Proteinase K-resistant material in ARR/VRQ sheep brain affected with classical scrapie is composed mainly of VRQ prion protein.

Classical scrapie is a prion disease in sheep and goats. In sheep, susceptibility to disease is genetically influenced by single amino acid substitutions. Genetic breeding programs aimed at enrichment of arginine-171 (171R) prion protein (PrP), the so-called ARR allele, in the sheep population have been demonstrated to be effective in reducing the occurrence of classical scrapie in the field. Understanding the molecular basis for this reduced prevalence would serve the assessment of ARR adaptation. The prion formation mechanism and conversion of PrP from the normal form (PrP(C)) to the scrapie-associated form (PrP(Sc)) could play a key role in this process. Therefore, we investigated whether the ARR allele substantially contributes to scrapie prion formation in naturally infected heterozygous 171Q/R animals. Two methods were applied to brain tissue of 171Q/R heterozygous sheep with natural scrapie to determine the relative amount of the 171R PrP fraction in PrP(res), the proteinase K-resistant PrP(Sc) core. An antibody test differentiating between 171Q and 171R PrP fragments showed that PrP(res) was mostly composed of the 171Q allelotype. Furthermore, using a novel tool for prion research, endoproteinase Lys-C-digested PrP(res) yielded substantial amounts of a nonglycosylated and a monoglycosylated PrP fragment comprising codons 114 to 188. Following two-dimensional gel electrophoresis, only marginal amounts (<9%) of 171R PrP(res) were detected. Enhanced 171R(res) proteolytic susceptibility could be excluded. Thus, these data support a nearly zero contribution of 171R PrP in PrP(res) of 171R/Q field scrapie-infected animals. This is suggestive of a poor adaptation of classical scrapie to this resistance allele under these natural conditions.

Use of a new immunoassay to measure PrP Sc levels in scrapie-infected sheep brains reveals PrP genotype-specific differences.

The diagnosis of prion diseases, such as scrapie and BSE, has traditionally relied upon the identification of the disease-associated form of the prion protein, PrP(Sc), based on its resistance to digestion by proteinase K (PK). A more recent development is the conformation-dependent immunoassay (CDI), which distinguishes between PrP Sc and normal PrP (PrP C) based on their differing solubility in guanidine hydrochloride rather than resistance or sensitivity to PK. We have developed a CDI-formatted sandwich immunoassay for the measurement of PrP Sc in sheep brain, which discriminates between clinically affected scrapie cases (natural or experimental) and uninfected controls of the same PrP genotype. Using this method, we have shown for the first time that, in sheep, the PrP genotype has a significant influence on the amount of PrP Sc deposited in the brains of animals experimentally infected with scrapie.

Responses of young energy-restricted sheep to chronically administered insulin-like growth factor I (IGF-I): evidence that IGF-I suppresses the hepatic growth hormone receptor.

We have shown previously that chronic administration (8 weeks) of insulin-like growth factor-I (IGF-I) has little growth-promoting effect in well fed sheep. The aim of this study was to investigate the anabolic effects of IGF-I in energy-restricted conditions in which circulating concentrations of IGF-I in control animals were expected to be low. Young castrate male sheep were offered chaffed lucerne at a rate equivalent to 110% maintenance and were treated by sc injection three times per day for either 8 or 12 weeks with recombinant human IGF-I (150 micrograms/kg live wt x day) or saline in a 2 x 2 factorial design (eight animals per cell). IGF-I treatment significantly increased plasma IGF-I concentrations, but reduced plasma concentrations of IGF-II, GH, urea, and creatinine. Treatment with IGF-I also decreased (P < 0.1) GH secretion in response to a GRF load, but significantly (P < 0.05) increased the nonesterified fatty acid response to an epinephrine load. The reduction in circulating GH levels was accompanied by a suppression of [125I]oGH binding to hepatic microsomal membranes. This effect, if apparent in other tissues, may act as a feedback mechanism to limit the local synthesis of IGF-I and could explain why IGF-I treatment had little effect on the growth rate of the sheep, although it did increase nitrogen digestibility of the feed consumed and decreased the fat content of the hind leg. It also differentially promoted the growth of the spleen, thymus, and mandibular salivary gland and increased blood counts of eosinophils. It is concluded that IGF-I does not have marked effects on growth rate or body composition in sheep fed a near-maintenance diet. Possible reasons include the associated suppression of GH secretion and action, which limits the ability of treated animals to repartition absorbed nutrients.

