cows. Sixty postpartum Angus beef cows were stratified by age, body condition, and calving date and were randomly assigned to one of two treatment groups. Group 1 cows (n = 24) received three silastic implants, each containing 150 mg of enclomiphene, on d 20 postpartum. Implants were removed on d 30 postpartum. Group 2 cows (n = 28), received empty implants and served as controls. Cows were artificially inseminated at first detected estrus. Estrus detection and ovulation were further verified by increased serum progesterone. Concentrations and pulse frequencies of LH were determined from blood samples collected at 15-min intervals for 6 h on d 20, 25, 30, and 40 postpartum. Hypothalami and pituitaries were collected from four cows in each treatment group on d 30 postpartum and analyzed for concentrations of estradiol receptors. Concentrations of total and unoccupied hypothalamic and pituitary estradiol receptors were reduced by enclomiphene. Neither concentrations nor pulse frequencies of LH differed significantly between treatment groups on any of the 4 d. Days to first estrus did not differ (P greater than .05) between enclomiphene-treated (57 +/- 6; n = 24) and control (56 +/- 4; n = 28) cows. Days to conception did not differ between treated (81 +/- 9) and control (79 +/- 8) cows. The dose of enclomiphene used in this study reduced hypothalamic and pituitary estrogen receptors but did not alter secretion of LH or days to first estrus in the postpartum beef cow.     

Effects of lasalocid on circulating concentrations of leptin and insulin-like growth factor-I and reproductive performance of postpartum Brahman cows

Objectives were to determine effects of lasalocid on reproductive performance and serum concentrations of leptin and IGF-I, and to correlate concentrations of leptin and IGF-I with reproductive performance of beef cows. Forty-one purebred, multiparous Brahman cows were blocked to control (C; n = 20) or lasalocid (L; n = 21) treatments by BW, BCS, and predicted calving date. Treatment began 21 d before expected calving. Cows were each fed 1.4 kg daily of an 11:1 corn:soybean meal supplement, with the L group receiving 200 mg of lasalocid/cow daily. Cows and calves were weighed, and cow BCS was assessed at calving and at 28-d intervals thereafter. Blood samples were collected weekly precalving, at parturition, and twice weekly thereafter. Sterile marker bulls were maintained with cows for estrous detection. Six days after estrus, ovaries were evaluated for corpus luteum formation, and blood samples from d 6, 7, and 8 after estrus were collected. Serum samples were assayed for progesterone (P4), IGF-I, and leptin concentration. Progesterone concentrations > 1 ng/mL were considered indicative of a functional corpus luteum. Treatment ended after completion of a normal estrous cycle, and cows removed from treatment were placed with a fertile bull equipped with a chinball marker. There were no treatment differences in calving date, calf sex, cow BW, BCS, calf BW, calf ADG, or in serum concentrations of P4, IGF-I, or leptin. Prepartum cow ADG was increased (P < 0.01) in L cows and tended (P < 0.011) to be increased from calving to d 56 after calving in L cows. Postpartum interval (PPI) was not affected by treatment; however, a greater percentage (P < 0.05) of L cows conceived by 90 d after calving (43% L vs. 15% C). First-service conception rate tended (P < 0.08) to be greater in L vs. C cows (68 vs. 40%), but pregnancy rate was not different (P < 0.12; 86% for L vs. 65% for C). There were no treatment differences (P > 0.18) for serum IGF-I concentrations. At calving, leptin was positively correlated with IGF-I (P < 0.04; r = 0.32), BCS (P < 0.06; r = 0.29), and cow BW (P < 0.02; r = 0.36), and was negatively correlated with PPI (P < 0.06; r = -0.29). These results provide evidence that feeding an ionophore before calving and during the postpartum period may increase the number of cows that rebreed to maintain a yearly calving interval. Cows with higher concentrations of leptin postpartum may exhibit shorter PPI.     

