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Department of Animal Science, Cornell University, Ithaca, New York 14853-4801
8 Correspondence:
phone: 607-255-5497; fax: 607-255-9829; E-mail:
awb6{at}cornell.edu..
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionImplicationsLiterature Cited |
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Key Words: Growth • Hormones • Lambs • Metabolism • Nutrition
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Introduction |
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Metabolic adaptations are necessitated by major changes in the quantity and composition of nutrients supplied to the perinatal animal. In well-fed neonates, the total nutrient supply greatly exceeds that of the late-gestation fetus, in which nutrient supply and growth are increasingly constrained by the placenta (Mellor and Murray, 1982). Also, the composition of nutrients changes from primarily glucose and amino acids to a diet containing high fat and less carbohydrate (Girard et al., 1997). In precocial mammals, birth is associated with maturation of the somatotropic/IGF axis, which has an important role in regulating anabolism and catabolism (Gluckman et al., 1999). This axis is influenced by nutrition in later life (McGuire et al., 1992; Bauman et al., 1995). However, adaptations to postnatal life and longer-term consequences of prenatal growth retardation in low-birth-weight lambs, and effects of rearing at different growth rates during early postnatal life on endocrine and metabolic development, remain to be elucidated.
The objective of this study, therefore, was to investigate the hypotheses that prenatal and postnatal nutrition affect regulation of energy metabolism in neonatal lambs using our previously described fetal growth retardation model (Greenwood et al., 1998; 1999; 2000b).
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Materials and Methods |
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Blood Sampling Procedures.
Blood samples (5 mL) were obtained by jugular venepuncture (0.9 x 25 mm needles) into heparinized 6-mL syringes. Baseline lambs were sampled within 2 h of birth, and all reared lambs were sampled prior to provision of feed at 0830 (see Greenwood et al., 1998) every second day thereafter. Samples were immediately transferred to glass test tubes placed on ice, then centrifuged at 4°C and 2,500 x g for 15 min. Two aliquots of plasma were transferred into Eppendorf tubes and stored at -20°C.
Weekly pooled plasma samples for each lamb were prepared from 300-µL aliquots of individual samples when first thawed. The pools alternately comprised aliquots from d 1, 3, 5 and 7, and d 2, 4, and 6 of each week until slaughter, which provided a representative plasma sample for the mean age and LW for each lamb in each week. Weekly pools were prepared immediately prior to measurement of glucose and ST, and the remaining pooled plasma was stored frozen prior to being thawed for assay of urea nitrogen, insulin, and IGF-I.
Blood samples were also obtained from 36 reared lambs every 15 min for 6 h immediately following morning feeding (0830) on the day prior to slaughter at various LW from approximately 5 to 20 kg. The lambs comprised two BWE-L and two BWE-H lambs (two BWE-L and BWE-H lambs were not sampled), and all LL, LH, HL, and HH lambs (LL and LH, n = 10/group; HL and HH, n = 8/group). An in-dwelling jugular catheter (polyvinyl chloride, 0.80 mm i.d./1.20 mm o.d. or 0.86 mm i.d./1.27 mm o.d.; Dural Plastics and Engineering, Silverwater, NSW, Australia) was inserted on the day prior to sampling and filled with heparinized saline (100 U/mL). Small lambs were restrained with minimal stress during sampling, and larger lambs were allowed to remain in the pen while samples were drawn from the catheter through the top of the pen. At each sampling, at least 2 mL of blood was drawn and placed into a glass test tube on ice containing 1 drop of heparin (10,000 U/mL). Immediately prior to each sampling, heparinized 0.9% saline (20 U heparin/mL), injected into the catheter following the previous sampling, was removed. Following each sampling, catheters were flushed with 0.3 mL of 0.9% saline and 0.3 mL of heparinized 0.9% saline, equivalent to the void volume (approximately 0.3 mL) of the catheter. Chilled blood samples were centrifuged at 2,500 x g and 4°C for 15 min. Plasma was stored at -20°C in two aliquots.
Blood Analyses.
