Serum leptin in neonatal lambs is associated with temperature, plasma lipids and metabolites

In this study we investigated changes of serum leptin in 74 newborn lambs and associations with environmental temperature (from - 8°C to + 25°C), body temperature, and concentrations of plasma lipids, 3-beta-hydroxybutyric acid and blood glucose. A leptin radioimmunoassay was established, using an antiserum (rabbit) produced against a partial sequence of ovine leptin (31-44). Before measurement, serum samples were denatured. The sensitivity of the assay was 0.4 µg l(-1) and intra- and interassay coefficients of variation were 5.1% and 2.5%, respectively. Blood samples were collected immediately after birth up to 24 h postnatally (pn). Median leptin concentrations at birth and 24 h pn were 20.9 and 52.7 µg l(-1), respectively. Because of non-normal distribution, leptin concentrations were converted to log(leptin) before further statistical processing. The change in log(leptin) during the first 24 h was highly significant (p<0.0001). Correlation analysis showed significant associations between serum leptin and the following variables: environmental temperature 24 h pn (r=0.34, p<0.005), log(plasma triglycerides) 24 h pn (r=0.50, p<0.001), log(plasma 3-beta-hydroxybutyric acid) 24 h pn (r=-0.50, p<0.001), blood glucose 6 h pn (r=0.43, p<0.001) and plasma cholesterol 12 h pn (r=0.38, p=0.001). We conclude that this radioimmunoassay is suited to measure total serum ovine leptin and that total leptin is already regulated in the very early postnatal phase. Leptin is increased at higher environmental temperatures, consistent with leptin's suppressive effect on energy expenditure and appetite. Furthermore, leptin levels are associated with plasma concentrations of lipids and lipid metabolites.