The effects of protein restriction on insulin-like growth factor-I and IGF-binding proteins in chickens

Evidence That Growth Hormone Exerts a Feedback Effect on Stomach Ghrelin Production and Secretion

Ghrelin is a recently discovered stomach hormone that stimulates pituitary growth hormone (GH) secretion potently. The purpose of these experiments was to test the hypothesis that a stomach-ghrelin-pituitary-GH axis exists in which either an elevation or reduction in systemic GH levels will exert a negative or positive feedback action, respectively, on stomach ghrelin homeostasis. In rats, GH administration decreased stomach ghrelin mRNA levels and plasma ghrelin levels significantly. In GH-releasing hormone (GHRH) transgenic mice, GHRH overexpression decreased stomach ghrelin peptide levels when compared with control mice. In aged rats (25 months) stomach ghrelin mRNA and peptide levels and plasma ghrelin levels were decreased when compared with young rats (5 months). Because GH secretion is reduced in aged rats, the elevated stomach ghrelin production and secretion may reflect a decreased GH feedback on stomach ghrelin, homeostasis, and secretion. Together, these findings suggest that endogenous pituitary GH exerts a feedback action on stomach ghrelin homeostasis and support the hypothesis that a stomach-ghrelinpituitary GH axis exists.

Early growth conditions, phenotypic development and environmental change

Phenotypic development is the result of a complex interplay involving the organism's own genetic make-up and the environment it experiences during development. The latter encompasses not just the current environment, but also indirect, and sometimes lagged, components that result from environmental effects on its parents that are transmitted to their developing offspring in various ways and at various stages. These environmental effects can simply constrain development, for example, where poor maternal condition gives rise to poorly provisioned, low-quality offspring. However, it is also possible that environmental circumstances during development shape the offspring phenotype in such a way as to better prepare it for the environmental conditions it is most likely to encounter during its life. Studying the extent to which direct and indirect developmental responses to environmental effects are adaptive requires clear elucidation of hypotheses and careful experimental manipulations. In this paper, I outline how the different paradigms applied in this field relate to each other, the main predictions that they produce and the kinds of experimental data needed to distinguish among competing hypotheses. I focus on birds in particular, but the theories discussed are not taxon specific. Environmental influences on phenotypic development are likely to be mediated, in part at least, by endocrine systems. I examine evidence from mechanistic and functional avian studies and highlight the general areas where we lack key information.

Effects of methionine deficiencies on plasma levels of thyroid hormones, insulin-like growth factors-I and -II, liver and body weights, and feed intake in growing chickens

We showed previously that Met deficiency at 0.25% of the diet causes elevations in plasma triiodothyronine (T3) in broilers. In the present study, plasma levels of thyroid hormones as well as insulin-like growth factors (IGF)-I and -II were measured in chicks fed 3 deficient levels of total Met. Control (0.5%) and Met-deficient diets (0.4, 0.3, and 0.2%) were fed to male broilers from 8 to 22 d of age. Additional groups of control chicks were pair-fed with the Met-deficient ones. Chicks receiving 0.4% Met increased feed intake by 10% with no significant change in body weight. The more severe Met deficiencies of 0.3 and 0.2% caused graded reductions in feed intake and weight gain. However, corresponding pair-fed control chicks were significantly heavier. These changes suggest more marked alterations in metabolic processes with 0.3 and 0.2% Met than with 0.4% Met. Liver weights were heavier in chicks fed 0.3 and 0.2% Met but not 0.4%. Plasma T3 was higher in all deficient chicks compared with the free-fed control, which was significant only with 0.3% Met. However, with 0.3 and 0.2% Met, plasma T3 was significantly elevated compared to pair-fed controls. Plasma thyroxine (T4) was lower in all deficient groups, which was significant only with 0.2% Met, whereas no significant differences occurred between deficient chicks and their pair-fed controls. Plasma IGF-I levels were not significantly different, but they were consistently lower in deficient chicks and deserve further study. Plasma IGF-II was significantly less in chicks fed 0.2% Met compared to pair-fed controls suggesting that Met deficiency interferes with IGF-II metabolism. We concluded that a deficit of dietary Met altered plasma T3 and IGF-II levels, but the effect was dependent on the degree of deficiency.

