Ontogenic maturation of the somatotropin/insulin-like growth factor axis.

Abstract

The ontogeny of the somatotropin/insulin-like growth factor system was examined in well-fed pigs under basal conditions and during a short-term challenge of porcine ST (pST). The study was conducted with two replicates of eight castrate male pigs from 3.8 kg BW (10 d of age) to 92 kg BW (129 d of age). Pigs were reared individually with ad libitum access to milk replacer through 23 d of age. Thereafter, pigs were fed a corn, soybean meal, and dry skim milk diet formulated to exceed nutrient requirements by approximately 30%. Pigs were randomly assigned to receive daily i.m. injections of either 0 (buffer) or 120 microg/kg BW of pST for a duration of 4 d starting at 10, 19, 33, 43, 63, 83, and 125 d of age. Blood was collected via jugular venipuncture on d 0 and 4 of the challenge. Circulating levels of IGF-I were not dramatically affected by age, but levels of IGF-II were low from 10 to 19 d of age and then increased through later stages of growth. Circulating concentrations of IGF binding protein (BP)-3 increased with age (P < .05), but levels of IGFBP-2, a 30-kDa IGFBP, and IGFBP-4 were unchanged (P > .10). The pST challenge reduced plasma urea nitrogen at all ages, but the magnitude of the response was less in younger pigs compared with the maximum response in pigs greater than 30 kg BW (63 d of age). The IGF-I response to the pST challenge also increased from approximately 30% in young pigs to a threefold increase in older pigs. Regardless of age, concentrations of IGF-II were minimally affected by the pST challenge. Circulating levels of IGFBP-3 increased and IGFBP-2 levels decreased in response to the pST challenge, and the magnitude increased with age. The high nutritional status of pigs in the early phases of growth diminished the postnatal changes in IGF-I and IGFBP-2, but not IGF-II or IGFBP-3. Overall, data demonstrate a developmental regulation of the ST/IGF system, with pST challenges altering circulating concentrations of IGF-I, IGFBP-3, and IGFBP-2 coincident with changes in amino acid metabolism.

Seasonal response of ghrelin, growth hormone, and insulin-like growth factor I in the free-ranging Florida manatee (Trichechus manatus latirostris)

“…Most species exhibit an age related decline in GH while IGF-I generally increases with age (Comeau et al, 2008;Govoni et al, 2003;Harrell et al, 1999). Importantly, both GH and IGF-I are elevated during periods of rapid growth such as the early post-natal period as well as puberty (Govoni et al, 2003;Harrell et al, 1999).…”

Growth and development of adipose tissue and gut and related endocrine status during early growth in the pig: impact of low birth weight

“…GH receptors increase over the first 10 days of life in liver and IGF-I receptors decrease in the skeletal muscle and liver (Breier et al, 1989;Lee et al, 1993;Louveau et al, 1996;Schnoebelen-Combes et al, 1996). The somatotropic axis appears to be functional and responsive to GH administration in neonatal pigs although the responsiveness is reduced compared with older pigs (Harrell et al, 1999). Administration of exogenous GH results in a 20% to 30% increase in plasma IGF-I concentration and a 20% decrease in amino acid oxidation in 10-day-old piglets although the magnitude of IGF-I and amino acid responses is of 300% and 60%, respectively, in 123-day-old pigs.…”

Fed levels of amino acids are required for the somatotropin-induced increase in muscle protein synthesis

“…Treatment with pST also decreased circulating levels of PUN and increased plasma IGF-I. Treatment with pST has been demonstrated to reduce the activity of urea cycle enzymes (4) and hence urea synthesis (48), whereas increases in IGF-I plasma concentrations are well documented with treatment of pST (3,26,48).…”