DIFFERENTIAL RESPONSE TO THE 3-ADRENERGIC AGONIST CIMATEROL IN MICE SELECTED FOR RAPID GAIN AND UNSELECTED CONTROLS 1'2

Male mice selected for rapid 3 to 6 wk postweaning gain (M16) and unselected controls (ICR) were ad libitum fed a stock diet containing 0, 50 or 200 ppm cimaterol, a 3-agonist, from 4 to 7 or 4 to 10 wk of age. Mortality rate was higher in M16 than in ICR mice fed cimaterol (12.5 vs 1.3%; P<.01). No mortalities occurred in either line fed the control diet. Line M16 exceeded (P<.01) ICR in growth rate, feed intake, feed efficiency and lean index. Line  cimaterol level interactions (P<.O1) were found for the first three of these traits, although cimaterol level did not change line ranking. Epididymal fat as a percentage of empty body weight decreased at a faster rate in M16 than in ICR as cimaterol level increased. At 0 and 50 ppm, M16 exceeded ICR (P<,05), but at 200 ppm there was no line difference (P>.05). Line M16 exceeded (P<.05) ICR in blood glucose (5%), nonesterified fatty acids (4%) and lactate at 7 wk (9%), but lactate was higher in ICR at 10 wk (13%). Lines were not different in blood urea-N. Compared to zero cimaterol level, at 50 and 220 ppm glucose decreased (14% and 23%; P<.05), nonesterified fatty acids decreased (3% and 29%; P<.05), lactate increased (9% and 11%; P<.05) and blood urea-N increased (3% and 16%; P<.05). There were no line  cimaterol level interactions for blood metabolites. Differences in mortality rate, growth, feed consumption, feed efficiency and epididymal fat pad percentage between the high-growth and control lines in response to cimaterol may reflect genetic differences in mecha- nisrns of metabolic regulation.