Effect of beta(3)-adrenergic agonist (Zeneca D7114) on thermoregulation in near-term lambs delivered by cesarean section

We investigated the effect of a beta(3)-adrenergic agonist, Zeneca D7114, on thermoregulation in near-term lambs delivered by cesarean section. Lambs were delivered into a cool ambient temperature of 15 degrees C, and between 60 and 80 min of life were given an oral dose of Zeneca D7114 (10 mg . kg body weight(-1)) dissolved in 20 mt of milk, or milk alone. During the first 0.5 h of life colonic temperature decreased in all lambs, and then increased to plateau levels (39.6-40.4 degrees C) after 120-150 min of life, in 19 out of 23 lambs studied. In the remaining lambs, colonic temperature failed to return to normothermic values, plateauing at 34.3 degrees C. All control lambs were observed to shiver throughout the study, but after Zeneca D7114 treatment 7 out of 10 normothermic lambs stopped shivering, and plateau colonic temperature was 0.8 degrees C higher. Hypothermic beta(3)-agonist-treated lambs had significantly lower rates of heat production, breathing frequency, and plasma triiodothyronine and cortisol concentrations than normothermic lambs. The level of GDP binding and norepinephrine content of brown adipose tissue (BAT) sampled from hypothermic beta(3)-agonist-treated lambs was significantly lower than in normothermic lambs, There was no difference in GDP binding in BAT between control and Zeneca D7114-treated groups, but the Hb content was higher in the latter group. It is concluded that administration of Zeneca D7114 to euthyroid lambs enhances their ability to thermoregulate and restore colonic temperature without altering the thermogenic activity of BAT. This response may be mediated by increasing blood flow to BAT and/or an improvement in the animal's thermal efficiency (i.e. decreased heat loss) due to a reduced reliance on shivering thermogenesis.