Leptin is the product of the obese gene and is primarily secreted from adipose tissue. It is a 16 kDa protein whose abundance in ovine fetal adipose tissue is positively correlated with fetal weight (Yuen et al. 1999). However, it is not yet known whether similar relationships occur in early postnatal life. This study examined the neonatal ontogeny of leptin in both sheep and pigs.
Fifteen lambs (birth weight 5.0 ± 0.6 kg, mean ± S.E.M.) born to Bluefaced Leciester cross Swaledale ewes and ten piglets (birth weight 1.5 ± 0.1 kg, mean ± S.E.M.) born to Meishan cross Large White sows were entered into the study. Birth weights were measured and the animals randomly assigned to each sampling date, i.e. for the lambs within 1 h of birth, 1, 2, 4, 7 and 14 days after birth, and for the piglets 1, 9, 12 and 16 days of life. The neonates remained with their mothers and growth rates were recorded until the animals were humanely killed with an overdose of barbiturate (100 mg kg-1 pentobarbitone sodium: Euthatal) to enable adipose tissue sampling to take place. Experiments were carried out in accordance with UK legislation. Total RNA was isolated from the adipose tissue and leptin mRNA abundance was examined by RT-PCR, using oligonucleotide primers homologous to both species (forward 5Ì-CACCAAAACCC-TCATCAAGACG-3Ì and reverse 5Ì-ACATTTCTGG-AAGGCAGACTGG-3Ì), the PCR conditions were optimised for each species. Leptin results are expressed in arbitrary units as a ratio of an 18S rRNA internal control. Relationships were investigated using regression analysis.
Growth rates were similar (0.15-0.25 kg day-1) in both the lambs and piglets. The abundance of leptin mRNA increased from birth (41.7 ± 6.5, mean ± S.E.M.) to 7 days (73.0 ± 10.0, mean ± S.E.M.) of life and was positively correlated with body weight during this period in lambs (r 2 = 83.6 %) but not piglets (r 2 = 0 %). This relationship did not exist after 7 days of age in either species and thereafter the expression of leptin was similar, irrespective of sampling time.
In conclusion, the relationship between body weight and leptin mRNA during the first week of life appears to be species dependent. This difference may be partly attributed to the higher growth rates, relative to birth weight, observed in pigs compared with sheep.
The authors would like to thank Cotswold Pig Development Company for their support in this study. J.C.L. is funded by a Wye College PhD studentship. This study is also part funded by the Kent County Agricultural Society.