Response of broilers selected on carcass quality to dietary protein supply: live performance, muscle development, and circulating insulin-like growth factors (IGF-I and -II)

The effect of dietary protein supply on muscle development and circulating concentrations of insulin-like growth factors (IGF)-I and -II was examined in chickens selected for increased breast yield and decreased fatness (quality, QL) and in its control line (CL). CL and QL chickens were fed isoenergetic diets containing 121.5 or 215.8 g CP/kg during a 12-d period; comparisons were performed at 33 d of age. Birds given the high protein diet grew faster, ate less feed, had lower feed conversion ratio (FCR), and higher muscle weights than their counterparts given the low protein diet. The muscle weight response to protein supply differed between muscles in both lines, with pectoralis major appearing more sensitive than sartorius. The response of the gastrocnemius muscle depended on the line. Selection for carcass quality increased (P < 0.01) body weight, growth rate, feed intake, pectoralis major and sartorius muscle weights, and pectoralis major muscle proportion. There was, however, no line difference in FCR or in sartorius muscle proportion. The weight and proportion of the gastrocnemius muscle were higher (P < 0.05) in the QL than the CL chickens on the high protein diet, but there was no line difference for the low protein diet. Plasma levels of IGF-I, and to a lesser extent IGF-II, were lower (P < 0.01) in protein-restricted chickens. No difference in circulating IGF-II was observed between the lines. Concentrations of IGF-I were higher (P < 0.05) in QL than CL chickens, which may contribute to improved body composition for this genotype.

Influence of dietary protein on insulin-like growth factor binding proteins in the chicken

We determined the effect of dietary protein on the distribution of insulin-like growth factor (IGF) binding proteins in chicken plasma. Three groups of male broilers (n = 6 per group) were fed (ad libitum) isocaloric diets containing 12, 21 or 30% dietary protein. Birds were fed respective diets beginning at 7 days of age and killed at 28 days. No differences were observed between adequate (21%) and high (30%) protein intakes for any of the parameters investigated (growth criteria, plasma levels of IGF-I, growth hormone or IGF-binding proteins). Feeding protein deficient diets (12%) resulted in a 34% decrease in body weight, 17% decrease in feed intake and a 39% increase in feed/gain ratio. IGF-binding proteins in plasma samples were separated by SDS-PAGE and transferred to nitrocellulose sheets. Nitrocellulose blots were probed with [125I]chicken IGF-II. Four regions of binding activity corresponding to 70, 43, 30 and 24 kDa were observed in all samples. Birds consuming 12% dietary group protein had less than 50% of the 43-kDa binding activity of birds consuming 21 or 30% dietary protein. The 30-kDa binding activity was 42% lower in the 12% dietary protein group compared to birds consuming adequate protein. In contrast, 70- and 24-kDa binding activities were not influenced by dietary protein. Chickens consuming 12% dietary protein had higher levels of growth hormone and lower levels of IGF-I than those consuming 21 or 30% dietary protein. These data indicate that in chickens, the circulating levels of at least two independent IGF-binding proteins are influenced by dietary protein.

Genetic models for the study of insulin-like growth factors (IGF) and muscle development in birds compared to mammals

IGFs are important positive modulators of overall body and muscle growth in different species. Genetic variation in IGF-I gene expression exists as shown by the possibility of genetic selection for high or low circulating IGF-I concentrations in mice and the associated variations in growth potential. Targeted over-expression of IGF-I in transgenic mice results in muscle hypertrophy, but it is yet unknown whether genetic variability in muscle IGF-I gene expression exists. Much less data are available in birds. This review is focussed on the potential role of IGFs on chicken muscle development. Apart from the absence of a type 2 IGF receptor (IGF-II receptor), the general characteristics of the chicken IGF system seem to be similar to mammalian species. In different genetic models with altered growth rates or body composition, differences in the IGF system are observed suggesting its importance in regulating body growth in those species. The components for a paracrine action of IGF are also present in chicken muscle but it has not yet been demonstrated if they contribute to differences in muscle development.