Age-related changes in serum insulin-like growth factor-I, insulin-like growth factor-I binding protein-3 and articular cartilage structure in Thoroughbred horses

REASONS FOR PERFORMING STUDY: Structural changes in articular cartilage associated with the ageing process require definition for investigators performing developmental and age-related studies, for which information is lacking. OBJECTIVES: To 1) determine the onset and end of puberty as defined by serum insulin like growth factor (IGF-I) and IGF-binding protein-3 (IGFBP-3) concentrations and 2) correlate articular-epiphyseal cartilage complex structural changes with the onset and end of puberty. METHODS: IGF-I and IGFBP-3 were measured in serum samples from normal female and male horses age 9-715 days to determine peak and steady-state values for horses transitioning through puberty. Osteochondral tissue sections were obtained from horses age 120-840 days (4-28 months) and examined histologically for cartilage canals and tidemark formation. RESULTS: In male and female horses, serum IGF-I/IGFBP-3 concentrations peaked at approximately 225 days, defining the onset of puberty. Cartilage canals were absent from articular cartilage just prior to this time point. IGF-I/IGFBP-3 concentrations declined to steady-state levels at approximately age 450 days, signalling exit from puberty and therefore the beginning of ageing. This time point correlated to initial formation of a tidemark in the osteochondral tissue sections. CONCLUSIONS: Horses may be considered pubescent at age 225-450 days, and post pubescent and ageing after age 450 days. POTENTIAL RELEVANCE: Defining the normal post natal to post pubescent concentrations for serum IGF-I and serum IGFBP-3 establishes subsets of animals for age-related studies and may be used to monitor horses for abnormally high IGF-I concentrations due to natural disease or subsequent to systemic growth hormone administration.     

Influence of genotype and yield and composition of milk on interval to first postpartum ovulation in milked beef and dairy cows.

Our objectives were to investigate the effects of genotype, yield and composition of milk, and changes in BW and body condition on the initiation of cyclic ovarian activity in nonsuckled beef and dairy cows milked twice daily. Nulliparous heifers were selected from three breeds, 12 Angus, 13 Simmentals, and 26 Holsteins, based on their EBV for milk yield. Cows were machine-milked twice daily, and daily milk yield was recorded; composite samples of milk were collected twice weekly for analyses of milk components. Blood was collected from all cows thrice weekly for 75 d postpartum, and concentrations of progesterone in serum measured by RIA were used to estimate day of first postpartum ovulation. Holstein cows produced more (P less than .05) milk (unadjusted or 3.5% fat-corrected) than Angus or Simmental cows during the first 30 d of lactation. Holstein cows had higher (P less than .01) peak yield of milk than Simmental and Angus cows. Days to peak milk yield were similar for Simmental and Holstein cows, and both were more (P less than .05) than those for Angus cows. Percentages of fat, protein, and total solids in milk were highest (P less than .05) for Simmental cows, whereas milk of Holstein cows had the highest (P less than .05) percentage of lactose and lowest (P less than .05) concentration of somatic cells. Average BW at ovulation differed (P less than .05) among breeds. Estimated daily changes in BW from calving to first ovulation were different (P less than .05) for Holstein and Simmental cows but were similar to those of Angus cows.(ABSTRACT TRUNCATED AT 250 WORDS)     

Relationships between the plasma concentrations of insulin-like growth factor-I in dairy cows and their fertility and milk yield

The relationships between insulin-like growth factor-I (IGF-I) and the fertility and milk yield of Holstein-Friesian dairy cows were investigated. The concentration of IGF-I in blood was measured weekly from one week before to 12 weeks after calving in 177 multiparous cows and at four times during this period in 142 primiparous cows; the concentration of IGF-I in milk was measured in 50 of the multiparous cows. The plasma concentrations of IGF-I were higher in the primiparous than in the multiparous animals. In the primiparous cows, high concentrations of IGF-I before calving were associated with longer calving to conception intervals. Conversely, in the multiparous cows low concentrations of IGF-I before and after calving were associated with a failure to conceive, despite repeated services. Multiparous cows with IGF-I concentrations of greater than 25 ng/ml in the week after calving were 11 times more likely to conceive to first service than those with lower concentrations. Concentrations of IGF-I greater than 50 ng/ml at first service increased the likelihood of conception five-fold. Cows with higher peak milk yields had lower plasma concentrations of IGF-I and took longer to return to ovarian cyclicity. The negative relationship between milk yield and return to cyclicity was stronger in the multiparous cows (P < 0.002) than in the primiparous cows (P < 0.04). The concentrations of IGF-I in milk followed a different pattern and were not associated with the changes in plasma IGF-I or fertility.     

