Prolactin (PRL) and insulin-like growth factor (IGF)-I acting through class I cytokine receptors (R) regulate fetal growth and development and may also control adipose tissue deposition. After birth rapid growth of adipose tissue occurs as the newborn establishes independent feeding (Clarke et al. 1997). Plasma prolactin and IGF-I concentrations peak around the time of birth, coincident with the initiation of non-shivering thermogenesis in brown adipose tissue. Then over the first week of life abundance of the brown adipose tissue-specific uncoupling protein 1 decreases deposition and white adipose tissue is promoted. The following study aimed to determine whether the abundance of mRNA species for IGF and prolactin receptor (PRL-R) increase in adipose tissue over the first week of neonatal life.
Twelve twin-bearing Bluefaced Leicester X Swaledale ewes of known mating date and of similar body weight and parity were entered into the study. All ewes were allowed to give birth normally at term and were randomly assigned to one of the lamb sampling times (i.e. within 1 h of birth, 2, 4 and 7 days post-lambing (n = 3 per group)). The lambs were humanely killed by intravenous overdose of sodium pentobarbitone to allow perirenal adipose tissue sampling. All samples were weighed and snap-frozen in liquid nitrogen before being stored at -80 °C until analysis. Total RNA was extracted. All work performed was carried out in accordance with both national and local guidelines. The expression of mRNA species for IGF-I and both the long and short forms of PRL-R were examined by RT-PCR using specific oligonucleotide primers: IGF-I (Genbank M31735: forward 5Ô-CCC-ATC-TCC-CTG-GAT-TTC-TT-3Ô and reverse 5Ô-ACA-TCT-CCA-GCC-TCC-TCA-GA-3Ô product 401 bp), long form of PRL-R (Genbank AF041257: forward 5Ô-CCA-GAT-ACC-TAA-TGA-CTT-CCC-3Ô and reverse 5Ô-TCT-TCG-GAC-TTG-CCC-TTC-TCC-3Ô product 200 bp), short form of PRL-R (Genbank AF041977: forward 5Ô-CCA-GAT-ACC-TAA-TGA-CTT-CCC-3Ô and reverse 5Ô-GCC-CTT-CTA-TTA-AAA-CAC-AGA-3Ô product 229 bp). Results, in arbitrary units (a.u.; mean ± S.E.M.) are a ratio of an 18S rRNA internal control. Differences between ages were analysed using Kruskal-Wallis and Mann-Whitney U tests.
During the first week of life adipose tissue weight steadily increased (0.1 days: 20 ± 2.8 g, 2 days: 26 ± 3.7 g, 4 days: 41 ± 3.9 g, 7 days: 70 ± 9.1 g; P = 0.01).
IGF I mRNA abundance increased up to 2 days before reaching a plateau (0.1 days: 62.1 ± 12.7 a.u., 2 days: 117.7 ± 2.1 a.u., 4 days: 104.1 ± 2.2 a.u., 7 days: 117.1 ± 10.7 a.u.; P = 0.05), whereas IGF-II abundance remained unchanged.
In contrast, abundance of mRNA for both the long and short forms of PRL-R peaked at 4 days of age and then declined (e.g. short form: 0.1 days: 3.9 ± 1.4 a.u., 2 days: 4.6 ± 0.9 a.u., 4 days: 8.5 ± 2.7 a.u., 7 days: 5.3 ± 1.3 a.u.).
The peak in mRNA abundance for IGF-I and PRL-R between 2 and 4 days after birth may be important in enhancing adipose tissue up to 1 week after birth.