Genetic selection for insulin-like growth factor-1 in growing mice is associated with altered growth.

Substantial responses in the 6-week and mature body-weights of mice occurred after 7 generations of selection for or against plasma levels of Insulin-like Growth Factor-1 (IGF-1). Plasma levels of IGF-1 were also significantly different after 7 generations of selection (high line = 85 +/- 2 ng/ml, low line = 58 +/- 2 ng/ml). The average 6-week weight in the line selected for high plasma IGF-1 was 22.5 +/- .2 g compared with 18.5 +/- .2 g in the low plasma IGF-1 line, after 7 generations of selection. The difference between lines was maintained at 20 weeks of age. These data provide further evidence for the roles of IGF-1 in the regulation of somatic growth and as a mediator of a genetic component of growth.

Body growth, carcass composition, and endocrine changes in lambs chronically treated with recombinantly derived insulin-like growth factor-I.

Castrate yearling male sheep were treated for 8 weeks with either 50 micrograms/kg body wt/8 hourly sc insulin-like growth factor-I (IGF-I) (n = 10) or with saline (n = 9). IGF-I treatment increased plasma IGF-I from 235 +/- 17 to 347 +/- 16 ng/ml (P less than 0.001). There was a gradual divergence in body wt (P less than 0.10) between treatment groups. Food intake did not change significantly. The weight of the spleen corrected for body wt increased by 40% (P less than 0.001) and there was a marginal increase in adjusted kidney wt (P less than 0.1). There was no effect of IGF-I on carcass weight or dimensions, or on long bone length, although the weight per unit length of the tibia (P less than 0.05) and femur (P less than 0.10) were increased. There was no effect on wool growth. Plasma IGF binding proteins (IGFBPs) were quantified by ligand blot analysis. In the IGF-I treated group, IGFBP-1 showed a transient increase (P less than 0.05) at day 3 but was similar in both groups at day 55 of treatment. IGFBP-2 was suppressed (P less than 0.05) by day 55 and IGFBP-3 and 4 did not change. Plasma glucose was elevated (P less than 0.05) and plasma insulin was suppressed (P less than 0.01) from 280 +/- 32 pg/ml to 124 +/- 30.4 pg/ml, plasma urea (P less than 0.01) and creatinine (P less than 0.05) were reduced in the IGF-I treated group. The somatogenic effect of IGF-I in this study was minimal suggesting that in the well fed animal with an intact somatotropic axis IGF-I treatment at doses which double plasma IGF-I does not enhance somatic growth performance. However, the marked splenomegaly shows the sensitivity of splenic growth to systemic IGF-I. The suppression of insulin with chronic IGF-I treatment was accompanied by hyperglycaemia--this may explain in part the lack of a significant anabolic response and may limit the utility of IGF-I therapy unless higher doses with insulin-like effects are used.

Insulin-like growth factor-1 (IGF-1) in mice reduces weight loss during starvation.

Plasma concentrations of IGF-1 decrease markedly during starvation secondary to a reduction in somatotropic receptors in the liver. We investigated whether IGF-1 administration during starvation in mice inhibits the catabolic state normally observed. Plasma concentrations of IGF-1 in starved mice receiving IGF-1 therapy were similar to values from non-starved mice, whereas bGH treatment failed to increase plasma IGF-1 levels. The degree of weight loss during 36 hours of starvation was reduced (p less than 0.01) by frequent treatment with subcutaneous IGF-1 but not by bGH therapy. The effect was restricted to the period 28 to 36 hours after commencement of the fast. These results suggest that a fall in circulating IGF-1 may play a role in the metabolic adaptation during malnutrition.