Effect of morphine on serum gonadotropin concentrations in postpartum beef cows

Morphine (M), an opioid agonist, was administered to postpartum (PP) Angus cows to investigate opioid modulation of gonadotropin secretion. In Exp. 1, eight PP cows (36.9 +/- 2.3 d) received either M (1 mg/kg; n = 4) or saline solution (S) (n = 4) via i.v. injection 36 h after calf removal. Morphine decreased (P less than .01) the number of serum LH pulses (3.0 +/- 1.1 pre- vs .3 +/- .3 post-pulses/h) and, compared with pretreatment values (3.3 mg/ml), decreased (P less than .05) mean LH at 105 min (2.1 ng/ml) through 270 min 1.9 ng/ml +/- .4). Serum prolactin (PRL) increased (P less than .01) following M from 16.4 ng/ml to a peak of 59.3 ng/ml (+/- 3.9). Serum FSH concentrations were unaffected. In Exp. 2, M (.31 mg/kg i.v. injection followed by .15 mg/(kg.h) infusion; n = 6) or S (n = 6) treatments were given for 7 h beginning 36 h after calf removal. Serum LH was similar between groups during the pretreatment and the first 6 h of infusion, but M decreased (P less than .001) the number of serum LH pulses (.44 +/- .09 vs .06 +/- .04 pulses/h). Morphine increased (P less than .05) serum PRL. It is concluded that M differentially modulated gonadotropin secretion in the cow such that PRL increased, LH decreased and FSH was unchanged.     

The relationship of insulin-like growth factor-I with postweaning performance in Angus beef cattle

This study was designed to evaluate the ontogeny of serum IGF-I (SI) concentrations and its relationship to animal performance in a 140-d postweaning feeding trial. Ninety-eight progeny representing six sires (three high and three low feed conversion) and two sexes (43 bulls and 55 heifers) with ad libitum access to feed were allocated by sire and sex to monitor individual weights and pen feed consumption. Blood serum samples were obtained at the beginning of test (average age of 230 d) and every 28 d thereafter until each animal reached a fat thickness (estimated by sonoray) of 8.9 mm. Individual serum samples were acid-ethanol extracted and measured for IGF-I peptide by heterologous RIA. Serum IGF-I concentrations differed (P less than .10) between high (H) and low (L) feed conversion progeny groups at the end of the first 28-d period (125.12 vs 89.52 ng/ml) and tended to differ at the conclusion of the second 28-d period (P less than .15). Weight gains of H and L groups tended to differ in the second and third 28-d periods (P = .11 and .10, respectively). Serum IGF-I concentrations differed (P less than .05) between bulls and heifers for the first through fourth 28-d periods (P less than .01, P less than .05, P less than .10 and P less than .01, respectively). Phenotypic correlations indicated that pens with higher mean SI concentrations at the beginning of the test consumed less feed and had lower cumulative feed:gain ratios.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of energy intake on the postpartum interval in beef cows and the adequacy of the cow's milk production for calf growth

Lactating Hereford and Hereford X Angus cows, 2 to 6 yr old, and their calves were used in two trials to study the effects of energy intake on the postpartum interval (PPI) and the adequacy of the cow's milk production for calf growth. In trial 1, 23 cows were fed either 120 (high energy, HE) or 100% (adequate energy, AE) of National Research Council energy recommendations. Cow weights, calf weights and milk production were determined 1 wk after calving and every 2 wk until 17 wk postcalving . One-half of the calves received creep feed from the start of the study, whereas the other half did not receive creep feed until their average daily gain was below .23 kg/d. In trail 2, 19 cows were fed energy levels of 100 (AE) or 85% (low energy, LE). Estimated percentage body fat ( EPBF ) was estimated twice during the study; other variables were determined as in trial 1. In trial 1, the HE cows gained more weight (P less than .01) and produced more milk (P less than .01). In trial 2, the AE cows gained more weight (P less than .01) and produced calves 17 kg heavier (P less than .01) at the end of the study than did the LE cows. Milk production was not influenced by energy treatment (P greater than .2). It required 7.5 kg of milk and 2.3 kg of creep feed to produce 1 kg of calf gain in trial 1.(ABSTRACT TRUNCATED AT 250 WORDS)     

Increased degradation of insulin-like growth factor-I in serum from feed-deprived steers