Analyses for metabolite and hormone concentrations were undertaken on samples from birth to 2 wk of age (Figure 1
) for the baseline, BWE-L, and BWE-H lambs, and on samples for the HL and HH lambs reared to 15 and 20 kg LW (n = 4/group). For glucose, plasma urea nitrogen (PUN), insulin, and ST, laboratory analyses were also performed on pooled weekly plasma samples from all animals until slaughter (LL and LH, n = 12/group including BWE-H and BWE-L lambs; HL and HH, n = 8/group; Figures 2 and 3). Analyses for IGF-I concentration were undertaken on all pooled weekly plasma samples for the LL, LH, HL, and HH animals (n = 4/group) reared to 15 and 20 kg LW (Figures 2 and 3). Statistical analyses of the pooled weekly plasma samples were performed from 2 wk of age, when postpartum concentrations had become more stable, to completion of rearing at approximately 20 kg LW (see Statistical Analyses subsection of Materials and Methods). All samples from all animals that underwent frequent sampling the day prior to slaughter (n = 36) were assayed for ST concentration.
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Insulin.
Concentration of insulin in plasma was determined by double antibody immunoprecipitation RIA using methods and components of a porcine insulin RIA kit (Linco Research, St. Charles, MO), except that bovine insulin (Ely Lilly and Co., Indianapolis, IN; Lot # 615-70N-80) was used for preparation of standards and radiolabeled insulin. The Iodogen method (Pierce, Rockford, IL), with 5 µg insulin, 1 mCi 125I, 20-s reaction time, and Sephadex G-50 medium (Sigma, St. Louis, MO) column, was used to iodinate and purify the hormone. Incorporation of radioactivity into insulin was 102 µCi/µg (samples to 2 wk of age) and 45 µCi/µg (pooled weekly samples). The primary antibody was guinea pig antiporcine insulin (Linco Research). Bound antibody–antigen complexes were precipitated using normal guinea pig IgG carrier and goat anti-guinea pig IgG (Linco Research). Plasma (100 µL in duplicate), and insulin standards (100 µL in triplicate) in the range 0.1 to 20 ng/mL, were assayed. The within-assay CV was 6.3%, and between-assays 6.8%.
Urea Nitrogen.
Plasma urea nitrogen concentration was measured by the urease/Berthelot colorimetric procedure (Fawcett and Scott, 1960; Chaney and Marback, 1962). Plasma and standards (20 µL) in the range of 0 to 21.3 mM (0 to 60 mg/dL) were assayed (all reagents from Sigma Diagnostics) and absorbance was read at 570 nm. The within-assay CV was 2.5% and the between-assay CV 4.5%
Somatotropin.
Concentration of ovine ST in plasma was determined by a specific double-antibody immunoprecipitation RIA developed from the procedures described by Gorewit (1981). The primary antibody was rabbit anti-ovine ST (NIADDK-NIH, Bethesda, MD) and the secondary antibody was sheep anti-rabbit gamma globulin. Preparation and purification of [125I]ST was performed using the Iodogen method (Pierce, Rockford, IL) as described by Cohick et al. (1989), using 5 µg ST and 1 mCi 125I. Incorporation of radioactivity into ST (bovine ST; The Upjohn Co., Kalamazoo, MI; Lot # 12, Code # 77-001) was 89 µCi/µg (window bleeds) and 93 and 123 µCi/µg (samples to 2 wk of age and pooled weekly samples). Plasma (100 µL) and standards (100 µL, in triplicate) prepared from bovine ST in the range of 0 to 100 ng/mL were assayed. Diluent (400 µL; 1% BSA in pH 7.1 PBS) was added to samples and standards, followed by 200 µL of primary antibody and 100 µL radiolabeled ST, and the tubes were incubated at 4°C for 48 h. Secondary antibody (200 µL) was then added and the tubes were incubated at 4°C for a further 48 h. Following incubation, 2 mL of cold PBS was added to the tubes, which were then centrifuged for 30 min at 1,000 x g, the supernate removed, and the radioactivity of the precipitate determined. Samples to 2 wk of age and weekly pooled plasma samples were assayed in duplicate, and one sample from each 15 min collection on the day prior to slaughter was assayed. The within-assay CV was 8.1%, and the between-assay CV was 7.4%.
Insulin-Like Growth Factor-I.