P34(cdc2) kinase is associated with cortical microtubules from higher plant protoplasts.

The cell cycle regulatory enzyme p34(cdc2) kinase is known to be localized to the preprophase band, the spindle and the phragmoplast, but not to interphase cortical microtubules. This was investigated further by mechanically cleaving substrate-attached protoplasts to leave plasma membrane disks bearing microtubules freed of nuclear and cytosolic signal. Antibodies to PSTAIRE and to specific C-terminal peptides of cdc2a, were used in immunofluorescence, protein blotting and immunogold electron microscopy to demonstrate that antigen is located on the cortical microtubules of carrot, tobacco BY-2 and Arabidopsis cells.

A homologous radioimmunoassay for ovine insulin-like growth factor-binding protein-2: ontogenesis and the response to growth hormone, placental lactogen and insulin-like growth factor-I treatment in sheep.

Although insulin-like growth factor-binding protein-2 (IGFBP-2) is an abundant IGFBP in fetal and postnatal plasma, its regulation is not yet clearly understood. To address this question in sheep, we purified ovine IGFBP-2 and developed a homologous radioimmunoassay. We have studied its ontogenesis and measured serum concentrations of ovine IGFBP-2 after bovine growth hormone (bGH), ovine placental lactogen (oPL) and IGF-I treatment. Concentrations of IGFBP-2 were high at 125 days of gestation (550 +/- 15 micrograms/l) but fell after birth (P < 0.05) and plateaued after 1 year of age (340 +/- 20 micrograms/l). In lactating ewes, bGH treatment for 7 days significantly reduced (21%; P < 0.05) IGFBP-2 relative to the saline-treated group. Similarly, in neonatal lambs, bGH treatment from day 3 to day 23 of life reduced (P < 0.05) IGFBP-2 by 23% relative to the saline-treated group. oPL had no effect on serum levels of IGFBP-2 in the ewe or the neonatal lamb. In well-fed yearling lambs, treatment with IGF-I reduced IGFBP-2 values by 27% (P < 0.05) relative to control animals. In yearling lambs, reduced nutrition increased plasma IGFBP-2 (41%; P < 0.05). However this increase was abolished by IGF-I treatment. The changes in plasma levels of IGFBP-2 were positively related to changes in IGF-II while there was a negative relationship between circulating IGF-I and IGFBP-2 such that both IGF-I and IGF-II may play a role in the regulation of IGFBP-2 in serum.

Somatotrophic receptors in hepatic tissue of the developing male pig.

The development of hepatic somatotrophic receptors and plasma concentrations of insulin-like growth factor-I (IGF-I) were investigated at five different ages (2, 20, 35, 105 and 165 days) in four male pigs per group. The specific binding of 125I-labelled porcine GH (pGH) to hepatic somatotrophic membranes was very low at 2 days of age (0.53 +/- 0.12%), and increased progressively (P less than 0.01) with advancing age to 3.60 +/- 0.95% at 165 days of age. Specific binding of 125I-labelled bovine GH (bGH) to the same membrane preparations was markedly higher than binding of 125I-labelled pGH; it also showed a distinct developmental increase (P less than 0.01) with age from 4.4 +/- 0.55% at 2 days of age to 24.0 +/- 1.90% at 165 days of age. Plasma concentrations of IGF-I increased significantly (P less than 0.01) from 79 +/- 14.0 micrograms/l at 2 days of age to 610 +/- 64.0 micrograms/l at 165 days of age. Non-linear regression analysis of the competitive binding data using bGH as labelled and unlabelled ligands showed linear Scatchard plots in the three youngest age groups, with an association constant (Ka) of approximately 3.5 litres/nmol. Curvilinear Scatchard plots were observed in the two oldest age groups. The Ka for the higher affinity binding site (approximately 5.0 litres/nmol) was very similar to that for the sole site observed in the younger animals. The Ka of the lower affinity binding site was approximately 0.35 litres/nmol.(ABSTRACT TRUNCATED AT 250 WORDS)

Elevating maternal insulin-like growth factor-I in mice and rats alters the pattern of fetal growth by removing maternal constraint.

