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The growth endocrine axis influences reproduction. The objectives of this study were to evaluate population genetic characteristics of SNP genotypes within genes of the GH-IGF axis in straightbred and crossbred Angus, Brahman, and Romosinuano heifers (n = 650) and to test the association of these genotypes with measures of reproduction. These objectives were achieved using 73 SNP within 7 genes on chromosome 5 and the pregnancy-associated plasma protein A2 (PAPP-A2) and GH-receptor genes, which map to chromosomes 16 and 20, respectively. The SNP were elucidated by resequencing conserved regions of each gene by using DNA from familial-unrelated cattle of a multibreed discovery population. A multiplex SNP assay yielded 59 biallelic SNP useful for evaluating genetic identity and distance. Specifically, the divergence of straightbred Brahman cattle was approximately 15.5% from 5 Bos taurus-influenced breed groups. In the straightbred groups used as a validation population, only 3 SNP had minor allele frequencies >10%. These SNP were in the genes PAPP-A2 (ss115492449-A/C and ss115492450-G/T within intron 10) and signal transducers and activators of transcription 2 (STAT2; ss252841035-A/G within the 5' untranslated region), and they met the conditions of Hardy-Weinberg equilibrium (P > 0.31). The other 56 SNP were useful for assigning each animal into ancestral clusters (n = 3 proportions) to account for population stratification in genotype to phenotype association analyses. The 2 SNP in the PAPP-A2 gene influenced (P < 0.05) traits indicative of first-calf heifer rebreeding (i.e., calving interval, days to calving, and pregnancy rate). A STAT2 SNP genotype (i.e., GG) × primary ancestral cluster interaction (P < 0.05) suggested heifers primarily of B. taurus ancestry had a reduction of approximately 16.4 ± 0.1% in calving interval and days to calving relative to heifers clustering primarily as Bos indicus ancestry. Even though additional research is needed to delineate the allelic variation attributed to genes of the GH-IGF axis, results of this study provide support for STAT2 and PAPP-A2 as potential candidate genes associated with first-calf heifer rebreeding traits.

Objectives were to (1) characterize the relationship of third-ventricle (IIIV) cerebrospinal fluid (CSF) concentrations of growth hormone-releasing hormone (GHRH) with concentrations of GH in the peripheral circulation; and (2) assess the influence of acute administration of appetite-regulating peptides leptin (anti-orexigenic) and neuropeptide Y (NPY; orexigenic) on the release of GHRH. Six mature beef cows fitted with IIIV and jugular vein cannulae were treated intracerebroventricularly with saline, and leptin (600 microg) and NPY (500 microg) in saline, in a replicated 3x3 Latin square design. Third-ventricle CSF and blood were collected 10 min before and continued 220 min after treatments. Mean concentrations of GHRH and frequency of pulses after treatments were 2.2+/-0.13 ng/mL and 1.2+/-0.15 pulses/220 min, respectively. These measures were not influenced by treatments. Concentrations of GHRH in CSF were weakly correlated (r=0.15; P<0.03) with serum concentrations of GH; however, 58% of the GH pulses were preceded by a pulse of GHRH and 90% of the GHRH pulses occurred within 20 min preceding a pulse of GH. Leptin tended (P<0.10) to suppress GH area under the curve (AUC) compared to saline. Concomitantly, NPY tended (P<0.10) to increase GH AUC, which appeared to be a consequence of increased (P<0.05) pulse amplitude. Infusion of NPY also increased (P<0.05) AUC of GHRH relative to saline. No differences were detected among treatments in serum concentrations of insulin-like growth factor-I or its AUC. Sampling CSF from the IIIV appears to be a viable procedure for assessing hypothalamic release of GHRH coincident with anterior pituitary gland secretion of GH in cattle. These data also demonstrate the differential responsiveness of the GH axis to appetite-regulating peptides.

Sequence polymorphisms in the growth hormone (GH) gene and its transcriptional regulators, Pit-1 and Prop-1, were evaluated for associations with growth and carcass traits in two populations of Brangus bulls Chihuahuan Desert Rangeland Research Center (CDRRC, N = 248 from 14 sires) and a cooperating breeding program (COOP, N = 186 from 34 sires). Polymorphisms were SNP mutations in intron 4 (C/T) and exon V (C/G) in GH, A/G in exon VI in Pit-1, and A/G in exon III in Prop-1. In the COOP population, bulls of Pit-1 GG genotype had a significantly greater percentage of intramuscular fat than bulls of the AA or AG genotype, and bulls of the Prop-1 AA genotype had significantly greater scrotal circumference than bulls of AG or GG genotypes at ~365 days of age. Also, heterozygous genotypes for the two GH polymorphisms appeared advantageous for traits of muscularity and adiposity in the COOP population. The heterozygous genotype of GH intron 4 SNP was associated with advantages in weight gain, scrotal circumference, and fat thickness in the CDRRC population. The two GH polymorphisms accounted for >/=27.7% of the variation in these traits in the CDRRC population; however, R(2) was <5% in the COOP population. Based on haplotype analyses the two GH SNPs appeared to be in phase; the haplotype analyses also paralleled with the genotype analyses. Polymorphisms in GH and its transcriptional regulators appear to be predictors of growth and carcass traits in Brangus bulls, particularly those with heterozygous GH genotypes.