Concentration of IGF-I in plasma was determined by separation of IGF-I from its binding proteins and a double-antibody immunoprecipitation RIA using the procedures of Beermann et al. (1991). Separation of IGF-I from its binding proteins was achieved by acidification of 300 µL of plasma in 360 µL of 0.1 M glycyl-glycine HCl (pH 2.0) and incubation at 38°C for 48 h. Acid gel permeation chromatography of 300 µL of the acidified plasma was then undertaken to elute the IGF binding proteins separately from the free IGF-I. The column comprised cross-linked dextran (G-100 Sephadex, Sigma) suspended in 1 M acetic acid. Coefficient of variation among columns, determined by extraction, elution, and assay of free IGF-I from pooled plasma, was 5.6%. The RIA was performed on the neutralized column fraction containing free IGF-I. The primary antibody used in the assay was a polyclonal rabbit antihuman IGF-I (NIADDK-NIH, Baltimore, MD). Preparation and purification of [125I]bIGF-I was performed using the Iodogen method (Pierce). Bovine IGF-I (5 µg; Monsanto Co., St. Louis, MO; Lot # GTS-2), 0.5 mCi 125I, a 20-s reaction time, and Sephadex G-50 medium (Sigma) column were used to iodinate and purify the hormone (see Cohick et al., 1989). Incorporation of radioactivity into IGF-I was 25 and 32 µCi/µg (pooled weekly samples and samples to 2 wk of age). Duplicate plasma eluate (100 µL) and triplicate bIGF-I standards (100 µL; range 0 to 6 ng/100 µL) were assayed. The within-assay CV was 5.0% and the between-assay CV was 9.2%.
Statistical Analyses.
At Birth.
Comparisons between birth-weight categories were made using Student’s t-test (one-tailed).
From Birth to 2 wk of Age.
Comparisons between birth-weight categories and nutritional treatments from birth to 2 weeks of age were performed for subsets of lambs (n = 4/treatment group; see Blood Analyses subsection of Materials and Methods) using data from samples obtained every second day during the first 2 wk of postnatal life.
Statistical analyses of these data sets were performed using repeated measures analyses within a linear mixed-effects model due to inherent random effects emanating from repeated measures upon individuals. The initial model shown below was fitted using the ASREML algorithm and software (Gilmour et al., 1999; Verbyla et al. 1999):
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where terms in italics are random effects and: variable = plasma glucose (mM), urea nitrogen (mM), insulin (ng/mL), ST (ng/mL), or IGF-I (ng/mL); birth = birth weight category (low = 1, high = 2); nutrition = postnatal nutritional category (low = 1, high = 2); and age = d.
The final models are presented in the Results section. Significance of the fixed and random effects was established using the F statistic, following determination of the random effects. The significance of random effects was tested by fitting the model in the absence of each random effect. As the same fixed effects were involved in both models, a likelihood ratio test was used to compare their log-likelihood. The likelihood ratio statistic used was -2 x 
log-likelihood and was compared to a X2
df. Where the variate under analysis was not normally distributed, it was transformed to one with a normal distribution using a logarithmic (loge) transformation.
From 2 Weeks to 20 Kilograms Live Weight.
Data from pooled weekly samples from wk 3 (mean age = 2.5 wk) to the completion of rearing at approximately 20 kg LW were analyzed for all lambs reared beyond 2 wk of age (glucose, insulin, PUN, and ST, n = 8/treatment group) or for a subset of lambs (IGF-I, n = 4/treatment group) within this age and weight range (see Blood Analyses subsection of Materials and Methods). Statistical analyses of these data sets were also performed using a linear mixed-effects model due to inherent random effects emanating from repeated measures upon individuals and the unbalance in the design. The initial model shown below was also fitted using the ASREML algorithm and software:
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where terms in italics are random effects, and variable = plasma glucose (mM), urea nitrogen (mM), insulin (ng/mL), ST (ng/mL), or IGF-I (ng/mL); birth = birth weight category (low = 1, high = 2); nutrition = postnatal nutritional category (low = 1, high = 2); LW = kg; age = d; stage = stage of postnatal development: early (17.5 d to 45.5 d), mid (52.5 d to 80.5 d) and late (87.5 to 115.5 d); relative feed intake = g/kg LW•d; and fractional growth rate = %LW gained/d.
Significance of the fixed and random effects was established as described above. Final models are presented in the Results section.
Serial Samples to Determine Somatotropin Profile.