Fetal growth is normally constrained by maternal factors. This constraint is demonstrated by the usual inverse linear relationship between litter size and mean fetal weight. Cross-breeding experiments between mice of lines selected for high or low plasma insulin-like growth factor (IGF-I) levels suggested that elevations in maternal IGF-I abolish (P less than 0.01) this constraining effect and reverse the usual positive relationship between fetal and placental size in late gestation. This was confirmed by treating mice and rats throughout pregnancy with IGF-I. In normal mice and in low IGF-I line mice treatment with IGF-I (10 micrograms 8-hourly s.c. from day 1 to 19 of pregnancy) abolished maternal constraint whereas 0.9% (w/v) NaCl treatment did not. In Wistar rats osmotic pumps were implanted to deliver IGF-I (1 microgram/g body weight per day), bovine GH (bGH; 0.6 microgram/g body weight per day) or saline from day 1 to 19 of pregnancy. IGF-I therapy but not bGH or saline abolished (P less than 0.01) maternal constraint and altered (P less than 0.01) the relationship between placental and fetal weight. When high or low IGF-I line mice embroys were transplanted into a normal line of mice, the expected negative relationship (P less than 0.05) between mean fetal weight and litter size was maintained. However, the embryos of the high line were heavier (P less than 0.05) than those from the low line irrespective of fetal number, suggesting a direct role for IGF-I in the regulation of fetal growth.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental patterns of plasma insulin-like growth factor-I (IGF-I) and body growth in mice from lines divergently selected on the basis of plasma IGF-I.

A study was conducted to investigate developmental patterns of plasma concentrations of insulin-like growth factor-I (IGF-I), body growth and body composition in mice from lines selected for seven generations on the basis of low (L) or high (H) plasma IGF-I, and in a random-bred control (C) line. Litter size was standardized to eight individuals with equal sex ratios (as far as possible) within 48 h of birth. Pups were weaned at an average of 21 days and separated on the basis of sex. Blood samples were collected from one male and one female of each litter on days, 21, 42, 63 and 105 for analysis of plasma concentrations of IGF-I. The animals were then killed and analysed for water, fat and crude protein content. The plasma concentration of IGF-I was influenced by line (P less than 0.05) but not by sex. Significant (P less than 0.001) differences in liveweight between mice from L and H lines were first evident at 21 days of age. From 28 until 105 days of age the H line was significantly (P less than 0.001) heavier than both L and C lines, but differences between C and L lines were inconsistent and mostly non-significant. The growth velocity of the H line was significantly greater than that of C or L lines between 14 and 42 days of age, but differences in growth velocities of C compared with L lines were generally non-significant.(ABSTRACT TRUNCATED AT 250 WORDS)

Placental development and fetal growth in growth hormone-treated ewes.

