The metabolism in mammalian is regulated by multiple levels of hormone action, with complex feedback and control mechanisms. The somatotropic axis, primarily consisting of growth hormone, insulin-like growth factors (-I and -II), their associated carrier proteins (GHBP and IGFBPs), and receptors, plays a key role in the control of the regulation of metabolism and physiological process. Among this axis, other hormones like insulin, leptine, glucocorticoïds or thyroid hormones are involved in this mechanism by modulating GH and/or IGF-I synthesis and availability. In the present instance, this review summarizes recent findings and the complexity of the regulation of the metabolism by the growth hormone (GH)-insulin-like growth factor (IGF) axis using different examples: special nutritional situations, use of growth promoters (including hormones and hormone analogs such as bST, corticoids, beta-agonists) and particular physiological status (genetic potential, puberty, growth alteration, ….). Feed restriction or postpartum negative energy balance leads to elevated GH and IGFBP-2 secretion, but it reduces hepatic GH receptor (GHR) number and plasma levels of IGF-I and IGFBP-3. Distribution of glucogenic compounds to these animals prevents alterations of the somatotropic axis. GH treatment in animals leads to up-regulation of GH-receptor, increases synthesis and plasma concentrations of IGF-I and IGFBP-3 whereas concentrations of IGFBP-2 decrease; this may lead to increased availability of IGF-I at target tissues. These endocrine interactions, together with metabolic changes, leads to increased protein anabolism, reduced fat deposition, and enhanced growth rate. However, GH treatment seems to differently altered IGFBP-2 and IGFBP-3 mRNA levels in mammary tissue than in hepatic tissue. Similar alterations of somatotropic axis is observed in finishing bulls treated with dexamethasone esters while betaagonist administration reduces IGF-I and IGFBP-3 synthesis and stimulates IGFBP-2 production. Numerous genetic mutations are reported for molecules of somatotropic axis. Some of them are directly related with animal performances, endogenous hormonal synthesis and metabolism. Puberty is another important physiological period characterised by a sharp increase of IGF-I and IGFBP-3 levels and decrease of IGFBP-2 independently to steroids production. In the bovine species, a significant correlation seems to exist between GH response to GHRH stimulation or IGF-I/IGFBPs status and animal metabolism. However, in pig, whereas no differences are observed in GH secretion parameters, IGF-I and IGFBP-3 levels in retarded growth piglets are remarkable lower than in normal weight animals. In this cases, an altered responsiveness of hepatic GH receptor is suspected. The physiological mechanisms of metabolism remains fairly understood in farm animals. However, with the development of new analytical methods, particularly the IGFBPs quantification, advances in research of the somatotropic axis will improve our understanding of the normal anim...

SummaryThirty-nine multiparous Holstein cows were used to measure the effect of propylene glycol treatment around parturition on milk yield, reproductive efficiency and some hormone and metabolite concentrations. Cows were assigned randomly to control (n = 19) or propylene glycol treated (n = 20) groups. Propylene glycol (300g) was administered directly mixed with the diet from day 10 prior to the expected calving date until parturition (day 0) and orally after dilution in 1 l water on days 3, 6, 9 and 12. Blood samples were collected on days −20, −5, 0, 3, 10, 25 and 50 while milk samples were taken weekly until 13 weeks post partum. Body condition scores, recorded on days −20, 15 and 50, were not affected by propylene glycol administration. Propylene glycol did not significantly affect milk yield or composition but linear somatic cell score measured from the first 13 weeks post partum was reduced by propylene glycol administration (P < 0·01). Moreover, propylene glycol reduced milk urea (−25 mg/l, P < 0·05), especially during the first 9 weeks post partum. Plasma insulin concentrations were similar in both groups during the experiment while insulin-like growth factor I (P < 0·05) and insulin-like growth factor-binding protein 3 (P < 0·001) levels were higher on days 10, 25 and 50 post partum in the propylene glycol group. Propylene glycol administration decreased plasma non-esterified fatty acid concentrations (P < 0·05 to P < 0·01) but increased total cholesterol levels (P < 0·01) after parturition while 3-hydroxy-butyrate levels were unaffected by the treatment. Changes in the hormone and metabolic concentrations after propylene glycol administration in the last few days of gestation and the first week of lactation seem to indicate that energy balance in the treated group was probably more positive than in the control group. There was also evidence that propylene glycol administration prevented fatty liver syndrome and hastened the resumption of oestrous cycles (P < 0·001).

