Relationships between Resting Metabolic Rate and Morphology in Lactating Mice: What Tissues are the Major Contributors to Resting Metabolism?

Abstract

We sought the relationship between resting metabolic rate (RMR) and gross tissue morphology in 59 lactating female MF1 mice. RMR was measured at peak lactation (Day 18 post partum) and the animals were then immediately killed and dissected into 18 separate components which were dried for 14 days at 60 °C. Individual variation in the RMR (CV) was 27%. The masses of the tissue components were similarly variable (CV ranging from 4.3% for the brain to around 150% for fat tissue). RMR was significantly related to overall body mass (r² = 0.458) with a scaling exponent of 1.55. Similar significant relationships were evident between masses of most (16/18) of the tissues and overall body mass. Accordingly there were significant relationships between RMR and all the tissue masses with the exception of those tissues poorly related to overall body mass (specifically the lungs, small intestine, brain and kidneys). When the shared variation due to total body mass was eliminated there were no significant links between residual RMR and residual mass of any tissue. Re-describing the morphology using principal components analysis revealed a link between a general size component (PC 1) and RMR (r² = 0.374) but this relationship disappeared when residual RMR was employed as the dependent variable. Despite other studies that indicate wide variations in the rates at which different tissues metabolise energy in vitro there was no indication in the present study that any particular component of the morphology was linked to high RMR in vivo.