The effects of recombinant bovine growth hormone (bGH) treatment of pregnant ewes on maternal metabolism, placental development and fetal growth were examined in two studies. In a preliminary study (experiment one), single-bearing ewes were treated by twice-daily subcutaneous injection for 7 days with bGH (n = 8) at a dose of 0.15 mg/kg LW/day or with saline (n = 8) between days 101 and 107 of gestation inclusive. In experiment two, single- and twin-bearing ewes were treated for 14 days with bGH (0.15 mg/kg L W/day) (n = 10) or saline (n = 10) between days 70 and 83 or days 98 and 111 of gestation inclusive. Ewes were killed on the day following termination of bGH treatment and fetal and placental measurements recorded. Maternal plasma concentrations of GH, IGF-I and insulin were higher (P < 0.001) in bGH-treated ewes relative to saline-treated ewes in both experiments. Consistent across experiments was an increase (P < 0.05) in the weight of the myoendometrium in bGH-treated ewes. Treatment with bGH also increased the total weight of the gravid uterus (P < 0.05) in both experiments. Weights of the uterine fluids were increased by bGH in experiment one (P < 0.05), but an effect of the same magnitude could not be repeated in experiment two. In experiment one, there was a tendency towards increased mean fetal body weights after growth hormone treatment, although the effect was non-significant. In experiment two, treatment with bGH was associated with significantly (P < 0.05) higher fetal weights, but only at the later stage of gestation (day 112). This effect was additive with that of fetal rank. Exogenous bGH treatment had little discernible effect on measures of placental size. It is concluded that administration of exogenous bGH to pregnant ewes can stimulate fetal growth, but only after about day 100 of gestation. This response seems most likely to reflect changes in maternal nutrient partitioning or placental function, rather than placental size. These studies suggest a role for growth hormone of maternal or placental origin in the regulation of fetal growth.

The effects of ovine placental lactogen and bovine growth hormone on hepatic and mammary gene expression in lactating sheep.

The ability of ovine placental lactogen (oPL) to bind to the growth hormone receptor (GHR) raises the possibility that oPL may exert a growth hormone (GH)-like action on galactopoiesis. We have compared the effects of treating lactating ewes for 5 days with an equimolar dose (0.1 mg/kg/day, administered as two equal doses 12 hourly) of either bovine growth hormone (bGH) (n = 10), oPL (n = 10) or saline (n = 9) on hepatic and mammary GHR, insulin-like growth factor-I (IGF-I) and IGF-binding protein-3 (IGFBP-3) gene expression and hepatic GHR number. Hepatic GHR and IGFBP-3 mRNA were unaltered by bGH or oPL treatment. Hepatic IGF-I mRNAs increased following bGH (P < 0.05) but not oPL treatment. GHR gene expression was greater in liver compared to mammary gland extracts. There was no effect of either bGH or oPL treatment on mammary GHR, IGF-I or IGFBP-3 mRNA or hepatic GHR number. These studies confirm the galactopoietic effects of bGH in lactating ruminants and suggest that the mechanism of this action is not via increased hepatic GHR number or gene expression. In addition, the increase in hepatic but not mammary IGF-I mRNA with bGH treatment suggests an endocrine action of IGF-I on milk synthesis. These studies also demonstrate that an equimolar dose of oPL is not galactopoietic or somatogenic in the lactating ewe.

Muscle growth, cell number, type and morphometry in single and twin fetal lambs during mid to late gestation.

Muscle growth, myofibre number, type and morphometry were studied in large hindlimb muscles of single and twin fetal lambs during mid to late gestation. Placental insufficiency, evident by lower total placentome weight and number per fetus, resulted in reduced fetal weights from 100 to 140 days gestation in twins compared with singletons (at 140 days: 5016 +/- 108 g v. 5750 +/- 246 g, respectively; P<0.05). However, competition between littermates did not consistently reduce muscle mass (15-22%) until 140 days gestation. Apparent myofibre number increased with age, indicating that the full complement of myofibres in some large hindlimb muscles may be achieved during early postnatal life. Litter size did not impact on apparent myofibre number in the semitendinosus, plantaris or gastrocnemius muscles. However, a transient effect on myofibre number in the adductor femoris muscle was observed from 80-120 days gestation. The phenotypic maturation of myofibres was unaffected by increasing litter size. Smaller muscle mass in twins was associated with smaller myofibre cross-sectional area in the semitendinosus, adductor femoris and gastrocnemius muscles at 140 days gestation. A similar trend was observed for the plantaris muscle. These results indicate that while competition between littermates for nutrients in late gestation can impact on both fetal and muscle mass, the fetus has the capacity to buffer against the effects of restricted nutrient supply on myofibre hyperplasia and phenotypic maturation, but myofibre hypertrophy is compromised.