Associations of DNA polymorphisms in growth hormone (GH) relative to growth and carcass characteristics in growing Brahman steers (N = 324 from 68 sires) were evaluated. Polymorphisms were an Msp-I RFLP and a leucine/valine SNP in the GH gene as well as a Hinf-I RFLP and a histidine/arginine SNP in transcriptional regulators of the GH gene, Pit-1 and Prop-1. Genotypic frequencies of the GH SNP, Pit-1 RFLP, and Prop-1 SNP were greater than 88% for one of the bi-allelic homozygous genotypes. Genotypic frequencies for the GH Msp-I RFLP genotypes were more evenly distributed with frequencies of 0.43, 0.42, and 0.15 for the genotypes of +/+, +/-, and -/-, respectively. Mixed model analyses of growth and carcass traits with genotype and contemporary group serving as fixed effects and sire fitted as a random effect suggested that sire was a significant source of variation (P < 0.05) in average daily gain, carcass yield, and marbling score. However, measures of growth and carcass traits were similar across GH Msp-I genotypes as steers were slaughtered when fat thickness was estimated to be approximately 1.0 cm. These polymorphisms within the GH gene and/or its transcriptional regulators do not appear to be informative predictors of growth and carcass characteristics in Brahman steers. This is partly due to the high level of homozygosity of genotypes. These findings do not eliminate the potential importance of these polymorphisms as predictors of growth and carcass traits in Bos taurus or Bos taurus x Bos indicus composite cattle.

Age-related changes in body composition, leptin, and hypothalamic-pituitary expression of the leptin receptor and associative relationships of these factors to constituents of the growth hormone (GH) axis were evaluated. Seventy wethers were randomly assigned at birth to one of four treatment groups: control; treatment 1 implanted with the estrogenic compound zeranol (12 mg, Ralgro on days 0, 45, and 90; treatment 2 received zeranol on days 45 and 90; and treatment 3 received zeranol on day 90. Serum and tissues were collected from wethers (n > or = 5) from each group on days 28, 73, 118, and 135. Percent body fat and leptin increased linearly (P < 0.01) with age, but were not influenced (P > or = 0.14) by zeranol. The leptin receptor in the pituitary appeared to be differentially (P = 0.097) expressed across days 73-135, but no differences (P > or = 0.43) were detected in expression of this receptor in the hypothalamus among treatments and ages. Leptin and % body fat were negatively correlated (r > or = -0.52, P < 0.05) to mRNA levels of factors involved in pituitary synthesis and secretion of GH. Serum leptin increased with age as did percent body fat, but zeranol did not influence body composition, serum leptin, or expression of the leptin receptor in the hypothalamus or pituitary; however, the leptin receptor appeared to be differentially expressed among the hypothalamus and pituitary with level of body fat and leptin being inversely associated to transcriptional-factors involved in somatotrope synthesis and secretion of GH., (Copyright 2002 Elsevier Science Inc.)

Neuropeptide Y (NPY) provides an important hypothalamic link between nutritional status and neuroendocrine mechanisms regulating growth and reproduction. The objective of the following series of experiments was to determine the effects of single or continuous administration of NPY on secretion of luteinizing hormone (LH) and (or) growth hormone (GH). In experiment 1, four ovariectomized (OVX) ewes and four OVX + estrogen-treated ewes each received, in a 4 x 4 Latin Square arrangement of treatments, a single injection of 0, 0.5, 5, or 50 microg NPY via an intracerebroventricular (i.c.v.) cannulae to determine the effects on secretion of GH. NPY significantly elevated serum GH at the 50 microg dose regardless of estrogen exposure (P = 0.003). In experiment 2, eight OVX ewes were infused i.c.v. with NPY or saline (n = 4/trmt) continuously for 20 h in a linearly increasing dose, ending at 50 microg/h NPY. Blood samples were collected via jugular cannulae every 10 min during hour -4-0 (interval 1, pre-treatment), hour 6-10 (interval 2) and hour 16-20 (interval 3) relative to the initiation of infusion (0 h). Mean LH and LH pulse frequency were lower in NPY- versus saline-infused ewes during intervals 2 and 3 (P < 0.01), but NPY had no discernable effect on serum GH (P > 0.10). In experiment 3, four OVX ewes were continuously infused with NPY as in experiment 2, except that the maximum 50 microg/h dose was achieved after only 10 h of infusion. Blood samples were collected every 10 min, beginning 4 h before and continuing until 4h after the NPY infusion. Mean serum LH changed significantly over time (P = 0.0001), decreasing below pre-treatment levels by hour 3 of NPY infusion (P < 0.01), and returning to pre-treatment concentrations following the end of infusion (P > 0.15). Serum GH also changed significantly over time (P < 0.001). Mean GH levels tended to be greater than pre-treatment levels by hour 2 of infusion (P < 0.08), but thereafter returned to basal levels. Serum GH also increased following the end of NPY infusion (P < 0.03). From these data we conclude that NPY exerts a persistent inhibitory effect on secretion of LH, and may stimulate the secretion of GH during the initiation and cessation of infusion of NPY. These observations support a role for NPY in mediating the effects of undernutrition on both LH and GH, and also provide evidence for potential mechanisms by which leptin, acting through NPY, may stimulate the secretion of GH., (Copyright 2002 Elsevier Science Inc.)