For data obtained during frequent sampling on the day prior to slaughter, ST profile characteristics were determined using an algorithm for the study of episodic hormone secretion (Merriam and Wachter, 1982) modified to function within PC Pulsar for the IBM-PC (Gitzen and Ramirez, 1986). Peaks were identified as individual subseries elevated above a baseline, determined for each animal using a robust smoothing technique, to account for longer-term trends. The G(n) parameters, where G = cut-off criteria based on peak width n, and n = peak duration in multiples of sampling points, were set at 1% error rate, i.e., G(1) = 4.40; G(2) = 2.60; G(3) = 1.92; G(4) = 1.46; and G(5) = 1.13. The cut-off points above baseline for peaks of various widths were established by multiplying G(n) by the assay SD (Y), determined as a function of ST concentration (X). The SD, calculated from 20 pairs of plasma duplicates spanning the assay standard curve, was Y = 0.126 + 0.043(X). The peak splitting cut-off and weight of assigned peaks used to resolve clustered peaks that may have initially been perceived by the program as a trend were 2.7 x SD and 0.5, respectively.
For the results of the ST profile analyses, comparisons between birth-weight categories and nutritional treatments from birth to slaughter were performed using ANCOVA on independent data for each lamb, with age and EBW (linear and, where significant, quadratic) as covariates.
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Results |
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Plasma Metabolite and Hormone Concentrations During the First 2 Weeks of Postnatal Life
Glucose.
Plasma glucose concentrations (Figure 1
and Table 1) increased rapidly during the first week of life. Rapidly reared lambs had higher plasma glucose concentrations than slowly reared lambs during this period, and glucose concentration increased with age.
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and Table 1). Well-nourished lambs had greater plasma insulin concentrations and there was a significant interaction between nutrition category and age.
Urea Nitrogen.
Plasma urea nitrogen increased rapidly from birth, peaking at d 1 in low-birth-weight lambs and at d 3 in high-birth-weight lambs, after which a rapid decline occurred between 3 and 5 d of age (Figure 1). Overall, during the first 2 wk of life, PUN concentration was higher in the well-nourished lambs and there was an interaction between age and nutrition category (Table 1).
Somatotropin.
Somatotropin concentration was higher overall in the low- compared to high-birth-weight lambs during the first 2 wk of postnatal life and declined with age (Figure 1). Significant effects of age, nutrition and birth weight, and age.nutrition, age.birth weight, and birth weight.nutrition interactions were also apparent (Table 1).
Insulin-Like Growth Factor I.
Plasma IGF-I concentration for the ad libitum fed lambs was maximal at d 9 to d 11 in the high-birth-weight group, and d 11 to d 13 in the low-birth-weight group (Figure 1). Overall, the high-birth-weight and rapidly reared lambs had higher IGF-I levels than their respective counterparts during the first 2 wk of life (Table 1). There were significant effects of age, birth weight and nutrition, and birth weight.nutrition, birth weight.age, nutrition.age, and birth weight.nutrition.age interactions.
Plasma Metabolite and Hormone Concentrations from 2 Weeks of Age to 20 Kilograms Live Weight
Glucose.
Glucose concentrations were significantly higher in the rapidly reared lambs than in the slowly reared lambs, and declined with increasing LW (Figures 2 and 3 and Table 2).
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and 3 and Table 2). Plasma insulin increased with increasing LW during this period and there was a significant positive association with relative feed intake.
Urea Nitrogen.
Rapidly reared lambs had higher PUN than the slowly reared lambs (Figures 2 and 3 and Table 2). There was also an increase in PUN with increasing LW, and a birth weight.nutrition category interaction.
Somatotropin.
Concentration of ST in the pooled weekly plasma samples declined with increasing LW (Figures 2 and 3 and Table 2).
Data for the serial samplings on the day prior to slaughter, obtained from about 5 kg LW onwards, showed that birth weight and postnatal nutrition did not influence mean ST concentration, ST pulse amplitude, ST interpeak interval, length of ST peaks, or number of peaks during the sampling period. There was a linear decline in mean ST concentration (from 8.23 ng/mL at 5 kg to 4.25 ng/mL at 17.5 kg EBW) and ST pulse amplitude (from 10.6 to 4.58 ng/mL) with increasing body weight (Table 3). Somatotropin interpeak interval increased in a linear manner with increasing age (from 107 min at 7 d to 209 min at 120 d). Length of ST peaks (mean ± SD, 37.4 ± 12.1 min) and the number of peaks during the sampling period (3.2 ± 1.1 peaks) did not differ due to age or body weight.
