Influence of thyrotrophin-releasing hormone on thermoregulatory adaptationafter birth in near-term lambs delivered by caesarean section

We investigated the hypothesis that exogenous stimulation with thyrotrophin -releasing hormone (TRH) immediately prior to umbilical cord clamping can i mprove thermoregulatory adaptation after birth in near-term lambs delivered by Caesarean section. Lambs received an umbilical vein injection of saline +/- TRH (8 mu g) prior to cord clamping. The rate of change in colonic tem perature and oxygen consumption after birth were not influenced by TRH, but TRH-treated lambs exhibited a greater incidence of shivering compared with controls over the first hour of neonatal life. Two and a half hours after birth, TRH-treated lambs possessed brown adipose tissue (BAT) with a higher thermogenic activity (i.e. GDP binding to mitochondrial protein), but thei r BAT had a reduced DNA content and they had less hepatic glycogen than con trol lambs. TRH administration had no effect on iodothyronine 5' deiodinase activity in BAT and liver, or on plasma concentrations of total triiodothy ronine, thyroxine, cortisol or free fatty acids. Three TRH-treated but no c ontrol lambs, failed to establish continuous breathing, so tissues from the se treated lambs together with time-matched controls were sampled 25 min af ter birth. These 'non-surviving' TRH-treated lambs had very high plasma cat echolamine concentrations, but their lung weights were similar to controls. 'Surviving' TRH-treated lambs possessed lungs with less DNA than non-survi ving TRH-treated lambs. It is concluded that umbilical vein injection of TR H prior to umbilical cord clamping increases the recruitment of both shiver ing and non-shivering thermogenesis after birth.