Phenotypic flexibility of traits related to energy acquisition in mice divergently selected for basal metabolic rate (BMR)

Abstract

SUMMARYTheoretical considerations suggest that one of the main factors determining phenotypic flexibility of the digestive system is the size (mass) of internal organs. To test this, we used mice from two lines selected for high and low levels of basal metabolic rate (BMR). Mice with higher BMRs also have larger internal organs and higher daily food consumption (C) under non-stressful conditions. We exposed animals from both lines to a sudden cold exposure by transferring them (without prior acclimation) from an ambient temperature of 23°C to 5°C. Cold exposure elicited a twofold increase in C and a 25% reduction of apparent digestive efficiency. For the same body mass-corrected C, small intestine, kidneys, heart and liver of cold-exposed low-BMR mice were smaller than those of the high-BMR line. Therefore, the internal organs of low-BMR animals were burdened with substantially higher metabolic loads (defined as C or digestible food intake per total mass of a particular organ). The mass-specific activity of citrate synthase (CS) in the liver and kidneys (but not heart) was also lower in the low-BMR mice. The magnitude of phenotypic flexibility of internal organ size and CS activity was strictly proportional to the organ mass (in the case of kidneys and liver, also mass-specific CS activity) prior to an increased energy demand. Thus, phenotypic flexibility had additive rather than multiplicative dynamics. Our results also suggest that variation in BMR positively correlates with the magnitude of an immediate spare capacity that fuels the initial response of internal organs to a sudden metabolic stress.

Milk output and composition in mice divergently selected for BMR

“…Measuring the size/weight of the internal organs involved in the lactation process has been considered a simple, yet effective proxy for estimating the functional capacity of these organs in a range of species (Hurley, 2001;Karasov and McWilliams, 2005;Bauchinger et al, 2009). Our selection lines differ not only in their BMR but also in the mass of viscera essential for nutrient and energy turnover (intestines, liver, kidneys and heart); all are significantly heavier in non-reproducing H-BMR mice (Książek et al, 2004;Książek et al, 2009;Brzęk et al, 2007;Gębczyński and Konarzewski, 2009a). In this study, we showed that differences in organ masses are preserved after the first as well as the second reproductive bout, with H-BMR females having heavier viscera (particularly intestines and liver; Table 1).…”

“…The lowest oxygen concentration that did not change by more than 0.01% for at least 4 min was defined as BMR. No more than three individuals per family with the highest (H-BMR line) and lowest (L-BMR line) body-mass corrected BMR were chosen as progenitors for further selection and mated outside their families (for details, see Książek et al, 2004;Książek et al, 2009;Gębczyński and Konarzewski, 2009b).…”

Metabolic correlates of selection on aerobic capacity in laboratory mice:a test of the model for the evolution of endothermy

“…of skeletal muscles and internal organs, such as liver, kidneys and the heart (Even et al, 2001;Legerlotz et al, 2008;Ksia˛zek et al, 2009). Furthermore, the masses of internal organs are heritable (Ksia˛zek et al, 2004 and references therein) and, therefore, are likely to respond to selection.…”

What causes intraspecific variation in resting metabolic rate and what are its ecological consequences?

“…Where resources are abundant or predictable, individuals with relatively high RMRs can therefore exhibit faster growth rates than low-RMR individuals (table 1). They may also have a greater physiological capacity for growth, as they can digest and process meals faster [90] and have higher digestive efficiency [31,91]. This may be advantageous in highly seasonal environments (e.g.…”