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and 3 and Table 2) due to a positive interaction between nutrition category and LW. |
Discussion |
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Transition from fetal to postnatal life is characterized by a decline in ST and an increase in IGF-I concentrations in plasma (Gluckman et al., 1999). Therefore, the higher ST and the lower IGF-I levels observed at birth in the low-birth-weight lambs suggest that the ST/IGF axis is less mature at birth in these animals. This notion is reinforced by the continuation of higher plasma ST and lower IGF-I in the low-birth-weight lambs during the first week or so of postnatal life (present study), and the reduced hepatic expression of the acid-labile subunit (ALS) of the circulating IGF system in low-birth-weight neonates (Rhoads et al., 2000).
A moderate inverse relationship between birth weight and plasma ST has been previously described for twin lambs, although a significant correlation between these parameters was not evident for triplet lambs (Peeters et al., 1991). Plasma ST concentrations as high as 85 ng/mL occurred in the severely growth-retarded newborns in the present study, suggesting a delay in the normal decline in ST concentration associated with birth in the small newborns.
High PUN concentrations at birth are assumed to indicate high rates of amino acid catabolism, particularly in the low-birth-weight newborns. This role for amino acids is likely to extend to their use as quantitatively important substrates for the high rate of gluconeogenesis (see Kalhan and Parimi, 2000) that occurs soon after birth in lambs (Warnes et al., 1977; Townsend et al., 1989) to support the rapid postnatal rise in plasma glucose concentration. As a consequence, glucose pool size, total entry rate, and concentration in plasma are higher during the early postnatal period than in later postnatal life, as are rates of irreversible loss and resynthesis of glucose (White and Leng, 1980).
Despite this probable role for amino acid catabolism to support energy needs and glucose homeostasis, the potential contribution of renal function to the high PUN levels must also be considered. In newborn lambs, a rapid rise in glomerular filtration rate occurs within 24 h of birth (Iwamato et al., 1985; Nakamura et al., 1987; Smith and Lumbers, 1989), and the urea gradient in renal tissue is steep and equivalent to that of adults within the first week of life (Stanier, 1972). However, the capacity of the newborn to deal with a rate of amino acid catabolism that may exceed that experienced in later postnatal life is unclear. In fetal life, catabolism of a substantial proportion of amino acids occurs (Bell, 1993). At birth, however, the placenta, which removes most urea from the fetal circulation (Battaglia and Meschia, 1978), is detached from the lamb. In well-fed neonates, there is also a substantial increase in the supply of amino acids over that in prenatal life. These factors suggest the capacity of the kidney to excrete urea may also make some contribution to the high PUN levels observed during the first days postpartum.
The degree to which hepatic responsiveness to ST may have contributed to the rapid decline from the peak PUN levels during the first week of life also warrants investigation. In young pigs, responsiveness of plasma IGF-I to exogenous ST administration increases with age, as does the ability of ST to reduce PUN levels (Harrell et al., 1999). In the present study, the rapid decline in ST levels during the postpartum period coincided with the rise in plasma IGF-I concentration. This finding is consistent with the notion of increased hepatic sensitivity and/or responsiveness to ST after birth compared to fetal life, resulting in a tighter coupling of the ST/IGF axis (Mesiano et al., 1987, 1989; Breier and Gluckman, 1991), despite the suggestion that hepatic ST receptors may not be fully functional until after 63 d of postnatal life in young lambs (Min et al., 1999).
Although the present study suggests a difference at birth between small and large newborns in hepatic responsiveness to ST and/or differences in the rate of IGF-I removal from circulation, there appears to be relatively little carry-over effect of birth size and, therefore, prenatal nutrition, on the ST/IGF system during rearing to 20 kg LW. Reduced circulating IGF-I in newborn low-birth-weight lambs is consistent with reduced liver IGF-I mRNA expression during late gestation in the severely growth-retarded fetal lamb (Rhoads et al., 2000). The lambs studied by Rhoads et al., which were a subset of those in the present study, also had reduced hepatic expression of the ALS gene but not altered hepatic expression of the IGFBP-2 and IGFBP-3 genes. During rearing of the neonates, differences due to birth weight in hepatic expression of IGF-I did not persist, although there was a tendency for ALS gene expression to remain lower to d 38 but not at 3 mo of age in lambs fed reduced levels of energy and protein.
