Andrzej K. Gębczyński

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

SUMMARY According to the aerobic capacity model of the evolution of endothermy, high levels of basal/resting metabolic rate (BMR/RMR) underlying endothermy have evolved as a correlated response to selection for high rates of aerobic metabolism (VO2max). To test the model we studied metabolic, behavioural and morphological correlates of replicated selection on maximum body mass-corrected metabolism elicited by swimming (VO2swim) in male laboratory mice. While 10 generations of selection did not change body mass, it resulted in a 12% difference in VO2swim between mice of selected and control line types and significant, correlated responses in maximum metabolic rates elicited by exposure to cold in a helium–oxygen atmosphere (VO2He), and during forced running on a motorized treadmill (VO2run). Selected and control lines also significantly differed with respect to duration of running (a measure of stamina, trun), and the distance run to exhaustion (de). However, the selection protocol did not result in elevated BMR and voluntary activity. Higher VO2max in selected animals was positively correlated with higher masses of gastrocnemius muscles and heart but not of other visceral organs (intestine, stomach, liver and kidneys). These findings provide a mechanistic explanation for the lack of correlation between basal and maximal metabolic rates in selected mice. Overall, our study does not support the assumptions of the aerobic capacity model for the evolution of endothermy.

Basal metabolic rate is positively correlated with parental investment in laboratory mice

The assimilation capacity (AC) hypothesis for the evolution of endothermy predicts that the maternal basal metabolic rate (BMR) should be positively correlated with the capacity for parental investment. In this study, we provide a unique test of the AC model based on mice from a long-term selection experiment designed to produce divergent levels of BMR. By constructing experimental families with cross-fostered litters, we were able to control for the effect of the mother as well as the type of pup based on the selected lines. We found that mothers with genetically determined high levels of BMR were characterized by higher parental investment capacity, measured as the offspring growth rate. We also found higher food consumption and heavier visceral organs in the females with high BMR. These findings suggested that the high-BMR females have higher energy acquisition abilities. When the effect of the line type of a foster mother was controlled, the pup line type significantly affected the growth rate only in the first week of life, with young from the high-BMR line type growing more rapidly. Our results support the predictions of the AC model.

Effects of oxygen availability on maximum aerobic performance in Mus musculus selected for basal metabolic rate or aerobic capacity

SUMMARY Maximum aerobic metabolism cannot increase indefinitely in response to demands for ATP production and, therefore, must be constrained by one (or many) of the steps of the oxygen transport and utilization pathways. To elucidate those constraints we compared peak metabolic rate elicited by running () in hypoxia (14% O2), normoxia (21% O2) and hyperoxia (30% O2) of laboratory mice divergently selected for low and high basal metabolic rate (L-BMR and H-BMR, respectively), mice selected for maximum metabolic rate elicited by swimming () and mice from unselected lines. In all line types was lowest in hypoxia, intermediate in normoxia and highest in hyperoxia, which suggests a ‘central’ limitation of oxygen uptake or delivery instead of a limit set by cellular oxidative capacity. However, the existence of a common central limitation is not in agreement with our earlier studies showing that selection on high (in contrast to selection on high BMR) resulted in considerably higher oxygen consumption during cold exposure in a He–O2 atmosphere than . Likewise, between-line-type differences in heart mass and blood parameters are inconsistent with the notion of central limitation. Although responses of to hypoxia were similar across different selection regimens, the selection lines showed contrasting responses under hyperoxic conditions. in the H-BMR line type was highest, suggesting that selection on high BMR led to increased cellular oxidative capacity. Overall, between-line-type differences in the effect of the oxygen partial pressure on and in the components of O2 flux pathways are incompatible with the notion of symmorphosis. Our results suggest that constraints on are context dependent and determined by interactions between the central and peripheral organs and tissues involved in O2 delivery.

Patterns of ultradian rhythms of activity and metabolic rate in relation to average daily energy expenditure in root voles

Microtine species are known to have an ultradian pattern of activity. Earlier study suggested that period of activity oscillations decreased after magnification of the average energy expenditure. Moreover, the lack of daily variation in nonshivering thermogenesis capacity suggests that the level of metabolic rate (MR) depends mainly on animal activity. Thus, (1) one could expect that ultradian oscillations of MR and activity were similarly dependent on the average level of energy expenditure. Moreover, I expected that animals at lower ambient temperature (Ta) should minimize their energy expenditure and the time of exposure to low Ta outside the nest. Thus (2) their mean activity near a feeder does not increase together with average level of energy expenditure. I tested these hypotheses using root voles Microtus oeconomus Pallas, 1776 acclimated to Ta of 20°C (12L∶12D). The MR and activity both inside (An) and outside the nest (Af), were measured continuously at Ta of 10 and 20°C. Average oxygen consumption differed between Tas. The period of MR and Af rhythm was shorter at lower Ta, but the period of An did not differ significantly. The level of energy expenditure did not affect mean Af. Close correlation between MR and Af length period suggests that oscillations of MR during the day are affected by activity related to feeding, rather than low cost An.