The growth hormone factor-1/pituitary-specific transcription factor Pit-1 is responsible for the expression of growth hormone in mammals. Mutations in Pit-1 have been found in growth hormone disorders of mice and humans. We studied the eventual association between Pit-1 polymorphism using the HinfI enzyme and the milk yield and conformation traits of 89 Italian Holstein-Friesian bulls. A strategy employing polymerase chain reaction was used to amplify a 451-bp fragment from semen DNA. Digestion of polymerase chain reaction products with HinfI revealed two alleles: allele A was not digested (451-bp fragment), and allele B was cut at one restriction site, generating two fragments of 244 and 207 bp. Three patterns were observed; frequencies were 2.2, 31.5, and 66.3% for AA, AB, and BB, respectively. Fixed and mixed linear models were fitted on daughter yield deviations for milk yields and on deregressed proofs for conformation traits. Predictions were weighted using the inverse of the estimated variance of records. The models used contained mean and gene substitution effects for Pit-1 A allele as fixed effects and random sire effect for the mixed model. The A allele was found to be superior for milk and protein yields, inferior for fat percentage, and superior for body depth, angularity, and rear leg set, which is difficult to explain. A canonical transformation revealed that Pit-1 had three actions, one linked to milk yield traits and angularity, a second linked to body depth and rear leg set, and a third linked to lower fat yields and to higher angularity. ( Key words: milk, conformation, Pit-1, restriction fragment length polymorphism) Abbreviation key: DRP = deregressed proofs, DYD = daughter yield deviations, GH = growth hormone, PCR = polymerase chain reaction, PRL = prolactin, RFLP = restriction fragment length polymorphism.

The effects of lactoferrin (LF) on the immune system have already been shown by many studies. Unfortunately, the current methods used to measure LF levels in milk do not permit the study of the genetic variability of lactoferrin or the performance of routine genetic evaluations. The first aim of this research was to derive a calibration equation permitting the prediction of LF in milk by mid-infrared spectrometry (MIR). The calibration with partial least squares on 69 samples showed a ratio of standard error of cross-validation to standard deviation equal to 1.98. Based on this value, the calibration equation was used to establish an LF indicator trait (predicted LF; pLF) on a large number of milk samples (n = 7,690). A subsequent study of its variability was conducted, which confirmed that stage of lactation and lactation number influence the overall pLF level. Small differences in mean pLF among 7 dairy breeds were also observed. The pLF content of Jersey milk was significantly higher than that in Holstein milk. Therefore, the choice of breed could change the expected LF level. Heritability estimated for pLF was 19.7%. The genetic and phenotypic correlations between somatic cell score and pLF were 0.04 and 0.26, respectively. As somatic cell score increases in presence of mastitis, this observation seems to indicate that pLF, or a function of observed pLF, compared with expected LF might have potential as an indicator of mastitis. The negative genetic correlation (−0.36) between milk yield and pLF could indicate an undesirable effect of selection for high milk production on the overall LF level.

One of the obstacles to progress in dairy cattle selection is that milk production traits are only expressed after the first calving. However, the use of the quantitative trait loci (QTL) technology will improve the efficiency of dairy industry with a positive image for the consumers. QTL are part of the genome showing a preponderant action and explaining the major part of variation of the trait production. At the present time, the two major strategies developed to detect such QTL are the candidate gene approach and the positional genetics approach. The somatotropic axis contains the most promising candidates in this respect, as it strongly regulates milk production. Then, the identification of favorable QTL associated with the somatotropic axis that are significantly correlated with genetic merits for milk production could lead to more effective selection programs.