BROWN ADIPOSE-TISSUE AND LIVER DEVELOPMENT DURING EARLY POSTNATAL LIFE IN HAND-REARED AND EWE-REARED LAMBS

This study examined the effects of modest changes in ambient temperatu re in hand-reared lambs (experiment one) and in ewe-reared lambs (expe riment two). Lambs were killed at either 8 or 31 days of age and perir enal adipose tissue was indentified as being brown adipose tissue (BAT ) from measurements of thermogenic activity (i.e. GDP binding to uncou pling protein in isolated mitochondria) or thermogenic capacity (i.e. detection of uncoupling protein by immunoblotting). In addition, type I and II iodothyronine 5'monodeiodonase (5'MDI) activities were assaye d in perirenal adipose tissue, plus type I 5'MDI activity in liver. Pl asma samples were also taken for measurements of glucose, lactate, ins ulin, triiodothyronine (T-3) and thyroxine (T-4) concentrations. In ex periment one, lambs were hand-reared at either warm (WR; 25 degrees C) or cool (CR; 10-15 degrees C) ambient temperatures. Mean growth rate over the first 8 days of life in CR lambs was 88 g/day and increased t o 128 g/day over the first month of life. Growth rate in WR lambs was constant at 141 g/day. Thermogenic activity of BAT was significantly h igher in CR than WR lambs, but total weight and tissue lipid content o f perirenal adipose tissue were significantly lower in the CR group. I n both WR and CR lambs, the thermogenic activity of BAT fell by an ave rage of 71% between 8 and 31 days. At 31 days of age, uncoupling prote in in mitochondria could be detected only by immunoblotting in adipose tissue sampled from CR lambs. There was no effect of ambient temperat ure on type I or type II 5'MDI activity in BAT or liver; it decreased in adipose but not liver tissue between 8 and 31 days of age. The plas ma concentrations of glucose, insulin and T-3 tended to decline with a ge in CR but not in WR lambs. In ewe-reared lambs perirenal adipose ti ssue weight and tissue lipid content more than doubled between 8 and 3 1 days of age, but the level of GDP binding decreased from 85 to 5 pmo l/mg mitochondrial protein over this period. Liver weight increased by 55% between 8 and 31 days of age, but hepatic 5'MDI activity remained unchanged. The plasma concentrations of T-3, T-4 and lactate, but not glucose or insulin, increased between 8 and 31 days of age. It is con cluded that hand-rearing lambs at a cool ambient temperature significa ntly delays postnatal development, to the extent that BAT characterist ics are retained. Ewe-rearing lambs enhances the rate at which BAT ado pts the characteristics of white adipose tissue, and it prevents the p ostnatal decline in plasma concentrations of thyroid hormones.