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 D71 14, on thermoregulation in near-term lambs delivered by cesarean secti on. 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 Zenec a D7114 (10 mg . kg body weight(-1)) dissolved in 20 mt of milk, or mi lk 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 r emaining lambs, colonic temperature failed to return to normothermic v alues, plateauing at 34.3 degrees C. All control lambs were observed t o shiver throughout the study, but after Zeneca D7114 treatment 7 out of 10 normothermic lambs stopped shivering, and plateau colonic temper ature was 0.8 degrees C higher. Hypothermic beta(3)-agonist-treated la mbs had significantly lower rates of heat production, breathing freque ncy, and plasma triiodothyronine and cortisol concentrations than norm othermic lambs. The level of GDP binding and norepinephrine content of brown adipose tissue (BAT) sampled from hypothermic beta(3)-agonist-t reated lambs was significantly lower than in normothermic lambs, There was no difference in GDP binding in BAT between control and Zeneca D7 114-treated groups, but the Hb content was higher in the latter group. It is concluded that administration of Zeneca D7114 to euthyroid lamb s enhances their ability to thermoregulate and restore colonic tempera ture without altering the thermogenic activity of BAT. This response m ay be mediated by increasing blood flow to BAT and/or an improvement i n the animal's thermal efficiency (i.e. decreased heat loss) due to a reduced reliance on shivering thermogenesis.