Severe feed restriction decreases serum insulin-like growth factor I (IGF-I) concentration in animals, and this decrease is thought to be due to reduced IGF-I production in the liver. The objective of this study was to determine whether feed deprivation also increases degradation of serum IGF-I and serum levels of IGF binding protein 3 (IGFBP-3) and acid-labile subunit (ALS), which inhibit IGF-I degradation and increase IGF-I retention in the blood by forming a ternary complex with IGF-I, in cattle. Five steers had free access to pasture, and another five were deprived of feed for 60 h. Serum concentration of IGF-I and liver abundance of IGF-I mRNA at the end of the 60-h period were 50% and 80% lower, respectively, in feed-deprived steers than in fed steers. Less ¹²⁵I-labeled IGF-I remained intact after a 45-h incubation in sera of feed-deprived steers than in sera of fed steers, suggesting that serum IGF-I is more quickly degraded in feed-deprived animals. Serum levels of IGFBP-3 and ALS were decreased by 40% and 30%, respectively, in feed-deprived steers compared with fed steers. These decreases were associated with more than 50% reductions in IGFBP-3 and ALS mRNA in the liver, the major source of serum IGFBP-3 and ALS. Taken together, these results suggest that feed deprivation reduces serum concentration of IGF-I in cattle not only by decreasing IGF-I gene expression in the liver, but also by increasing IGF-I degradation and reducing IGF-I retention in the blood through decreasing IGFBP-3 and ALS production in the liver.     

Biological activity of insulin-like growth factor-I purified from chicken serum

This study describes a rapid purification of insulin-like growth factor-I from chicken serum and the immunological, biological and receptor binding activity of the peptide. It was purified after initial extraction, by cation exchange chromatography, hydrophobic interaction chromatography and reverse phase chromatography up to 1.4 x 10(6)-fold with an overall yield ranging from 10-30%. The N-terminal amino acid sequence was the same as predicted from the nucleotide sequence of a chicken IGF-I cDNA and the partial sequence obtained from a previously reported purification. The material was both immunologically and biologically active. It had a 50% potency compared to human IGF-I in a radioimmunoassay using an antiserum raised against human IGF-I, stimulated the incorporation of [3H]-thymidine into DNA in cultured chick embryo myoblasts with a half-maximum effective dose of 5 ng/ml and displaced [125I]-labelled human IGF-I and IGF-II from binding sites in microsomal membranes prepared from both the chicken liver and the lactating rabbit mammary gland in a dose dependent manner.     

Insulin-like growth factor I in follicular development and function in postpartum beef cows

The objective of this study was to determine whether nutrition affects follicular growth and(or) steroid and insulin-like growth factor I (IGF-I) concentrations in follicular fluid. Beginning 6 d after calving, Hereford-cross cows (n = 28) were fed either 14 (ad libitum) or 7 (restricted) kg.animal1.d-1 of chopped alfalfa-brome hay. Half the cows in each treatment were ovariectomized on d 20 (OVX-20) and the remaining half on d 35 (OVX-35) postpartum. Cow weight and condition score were recorded weekly, and blood was collected thrice weekly for determination of insulin, IGF-I, glucose, and free fatty acid (FFA) concentrations. At ovariectomy, follicular fluid from each follicle greater than or equal to 4 mm in diameter was aspirated for determination of IGF-I, progesterone (P4), and estradiol-17 beta (E2) concentrations. Restricted cows lost more weight after calving than did ad libitum cows (P less than .0001), although all cows lost similar amounts of body condition (time postpartum, P = .008). Concentrations of FFA were elevated (P less than .0001) in restricted cows from wk 2 through 5 after calving but did not change with time in ad libitum cows. Plasma concentrations of glucose were lower in restricted than in ad libitum cows (59.6 +/- .4 vs 61.8 +/- .4 mg/dl; P = .05), but insulin and IGF-I were similar (P greater than .10) between dietary treatments.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of flax supplementation and a combined trenbolone acetate and estradiol implant on circulating insulin-like growth factor-I and muscle insulin-like growth factor-I messenger RNA levels in beef cattle

