GENOTYPIC EFFECTS ON NOREPINEPHRINE-INDUCED CHANGES IN THERMOGENESIS,METABOLIC HORMONES, AND METABOLITES IN NEWBORN CALVES

Heat production was measured in newborn Angus-, Brahman-, and Tuli-sir ed calves born to Angus (n = 20) and Brahman (n = 26) dams, before (th ermoneutral metabolic rate, TMR) and after norepinephrine (NE) infusio n (peak metabolic rate, PMR), to assess genotypic effects on nonshiver ing thermogenesis in brown adipose tissue. Calves were fed pooled colo strum, fitted with jugular catheters, and placed in a temperature-cont rolled (37 degrees C) water immersion system. Heat production, determi ned by indirect calorimetry, and tympanic temperature were measured co ntinuously in calves from approximately 3 to 6 h of age. Blood samples were collected at birth and at 0, 5, 20, 40, 60, 80, 100, and 120 min relative to NE infusion (35 mu g.min(-1).kg BW-1 for 4 min), and plas ma was analyzed for metabolites (glucose, NEFA, and urea nitrogen [PUN ]) and hormones (cortisol, triiodothyronine [T-3] and thyroxine [T-4]) . Weight-specific TMR (cal.min(-1).kg(-1)) was not affected by breed o f sire or dam, although weight-specific PMR (cal.min(-1).kg(-1)) was l ower (P < .01) in Brahman-sired calves than in Angus- or Tuli-sired ca lves and was lower (P < .001) in calves born to Brahman rather than An gus dams. The reduction in weight-specific PMR due to the maternal Bra hman influence was sire-breed dependent, and the reduction was largest (P < .01) for Tuli-sired (34.3%), intermediate (P < .05) for Brahman- sired (15.1%), and lowest (P > .25) for Angus-sired calves (4.1%). The PMR:TMR ratio was 1.80 and 2.21 +/- .06 in calves born to Brahman and Angus dams, respectively. Peak tympanic temperature was .6 degrees C lower (P < .01) in calves born to Brahman rather than Angus dams. At b irth, plasma NEFA concentrations were higher (P < .001) and glucose te nded (P = .13) to be higher in calves born to Brahman rather than Angu s dams. Cortisol, T-3, and T-4 concentrations at birth were higher in calves born to Brahman dams than in those born to Angus dams. These re sults suggest that calves born to Brahman dams may have less thermogen ically active brown adipose tissue than calves born to Angus darns, wh ich may contribute to the relative cold intolerance of calves with Bos indicus inheritance.