In contrast to the lack of a longer-term effect of birth weight, high levels of postnatal feeding elevated plasma IGF-I concentrations in the neonatal lambs in the present study and in that of Rhoads et al. (2000). The latter authors also analyzed plasma from a subset of the lambs in the present study for IGF-I concentration using the method described by McGuire et al. (1995). Furthermore, they reported altered hepatic gene expression of IGF-I, IGFBP-2, ALS, and somatotropin receptor, but not IGFBP-3, due to nutritional level in the early postnatal period. This demonstrates the increasing role of the ST/IGF axis in mediating postnatal nutritional influences on tissue energy metabolism following birth. Plasma ST concentration during rearing was not associated with postnatal nutritional status per se. Rather, it was indirectly associated with postnatal nutritional status because the age at which any given body weight was reached was dependent upon the nutritional regimen, and also because body weight was inversely related to ST concentration. Previous studies with older lambs have shown age-related declines in ST mean concentration, pulse amplitude, and pulse frequency (Klindt et al., 1985; Suttie et al., 1993). In the present study, which included animals of vastly different ages at the same LW or EBW and vice versa, mean ST concentration and pulse amplitude were found to decline with increasing body weight rather than with age, while pulse frequency declined in an age-related manner. Despite the significance of these relationships, it was evident from the daily samples, pooled weekly samples, and the results of the frequent sampling regimen, that much unexplained variation exists in the nature of ST secretion into and removal from circulation in the neonatal lamb.
Differences in circulating insulin concentration due to birth weight persisted over the duration of the rearing period, suggesting influences of prenatal and/or earlier postnatal development on subsequent circulating levels. Insulin resistance, hyperglycemia, glucosuria, and galactosuria have been observed in veal calves fed milk only (Hugi et al., 1997). However, in twin lambs born 20% lighter than their co-twins (mean ± SEM birth weight 4.1 ± 0.3 vs 5.1 ± 0.1 kg), glucose and insulin tolerance were not adversely affected during rearing to 12 mo of age (Clarke et al., 2000). In the present study, a positive association between relative feed intake and plasma insulin was evident. Small newborns had relative feed intakes 20 to 45% higher than their large counterparts during the first weeks of postnatal life (Figure 4), which resulted in the small newborns being fatter at 20 kg LW (Greenwood et al., 1998), at which weight they also had smaller muscles than their high-birth-weight counterparts (Greenwood et al., 2000a). We postulate, therefore, that high weight-specific levels of energy intake during the immediate postpartum period following severe intrauterine growth retardation may be a contributing factor to the reported predisposition of low-birth-weight animals to develop insulin resistance (see Hales et al., 1996), at least in lambs.
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Footnotes |
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2 The authors also wish to thank Mike Ashdown, Manager of the Cornell University Mt. Pleasant Sheep Farm, who provided lambs to specification, and Jenny Schuck, who assisted with lamb rearing. The assistance provided by Bill English, Manager of the Large Animal Research and Teaching Unit is also gratefully acknowledged. Susanne Pelton provided assistance in the conduct of radioimmunoassays, and Doug Hogue provided support and assistance throughout this study.
3 Recipient of Junior Research Fellowship from Meat Research Corporation (Australia).
4 Present address: NSW Agriculture Beef Industry Centre, University of New England, Armidale NSW 2351, Australia.
5 Present address: Nutritional Services, Fort Worth Zoo, Fort Worth, TX 76110.
6 Present address: NSW Agriculture Centre for Crop Improvement, Tamworth NSW 2340, Australia.
7 Present address: Department of Animal Science, University of Nebraska, Lincoln 68583-0908.
Received for publication October 16, 2001. Accepted for publication July 10, 2002.
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— and —•—, respectively, and for slowly and rapidly grown high-birth-weight lambs by —
— and —
—, respectively. Statistical models with significant effects on plasma metabolite and hormone levels, and the residual standard deviation for each model, for the period from 1 d to 2 wk of age are presented in Table 1