We evaluated effects of a 5% (dry matter basis) ground flaxseed supplement (flax) and a trenbolone acetate and estradiol-17beta implant, Revalor-S, on circulating IGF-I and muscle IGF-I messenger RNA (mRNA). Sixteen crossbred yearling steers (initial BW = 397 kg) were assigned randomly to one of four treatments: 1) flax/implant; 2) nonflax/implant; 3) flax/nonimplant; and 4) nonflax/nonimplant. Serum was harvested from blood collected on d 0 (before implant or flax addition), 14, and 28, and used in subsequent analyses of circulating IGF-I. Biopsy samples (0.5 g) were obtained from the longissimus muscle on d 0, 14, and 28. Total RNA was isolated from the muscle samples, and real-time quantitative-PCR was used to assess relative differences in IGF-I mRNA. Flax supplementation had no effect (P > 0.10) on circulating IGF-I concentrations. Following implantation, sera from implanted steers had 52 and 84% greater (P < 0.05) IGF-I concentrations than sera from nonimplanted steers on d 14 and 28, respectively. On d 28, local muscle IGF-I mRNA levels increased 2.4-fold (P < 0.01) in biopsy samples obtained from implanted compared with nonimplanted steers. Muscle biopsy samples from nonflax cattle had 4.4-fold higher (P < 0.01) levels of IGF-I mRNA than those from flax cattle on d 28. To determine whether a component of flax, alpha-linolenic acid (alphaLA), was directly responsible for IGF-I mRNA down-regulation, we incubated primary cultures of bovine satellite cells, from implanted and nonimplanted steers, in two concentrations of alphaLA (10 nM and 1 microM). An implant x dose interaction (P < 0.05) was observed for IGF-I mRNA concentrations in bovine satellite cells cultured for 72 h with alphaLA. Satellite cells from nonimplanted steers had similar (P > 0.10) IGF-I mRNA concentration regardless of the level of alphaLA exposure; however, satellite cells from implanted steers exposed to 10 nM and 1 microM alphaLA had 2.5- and 2.0-fold greater IGF-I mRNA levels, respectively, than cells from implanted steers that were not exposed to alphaLA (P < 0.05). Administration of a Revalor-S implant increased circulating IGF-I and local muscle IGF-I mRNA concentrations in finishing cattle. However, muscle IGF-I mRNA levels were decreased by flax supplementation. Muscle cell culture experiments suggested that alphaLA was not responsible for the IGF-I mRNA down-regulation.     

Temporal pattern of insulin-like growth factor-I response to exogenous bovine somatotropin in lactating cows

The effect of exogenous bovine somatotropin (bST) treatment on the temporal pattern of insulin-like growth factor-I (IGF-I) in serum of four multiparous Holstein cows was examined. Cows (190 +/- 24 days postpartum) were treated with daily subcutaneous injections of recombinant bST (40 mg) or excipient for 12-day periods in a crossover experimental design. During excipient treatment, concentrations of IGF-I in serum were relatively constant throughout the day and averaged 70 ng/ml. Following the first bST injection, serum IGF-I began increasing after a lag of 5 to 7 hr and progressively increased over the first 2 days of treatment. Serum IGF-I levels were approximately 2-fold greater than control values at the end of day 1 of bST treatment, with a 3-fold elevation observed at the end of day 2. Concentrations of IGF-I in serum plateaued by day 3 of bST treatment. Serum concentrations of IGF-I did not follow the oscillating pattern of bST in serum resulting from daily bST injections. Milk yield (3.5% fat-corrected) plateaued after 6 days of bST treatment and was increased 61% (+15.3 kg). Both IGF-I and milk yield remained essentially constant across days for the remainder of treatment. Following cessation of treatment, serum IGF-I and milk yield gradually declined, returning to control values after approximately 4 days. The temporal pattern of circulating concentrations of IGF-I is consistent with a role for IGF-I in mediating a portion of the effects of exogenous bST in lactating cows.     

Plasma insulin-like growth factor-I concentrations increase during the estrous phase in goats

The objective of this study was to characterize plasma insulin-like growth factor-I (IGF-I) profiles during the estrous cycle in goats. Frequent blood samples were drawn during the day of estrus and during the luteal phase on Day 10 after estrus, and plasma growth hormone (GH) and IGF-I profiles were examined. Then, daily blood samples were drawn throughout the estrous cycle or during induction of estrus by prostaglandin F(2alpha) (PGF(2alpha)) to further clarify the IGF-I profiles. GH was secreted in an episodic manner in the estrous and luteal phases in goats. There were no significant differences in the mean concentrations, pulse amplitude and pulse frequency of GH between the estrous and luteal phases. IGF-I concentrations during estrous phase were higher than those in the luteal phase (P<0.05). Plasma IGF-I increased approximately two days before behavioral estrus, and the IGF-I peak was observed in accordance with the appearance of estrus. The elevated IGF-I levels then declined to basal values 4 to 5 days after estrus. When estrus was induced by PGF(2alpha), plasma IGF-I concentrations increased after treatment, and the concentration 2 days after treatment (day of appearance of behavioral estrus) was significantly higher than concentrations before treatment (P<0.05). The elevated IGF-I levels then declined during the 3 days after treatment. These results indicate that plasma IGF-I concentrations increase during estrus in goats.     

