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Selection was practiced for high (MH) and low (ML) heat loss using direct calorimetry to create lines of mice differing in feed intake per unit BW (FI/BW). Selection occurred for both criteria in each of 3 replications (R); an unselected control (MC) was also maintained in each replicate. A total of 25 generations of selection was practiced; the 9 lines are presently maintained without selection. As a percentage of MC, mice of MH and ML lines differed in heat loss and FI/BW by 56 and 34%, respectively, when selection ceased. The 218 male mice measured in the present study came from R1-R3 and multiple generations. The purpose of this study was to determine hepatic mitochondrial efficiency in mice at maintenance across the genetic lines. Additionally, line comparisons of BW, FI/BW, liver weight per BW (LW/BW), and body composition were analyzed. Following measurement of FI and BW over a 2-wk period, mice were euthanized, and their livers were extracted and weighed. Livers were homogenized and mitochondria were isolated in buffer for measurement of oxygen consumption, and hence mitochondrial activity, using a Clark-type oxygen electrode. Mitochondria of ML mice expressed greater (P < 0.03) respiratory control ratio by 22% than that for MH mice, revealing greater degree of uncoupling, thus less efficient electron transport in MH mice. There were no line differences (P > 0.99) in ADP:Oxygen ratio. There was no difference in BW between MH and ML mice (P > 0.35), however MC mice were heavier (P < 0.02). Difference in FI/BW (P < 0.001) was large with MH mice consuming 34% more than ML mice. No line differences in LW/BW were found. Selection tended to affect on fat percentage (P < 0.07) with ML mice being 19% fatter than MH mice. RCR differed between mice of MH and ML lines as expected, however regression analysis revealed that it explained essentially no variation in feed intake per BW.
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