ADIPOSE-TISSUE DEVELOPMENT DURING EARLY POSTNATAL LIFE IN EWE-REARED LAMBS

This study examines the precise time course that brown adipose tissue (BAT) takes to adopt the characteristics of white adipose tissue in po stnatal lambs. Perirenal adipose tissue was sampled from ewe-reared la mbs within 1 h of birth and at 1, 2, 4, 7, 14, 21 and 30 days of age a nd analysed for the amount of mRNA for uncoupling protein (UCP), the a mount and activity of UCP, and protein, mitochondrial protein and lipi d content. This was combined with measurements of colonic temperature and jugular venous plasma concentrations of thyroid hormones and insul inlike growth factor-1 (IGF-1). Over the first 4-7 days of age, large quantities of UCP mRNA were associated with a peak in plasma triiodoth yronine concentration at 2 days of age followed by a maximal amount an d activity of UCP at 4 days and a basal colonic temperature of 39.3 de grees C. Between 7 and 30 days there was a large increase in lipid dep osition as the amount and activity of UCP and the amount of UCP mRNA d eclined to basal values and colonic temperature was maintained at 40 d egrees C. A significant positive relationship between perirenal adipos e tissue lipid content and plasma IGF-1 concentration was observed thr oughout the study period. It is concluded that ovine adipose tissue ma turation occurs in two distinct phases over the first month of life. T he precise rime scale of this process could be regulated in part by th e lamb's body temperature which determines whether adipose tissue is r equired for heat production (i.e. BAT) or as an endogenous energy sour ce (i.e. white adipose tissue).