Understanding mechanisms that regulate growth and reproduction are important for improving selection strategies in cattle. In this study, Angus, Brangus, and Brahman bulls (n = 7 per breed) of similar age were selected from a group of 65 weanlings. Bulls were evaluated after weaning (i.e., approximately 6 mo of age) for 112 d for serum concentrations of metabolic hormones and glucose, growth, and reproductive traits. Performance data and blood sera were collected on d 0, 28, 56, 84, and 112. Sera were also collected in periods from d 50 to 59 (56D) and 103 to 112 (112D). Angus bulls were heavier (P < 0.05) throughout the study than Brahman bulls and were heavier than Brangus bulls on d 56, 84, and 112. Initial and final BW for Angus, Brangus, and Brahman bulls were 292.7, 260.6, and 230.4 and 468.3, 435.6, and 350.7 +/- 12 kg, respectively. Conversely, Brahman bulls had greater hip height (P < 0.05) than Brangus, and Brangus were taller (P < 0.05) than Angus. Angus bulls had the greatest (P < 0.05) scrotal circumference (SC) and Brahman bulls the least. Mean SC across days was 31.5, 29.7, and 25.0 +/- 0.6 cm for the three respective breeds. Serum testosterone was greater (P < 0.01) in Angus and Brangus bulls (10.0 and 8.9 +/- 1.4 ng/mL) than in Brahman bulls (4.0 +/- 1.4 ng/mL) throughout the study. After d 112, 100, 86, and 57% of the Angus, Brangus, and Brahman bulls passed a breeding soundness exam (P = 0.51). Serum concentrations of IGF-I and leptin were greater (P < or = 0.06) in Angus bulls on d 56, 84, and 112 than in Brangus and Brahman bulls. Serum concentrations of GH (P < 0.08) and glucose (P < 0.03) were greater in Brangus bulls than in Angus or Brahman bulls throughout the study. Prediction analyses suggested that serum concentrations of leptin could be used to predict (P < or = 0.08) BW and SC (R2 > 0.82) in the 56D and 112D periods among these breeds. Leptin was also useful in predicting (P < or = 0.09) serum concentrations of GH and testosterone in the 112D period (R2 > 0.32). Residual correlation analyses with the effect of breed removed suggested that leptin was correlated (r > or => 0.53, P < 0.05) with both SC and serum testosterone. Angus and Brahman cattle differ in phenotype, level of adiposity, and rate of sexual development. Data herein suggest that these characteristics could be due to varying mechanisms by which metabolic hormones such as leptin, GH, and(or) IGF-I are regulated.

Two studies evaluated hormonal markers as indicators of the onset of puberty in Debouillet sheep selected for twinning. In Trial 1, 29 ewe lambs (50 +/- 0.5 kg, 159 to 187 d of age) were given 10 microg GnRH (i.v.) on September 15 and blood was collected at 30 min intervals after the injection for 2 h. Additional samples were taken twice weekly and progesterone (P4) was measured. The day that serum P4 was greater than 1 ng/mL for 2 consecutive sampling days was classified as the day of puberty. Average day of puberty was October 12 (average age at puberty was 199 d) and ewes with values less or greater than the average were classified as early or late, respectively. Average weight at GnRH challenge was 50 kg and ewes weighing less or more were classified as light or heavy, respectively. Early ewes weighed more (P = 0.01) and reached puberty sooner (P = 0.01) than late ewes. Heavy lambs reached puberty earlier, weighed more at GnRH challenge, and had more LH area under the curve (AUC, P < 0.05) than light ewes. In Trial 2, we gave 27 ewe lambs (54 +/- 0.9 kg, 173 to 189 d of age) a single i.v. injection of 10 microg GnRH and 10 microg GHRH on September 17. Average day of puberty was October 13, average weight was 54 kg, and average age at puberty was 208 d. Categories were designated as described for Trial 1. Early lambs reached puberty sooner (P = 0.01) and weighed more (P = 0.01) than late lambs, but the puberty groups had similar LH AUC (P = 0.64) and GH AUC (P = 0.75), whereas IGF-I was greater (P = 0.01) in early puberty ewes than in late puberty ewes. Heavy lambs reached puberty earlier (P = 0.06), weighed more (P = 0.01), and tended (P = 0.11) to have more GH AUC than light ewes. No difference was observed in LH AUC or IGF-I between weight groups (P > 0.15). Results suggest that serum LH after GnRH is not a reliable indicator of the onset of puberty in ewe lambs selected for twinning, but heavier ewes tended to produce more GH after a GHRH challenge and reach puberty earlier than lighter ewe lambs.

