EFFECT OF REARING TEMPERATURE ON PERIRENAL ADIPOSE-TISSUE DEVELOPMENTAND THERMOREGULATION FOLLOWING METHIMAZOLE TREATMENT OF POSTNATAL LAMBS

This study examined the effect of ambient temperature on perirenal adi pose tissue development and thermoregulation over the first month of p ostnatal life in lambs treated with a drug that reduces thyroid hormon e synthesis (methimazole; at a dose of 50 mg day(-1) (kg body weight)( -1)). Twin lambs were hand-reared at a fixed level of nutrition in eit her a warm (WR; 25 degrees C) or cool (CR; 10-15 degrees C) ambient te mperature. Oxygen consumption, heart rate and colonic temperature were measured during non-rapid eye movement sleep in different ambient tem peratures (6, 15, 25 and 34-36 degrees C) at 7, 14 and 28 days of age. Plasma thyroid hormone concentrations decreased with postnatal age an d were higher in CR than in WR lambs. AII lambs increased plasma thyro id hormone concentrations and O-2 consumption during cold exposure but this was not associated with a mean increase in colonic temperature i n WR lambs at 7 or 28 days. Colonic temperature increased with the ons et of panting al all ages when lambs were exposed to 34-36 degrees C, a response that was greatest at 28 days in CR lambs. An increase in co lonic temperature with age was observed in CR but not WR lambs. Heart rate declined with age only in the WR group. At 8 and 29 days there we re no significant differences between WR and CR groups in the occurren ce of shivering or in the thermogenic activity (i.e. GDP binding to mi tochondria) of perirenal adipose tissue. At 29 days WR lambs possessed more adipose tissue with a higher lipid and DNA content. It is conclu ded that modest changes in rearing temperature can have a large influe nce on the control of body temperature and thyroid hormone response to methimazole treatment. CR lambs are able to maintain higher plasma th yroid hormone concentrations and exhibit improved thermoregulation com pared with WR lambs without any detectable differences in brown adipos e tissue function.