Effect of Reproduction on the Consistency of the Between–Line Type Divergence in Laboratory Mice Selected on Basal Metabolic Rate

Artificial selection experiments are an effective tool for testing evolutionary hypotheses, because they allow one to separate genetic and environmental variances of the phenotype. However, it is unclear whether trait divergence typically selected early in life persists over an animal’s life and altered physiological states, such as reproduction. Here we analyzed the long-term consistency of the between–line type divergence in basal metabolic rate (BMR) selected at 12 wk of age in laboratory mice. We measured BMR in nonreproducing and reproducing females at the age of 22 wk and then at 27 wk of age. Our results show that within both the reproducing group and the control group, the between–line type separation in BMR is consistently retained over time and reproductive status. Metabolically active internal organs (heart, liver, kidneys, and small intestine) also consistently differed in size between the two line types with no significant long-term effect of reproduction. The observed consistency of the between–line type divergence in BMR suggests the existence of the persistent effect of the selection on metabolic traits applied early in life. Moreover, BMR variation achieved by means of artificial selection is considerably higher than that found in natural/unmanipulated populations. The latter may therefore be characterized by insufficient variance to statistically resolve correlations involving BMR.

Metabolic risk factors in mice divergently selected for BMR fed high fat and high carb diets

Factors affecting contribution of spontaneous physical activity (SPA; activity associated with everyday tasks) to energy balance of humans are not well understood, as it is not clear whether low activity is related to dietary habits, precedes obesity or is a result of thereof. In particular, human studies on SPA and basal metabolic rates (BMR, accounting for >50% of human energy budget) and their associations with diet composition, metabolic thrift and obesity are equivocal. To clarify these ambiguities we used a unique animal model—mice selected for divergent BMR rates (the H-BMR and L-BMR line type) presenting a 50% between-line type difference in the primary selected trait. Males of each line type were divided into three groups and fed either a high fat, high carb or a control diet. They then spent 4 months in individual cages under conditions emulating human “sedentary lifestyle”, with SPA followed every month and measurements of metabolic risk indicators (body fat mass %, blood lipid profile, fasting blood glucose levels and oxidative damage in the livers, kidneys and hearts) taken at the end of study. Mice with genetically determined high BMR assimilated more energy and had higher SPA irrespective of type of diet. H-BMR individuals were characterized by lower dry body fat mass %, better lipid profile and lower fasting blood glucose levels, but higher oxidative damage in the livers and hearts. Genetically determined high BMR may be a protective factor against diet-induced obesity and most of the metabolic syndrome indicators. Elevated spontaneous activity is correlated with high BMR, and constitutes an important factor affecting individual capability to sustain energy balance even under energy dense diets.

Milk output and composition in mice divergently selected for basal metabolic rate

From an evolutionary perspective, the high basal metabolic rate (BMR) of homeotherms is hypothesised to be a by-product of natural selection for effective parental care. We estimated daily milk output during two consecutive lactation bouts in mice divergently selected for high/low BMR and applied a cross-fostered design to control for potential differences in the between-line suckling abilities of nursed juveniles. Additionally, to remedy the potential limitation imposed by the ability of mother mice to dissipate excess heat, we exposed them to an ambient temperature of 17°C during the most energetically demanding second week of lactation. We found that the mice selected for high BMR produced significantly more milk in a 24 h period in both reproductive bouts. The milk samples obtained from the high BMR females had lower protein concentration and did not differ with respect to fat. However, the concentration of the primary milk carbohydrate – lactose – was higher. Although all the above between-line differences were statistically significant, their magnitude was too small to unambiguously ascribe them as stemming from a positive genetic correlation between the physiological traits underlying BMR and lactation performance. Nevertheless, our study lends such support at least at the level of phenotypic variation.

Selection for high aerobic capacity has no protective effect against obesity in laboratory mice

Aerobic capacity (VO2max measured during intensive physical exercise) both trained and intrinsic (i.e. genetically determined) has recently been deemed a good predictor of cardiometabolic risks. However, the underlying mechanisms linking VO2max and health risk factors are not entirely clear, as it seems that not VO2max per se, but rather some correlated traits, like spontaneous physical activity (SPA) are responsible for sustaining the lean phenotype. Here we investigated the link between genetically determined aerobic capacity, SPA and resistance to diet-induced health risks using replicated lines of mice selected for high aerobic capacity during swimming in mid-cold water (25°C) and Randomly Bred control mice. After four months of consumption of the western type HFat and HCarb diets and no forced nor voluntary training, we found no evidence of protective effects of intrinsic high VO2max. The Selected mice displayed similar levels of blood glucose, cholesterol, triglycerides and body fat as the Random Bred control animals. Most notably we found no correlation between VO2max and SPA levels. Our results therefore call into question the ubiquity of VO2max as a predictor of metabolic health and leanness, at least in animal models.

Long-Term Trait Consistency in Mice Selected for Swim-Induced High Aerobic Capacity

The majority of studies show that metabolic rates are usually repeatable at the individual level, although their repeatabilities tend to decline with time and to be strongly affected by physiological changes. Changes in individual repeatabilities may therefore affect putative differences between experimental groups or populations. This problem is particularly relevant to artificial selection experiments that apply the selection protocol at early life stages, running the risk of a poor correlation of the trait with itself throughout the life cycle of individuals. Moreover, significant physiological changes (e.g., induced by reproduction) may affect traits under selection and therefore their postreproductive differentiation between selected lines. Here, using a unique animal model—mice from four lines selected for V˙O2max during swimming in 25°C water and four random-bred control (reference) lines—we analyzed the long-term consistency of aerobic capacity as well as postswim hypothermia in primiparous and nonreproducing females at 12, 25, and 29 wk of age. Our results show that significant between–line type divergence in V˙O2max and hypothermia persists over time and is only weakly affected by past reproduction. Furthermore, both traits are also repeatable within lines at the individual level. More generally, our results suggest that past reproduction events are unlikely to significantly affect between-population and between-individual differences in V˙O2max and related traits.