Morphometric analysis of myofibre development in the adductor femoris muscle of single and twin fetal lambs.

Cellular development of the adductor femoris muscle from twin and single fetuses was studied at 140 days gestation to evaluate the effect of moderate fetal growth retardation on myofibre development. Twin fetuses had lower bodyweights (13%) and disproportionately small adductor femoris muscle weights (22%) compared with single fetuses. Reduced muscle mass was associated with smaller myofibre cross-sectional areas (CSA) and lower DNA content (22%), indicative of fewer myonuclei and retarded myofibre hypertrophy. Myofibre number and the phenotypic maturation of the myofibres were similar between twins and singletons. These results indicate that even modest growth restriction during fetal life can negatively influence myofibre hypertrophy, highlighting the importance of fetal nutrition for muscle growth. Large muscles, such as the adductor femoris, have intrafascicularly terminating myofibres, which necessitates accurate sampling of the muscle when investigating possible perturbations in morphological characteristics (e.g. between singletons and twins). The second objective of the present study was to investigate the impact of the sampling site on the morphological parameters of the adductor femoris muscle. The apparent total myofibre number decreased from the proximal to the distal region of the adductor femoris muscle. The apparent number of slow-twitch fibres also decreased from the proximal to the medial region, but was not different between the medial and distal regions of the muscle. Similarly, myofibre CSA differed between the medial and distal regions. These results indicate that, particularly with large muscles, such as the adductor femoris, which has intrafascicularly terminating myofibres, single site sampling for the determination of morphological fibre characteristics may generate misleading results and that careful selection of the sampling area may be necessary.

Maternal constraint influences muscle fibre development in fetal lambs.

The objective was to examine myogenesis in two situations expected to be characterized by maternal constraint: (i) in fetuses due to be born in spring (n=10) or autumn (n=10); and (ii) in single (n=16) and twin (n=20) fetal lambs. Maternal constraint operating through limitation of placental size, as measured by placentome weight per fetus, was evident in each study. Although a lower placental weight did not influence body and muscle weights of fetuses due to be born in the spring or autumn, twins had lower body and muscle weights than singles. Fibre number and average fibre cross-sectional (CS) area were differentially affected by season and fetal number. The differences in muscle fibre morphology between spring- and autumn-born fetuses suggest that muscle fibre development was influenced by maternal constraint in the absence of an effect on fetal weight. The differences in muscle fibre number and CS area in particular muscles from twin and single fetuses suggest that more severe maternal constraint, reflected in a lower placental size per fetus, not only influences fetal weight but can also affect muscle development.

Response to metabolic challenges in early lactation dairy cows during treatment with bovine somatotropin.

Milk production is increased in lactating cows treated with bovine somatotropin (bST) because a greater portion of absorbed nutrients are partitioned for milk synthesis. This homeorhetic action may be caused by alterations in response of key tissues to homeostatic signals. To examine this theory, acute metabolic challenges were administered to 8 multiparous Holstein cows (61 +/- 2 days postpartum) receiving daily subcutaneous injections of pituitary-derived bST (26.3 mg) or excipient during two 14-day treatment periods (crossover experimental design). Treatment with bST increased milk yield 12%. Feed intake did not change so that net energy balance decreased (+ .5 vs. -4.3 Mcal/day). Plasma concentrations of nonesterified fatty acids (NEFA) were chronically elevated in bST-treated cows, consistent with energy balance differences. However, baseline concentrations of glucose, insulin, and glucagon in plasma did not differ. On the last 3 days of treatment, individual metabolic challenges were administered via jugular cannulas: epinephrine (700 ng/kg BW), glucose (250 mg/kg BW), insulin (1.0 micrograms/kg BW), and glucagon (175 ng/kg BW). Plasma glucose was reduced after the insulin challenge to a lesser extent during bST treatment. In bST-treated cows, the increase in plasma NEFA in response to epinephrine was greater, and NEFA concentrations were lowered to a greater extent after insulin and glucose challenges. Glucose, insulin, and glucagon removal rates were not altered, nor was plasma glucose response to epinephrine or glucagon challenges. Treatment of lactating cows with bST primarily altered the response of adipose tissue to homeostatic signals which affect lipid metabolism.

