INFLUENCE OF THYROTROPIN-RELEASING-HORMONE ON THERMOREGULATION IN NEWBORN LAMBS

This study examined the effect of thyrotrophin-releasing hormone (TRH) administration on thermoregulation in the newborn, Twin lambs were ei ther delivered near-term by caesarean section or born vaginally at ter m. Colonic temperature, O-2 consumption, CO2 production, breathing and heart rates, plus plasma thyroid hormone and nonesterified fatty acid (NEFA) concentrations and thermogenic activity (i.e. GDP binding) of brown adipose tissue (BAT) win measured. In caesarean section delivere d lambs colonic temperature decreased rapidly after birth, a response that was greater in the group designated for TRH treatment: in which c olonic temperature fell to below 36.0 degrees C at 80 min of life, pri er to TRH administration. At this age colonic temperature had been res tored to a mean of 38.70 degrees C in controls. TRH had no influence o n the composition or thermogenic activity of BAT. The incidence of shi vering was not influenced by TRH, but treated lambs maintained a highe r rate of Or consumption and ventilation compared with controls after colonic temperature had been restored to 38.56 degrees C. TRH appeared to promote fat oxidation as O-2 consumption remained unchanged and CO 2 production declined by a greater rate in treated lambs, resulting in a lower respiratory quotient compared to controls. Heart rate and pla sma concentrations of NEFA increased following TRH administration alth ough this did not result in values greater than controls. Normothermic lambs born vaginally had BAT with a greater thermogenic activity, hig her plasma thyroid hormone and NEFA concentrations compared with caesa rean section delivered lambs, but a thermogenic response was not obser ved to TRH despite a rise in thyroid hormone concentrations. in conclu sion, TRH can improve thermoregulation, an effect that could be linked to an increase in fat oxidation.