Serum concentrations of IGF-I in postpartum beef cows

Four experiments assessed changes in serum IGF-I under various physiologic conditions in postpartum cows. In Exp. 1, anestrous suckled cows (n = 25) were infused for 6 d with either saline or glucose at two different infusion rates. In Exp. 2, anestrous cows (n = 29) received either a saline (weaned and suckled controls) or 3 g/d phlorizin (weaned phlorizin) infusion for 3 d. Calves from the weaned groups were removed from 15 h before and throughout infusions. In Exp. 3, cycling suckled cows (n = 20) received prostaglandin F2 alpha (PGF2 alpha) when the 5-d saline or phlorizin infusion began. In Exp. 4, suckled cows (n = 20) had ad libitum access to feed or received 50% of control feed consumption from 30 to 40 d postpartum. Increasing glucose availability (Exp. 1) increased (P less than .05) serum IGF-I by 30 to 35%. IGF-I remained stable after weaning (Exp. 2) in phlorizin-infused cows (128.8 +/- 12.7 ng/ml), but increased (P less than .05) by 3 d after calf removal in weaned control cows (152.2 +/- 7.5 ng/ml). IGF-I also remained stable in phlorizin-infused cows following PGF2 alpha injection (Exp. 3), but increased in control cows by 2 d after PGF2 alpha (156.8 +/- 18.3 on d 2 vs. 133.7 +/- 9.8 ng/ml pre-injection; P less than .05) and remained elevated (P less than .05) during the periovulatory period. In cows receiving restricted feed intake (Exp. 4), IGF-I decreased by approximately 50% within 4 d of feed restriction (71.3 +/- 9.4 vs 137.4 +/- 16.6 ng/ml; P less than .05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Ovulation in postpartum beef cows treated with estradiol

The effect of implants of estradiol on initiation of ovarian cycles postpartum was studied in 201 anestrous beef cows. Cows from four farms were used over a 2-yr period in a 2 x 3 factorial arrangement of treatments with estradiol implants and stage postpartum as main effects. Cows were assigned at random within date of calving within farm to receive an ear implant containing estradiol-17 beta (24 mg) for 21 d or to serve as controls. Stages postpartum at implantation were divided into less than or equal to 25, 26 to 39, and greater than or equal to 40 d, three stages that should reflect potential changes in hypothalmic-hypophysial sensitivity to estradiol. Blood samples for determination of progesterone were obtained and rectal examinations of the ovaries performed at implant insertion, 14 d after insertion, at implant removal (d 21), and 14 d after removal (d 35) to assess ovulatory response to treatment. Circulating concentrations of estradiol on d 14 of treatment averaged 3.2 +/- 1.0 and 23.1 +/- 4.7 pg/ml for control and estradiol-treated cows, respectively. Compared with control cows, treatment with estradiol initiated after d 26 postpartum increased the proportion of cows that ovulated during the experimental period. No differences were seen in the average days postpartum when cows were first determined to have ovulated.     

Opioid inhibition of luteinizing hormone secretion during the postpartum period in suckled beef cows

Recent studies have shown that naloxone (N), an opioid antagonist, increases concentrations of luteinizing hormone (LH) in the postpartum anestrous beef cow. However, the LH response to N was influenced by the postpartum interval. For example, a significant LH response to 200 mg of N occurred on d 42 but not on d 14 or 28 postpartum. The present study was conducted to determine the effect of different doses of N on LH secretion during the postpartum period of beef cows. Twelve cows were given 200, 400 or 800 mg of N on d 14, 28 and 42 postpartum in a Latin square design with repeat measures within cells. On d 14, serum concentrations of LH increased (P less than .01) from .5 +/- .1 ng/ml (mean +/- SE) before N to a peak of 2.0 +/- .5 and 1.4 +/- .5 ng/ml for cows given 400 and 800 mg of N, respectively. In contrast, 200 mg of N had no effect on serum concentrations of LH. On d 28 and 42 all three doses of N elevated (P less than .01) serum concentrations of LH. Therefore, a larger dose of N was required to increase serum concentrations of LH on d 14 postpartum compared with d 28 and 42. Based on these data we suggest that endogenous opioids participate in the regulation of LH secretion in the early postpartum period. The differential response to naloxone may be due to changes in endogenous opioid inhibition of LH secretion during the postpartum period.