Metabolic responses to exogenous bovine somatotropin in Friesian cows of low or high genetic merit.

Basal hormone/metabolite concentrations and responses to intravenous challenges of glucose, insulin and epinephrine were examined in Friesian cows from selection lines of low or high genetic merit treated with recombinantly-derived bovine somatotropin (bST) or control formulation in a 2 x 2 factorial design. Cows from the low genetic merit (low breeding index, LBI) line had previously been shown to be more responsive to the galactopoietic effects of bST (50 mg/day) than those from the high breeding index (HBI) line. Despite this, comparisons of metabolic differences were not confounded by differences in energy balance because bST treatment had also caused an increase in voluntary intake of cut pasture. Circulating levels of somatotropin, insulin-like growth factor-I (IGF-I) and insulin were greater in bST-treated than control cows but neither bST treatment nor selection line influenced basal concentrations of glucose, non-esterified fatty acids (NEFA), beta-hydroxybutyrate, urea or creatinine. Treatment with bST produced a small increase in sensitivity of cows to the lipolytic effects of epinephrine and this effect was similar in both selection lines. HBI cows had greater circulating insulin levels following the glucose challenge than LBI cows but bST treatment did not affect the insulin response to exogenous glucose. Whereas bST treatment retarded the glycogenolytic response to epinephrine and the clearance of blood glucose in response to insulin in LBI cows, it had no effect on epinephrine-stimulated glycogenolysis, and caused enhanced glucose clearance in response to insulin, in HBI cows. Results are consistent with bST altering the homeorhetic control of metabolism but do not adequately explain the greater responsiveness of LBI cows to the galactopoietic effects of bST.

Ontogeny of IGF-I responsiveness to bGH in young lambs.

The ontogeny of hepatic growth hormone (GH) receptors (GHR), as measured by responses of both plasma insulin-like growth factor-I (IGF-I) and hepatic GHR to an exogenous bGH stimulus, was examined using sheep of different ages (Days 1-7, 14-21, 28-35, and 56-63 of life, and yearlings). The IGF-I response to bGH was first examined in yearling sheep using two doses of bGH (0.1 and 0.2 mg/kg LW/d). Based on these results, lambs in four groups up to Day 63 of life were treated for 5 d with bGH (n = 10) at a dose of 0.15 mg/kg LW/d or with saline (n = 10). Jugular blood samples were taken once daily on Days - 1, 4, and 5 of treatment. bGH treatment in lambs up to Day 63 of life had little effect on plasma concentrations of GH, insulin, glucose or urea, but significantly (P < 0.05) increased circulating concentrations of IGF-I at all ages and of NEFA at Day 62/63 of life. In contrast, bGH treatment at either dose in yearlings significantly increased these parameters, except for plasma urea concentrations which were decreased in bGH-treated yearlings. However, the responses of plasma IGF-I concentration to bGH stimulus in lambs up to Day 63 of life were small compared to those in yearling sheep. Consistent with this, bGH treatment failed to affect hepatic GH binding in young lambs, but up-regulated it in yearling sheep. Furthermore, basal (unstimulated) GH binding did not differ between sheep of 7 vs. 63 vs. 365 d of age, despite the greater IGF-I responses to bGH in the latter group. It is suggested that hepatic GHR in lambs up to Day 63 of life are not fully functional compared to the situation in yearlings.