This experiment evaluated relationships between pituitary messenger RNA levels of the transcription factor Pit-1, the growth hormone releasing-hormone receptor (GHRHR), and synthesis and secretion of GH in growing wethers. The experiment also evaluated the influence of the estrogenic compound, zeranol, on these relationships. Seventy wethers that were 9.5 +/- 1 day of age were randomly assigned to a control group or to one of three zeranol treatment groups that were implanted (12 mg, Ralgro) at 0, 45, and (or) 90 days of age. Twenty-eight days after implantation (i.e., Days 28, 73, 118) and on Day 135, sera were collected serially from wethers (n > or = 5) from each group and then their pituitary was collected. As wethers gained weight with age, the pituitary increased in size and so did the relative message levels of Pit-1 and GH (effect of time, P < 0.01). However, as wethers reached 135 days of age, serum concentrations of GH had declined while concentrations of IGF-I had increased (linear contrast, P < 0.01). Additionally, zeranol increased serum concentrations of GH and IGF-I and this effect on GH appeared to be a consequence of increased pulse amplitude, particularly at 73 and 118 days of age (treatment x time, P

Hypothalamic neurons that control the luteinizing hormone (LH) and growth hormone (GH) axes are localized in regions that also express neuropeptide Y (NPY). Increased hypothalamic expression of NPY due to diet restriction has been associated with suppressed secretion of LH and enhanced secretion of GH in numerous species. However, these physiological relationships have not been described in cattle. Thus, two studies were conducted to characterize these relationships using ovariectomized (Experiment 1) or ovariectomized estrogen-implanted (Experiment 2) cows. In Experiment 1, four well-nourished, ovariectomized cows received third cerebroventricular (TCV) injections of 50 and 500 micrograms of NPY in a split-plot design. Venous blood was collected at 10-min intervals from -4 hr (pre-injection control period) to +4 hr (postinjection treatment period) relative to TCV injection. NPY suppressed (P < or = 0.04) tonic secretion of LH irrespective of dose and tended to stimulate (P < or = 0.10) an increase in tonic secretion of GH. In Experiment 2, six ovariectomized cows that were well nourished and implanted with estradiol received TCV injections of 0, 50, or 500 micrograms of NPY in a replicated 3 x 3 Latin Square. Both doses of NPY suppressed (P < 0.06) mean concentration of LH relative to the 0-microgram dose. The 50-microgram dose of NPY tended (P < 0.10) to increase the amplitude of GH pulses. In conclusion, TCV injection of NPY suppressed pituitary secretion of LH and simultaneously tended to increase pituitary secretion of GH.

Twenty-five nonlactating ewes were used to test the hypothesis that methscopolamine bromide (MB) blocks secretion of growth hormone (GH) by affecting hypothalamic rather than pituitary mechanisms. Ewes were randomly assigned to receive a s.c. injection of 96 mg of MB or 2 mL of saline at min = 0. Saline-treated ewes were assigned to receive a subsequent (at +60 min) i.v. injection of 10 microg of growth hormone-releasing hormone (GHRH) to test pituitary responsiveness or .3 mg of clonidine to test hypothalamic responsiveness. Methscopolamine bromide-treated ewes were assigned to receive a subsequent (at +60 min) i.v. injection of 10 microg of GHRH, .3 mg of clonidine, or 2 mL of saline. Jugular blood samples were collected at 10-min intervals from -120 min to +240 min, and serum concentrations of GH were quantified with a RIA. No difference was detected in serum concentrations of GH with respect to MB vs saline treatment (P = .20). Concentrations of GH increased in saline-pretreated ewes following injections of clonidine or GHRH (P < .01). Treatment of ewes with MB, however, limited the ability of clonidine-induced mechanisms to increase concentrations of GH, but did not affect pituitary responsiveness to GHRH (P < .01). These data support the hypothesis that MB inhibits hypothalamic and not pituitary mediated mechanisms that regulate the secretion of GH.