Spontaneous activity, economy of activity, and resistance to diet-induced obesity in rats bred for high intrinsic aerobic capacity

Abstract

Though obesity is common, some people remain resistant to weight gain even in an obesogenic environment. The propensity to remain lean may be partly associated with high endurance capacity along with high spontaneous physical activity and the energy expenditure of activity, called non-exercise activity thermogenesis (NEAT). Previous studies have shown that high-capacity running rats (HCR) are lean compared to low-capacity runners (LCR), which are susceptible to cardiovascular disease and metabolic syndrome. Here, we examine the effect of diet on spontaneous activity and NEAT, as well as potential mechanisms underlying these traits, in rats selectively bred for high or low intrinsic aerobic endurance capacity. Compared to LCR, HCR were resistant to the sizeable increases in body mass and fat mass induced by a high-fat diet; HCR also had lower levels of circulating leptin. HCR were consistently more active than LCR, and had lower fuel economy of activity, regardless of diet. Nonetheless, both HCR and LCR showed a similar decrease in daily activity levels after high-fat feeding, as well as decreases in hypothalamic orexin-A content. The HCR were more sensitive to the NEAT-activating effects of intra-paraventricular orexin-A compared to LCR, especially after high-fat feeding. Lastly, levels of cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C) in the skeletal muscle of HCR were consistently higher than LCR, and the high-fat diet decreased skeletal muscle PEPCK-C in both groups of rats. Differences in muscle PEPCK were not secondary to the differing amount of activity. This suggests the possibility that intrinsic differences in physical activity levels may originate at the level of the skeletal muscle, which could alter brain responsiveness to neuropeptides and other factors that regulate spontaneous daily activity and NEAT.

Figure 1

Low-capacity runners (LCR, n=9 rats on each diet) were more sensitive to the deleterious effects of a high-fat diet than high-capacity runners (HCR; n=9 on rats the high-fat diet, 8 rats on the regular diet). (A) LCR gained significantly more weight over one month on the high-fat or regular diets than HCR; *different from chow-fed controls. (B) LCR on a high-fat diet gained significantly more body fat than HCR compared to controls; *greater than HCR on same diet; **greater than same group on a regular diet. (C) Whereas LCR on the high-fat diet ate significantly more kilocalories than LCR on a regular diet for several weeks, HCR on the high-fat diet returned to baseline consumption within one week. (D) LCR had significantly higher circulating leptin levels than HCR, regardless of the diet; no significant effect of diet on leptin was seen in HCR or LCR; *greater than HCR.

Figure 2

Activity and energy expenditure in high- and low-capacity running rats (HCR and LCR, n=10/group). (A) HCR were consistently more active than LCR regardless of diet, and the high-fat diet significantly decreased 24-hr physical activity levels similarly in HCR and LCR. *significant decrease compared to regular diet. **greater than LCR. (B) Daily energy expenditure according to body weight of HCR (gray regression lines) and LCR (black regression lines) on regular and high-fat diets (diet × selected line interaction, p=0.099). Body weight gain (C) and cumulative food intake (D) over the course of high-fat feeding in HCR and LCR.

Figure 3

Energy expenditure of activity (EEA) and resting energy expenditure (REE) were measured in high-capacity rats (HCR) and low-capacity rats (LCR). (A) Energy expenditure during resting and activity in HCR and LCR (collapsed across diet) according to body weight. Energy expenditure of activity was significantly greater in the HCR (*significantly different effect of selected line after body weight, the covariate, was accounted for), indicating that they had a lower fuel economy of activity than LCR. (B) The energy expenditure during resting in the treadmill and while waling at 7 meters/min (arrow indicates start time) in one HCR (gray line; 216 g) and one LCR (black line; 218g); final EEA = EEA+REE − REE. (C) On the high-fat diet, respiratory exchange rate (RER) was significantly lower in the HCR than HCR on the high-fat diet, indicating increased use of lipid as fuel during activity in these rats.

Figure 4

Intra-paraventricular (PVN) microinjections of orexin-A at increasing doses resulted in heightened physical activity in high- and low-capacity runners (HCR and LCR; n=5 and 9, respectively). (A) On a regular diet, HCR were slightly more sensitive to intra-PVN orexin-A than LCR (HCR>LCR at 0.125 nmoles, p<0.05). (B) After one month on a high-fat diet, HCR were significantly more responsive to the intra-PVN orexin-A. *HCR were more active than LCR at the same dose *(HCR>LCR at 0.5 nmoles, p<0.05; HCR>LCR at 1.0 nmoles 0.01). (C) In a separate study, orexin-A concentrations in the perifornical lateral hypothalamic region (PeFLH) were greater in rats on a regular diet compared to a high fat diet. *greater than rats on a high-fat diet. No differences were seen between high- and low-capacity runners (HCR and LCR; n=8 HCR and 9 LCR on the regular diet, 9 HCR and 9 LCR on the high-fat diet). (D) Atlas figures [plates 42/49 and 49/66 from (Paxinos and Watson, 2005)] representing the micropunches taken from brain slices.

Figure 5

Skeletal muscle cytosolic phosphoenolpyruvate carboxykinase (PEPCK-C) in high-and low-capacity running rats (HCR and LCR). (A) PEPCK-C was higher in the skeletal muscle of HCR compared to LCR and the high-fat diet significantly decrease PEPCK-C in the skeletal muscle in both groups (n=9 in HCR on both diets and LCR on the regular diet, n=8 in LCR on the high-fat diet). *greater than LCR; **greater than rats on a high-fat diet. (B) SIRT-1 was significantly higher in HCR on a control diet compared to all other groups (n=5 HCR and 5 LCR on the regular diet, 6 HCR and 6 LCR on the high-fat diet. *p<0.05). (C) Western blots illustrating the effects of selected line and diet on PEPCK-C and SIRT-1, compared to tubulin. (D) When rats were housed in cages of reduced size, this resulted in reduced activity in HCR but not LCR; **greater than reduced housing. (E) This reduction in activity did not alter skeletal muscle PEPCK-C in HCR, demonstrating that skeletal muscle PEPCK-C was secondary to heightened activity in HCR; *greater than LCR in the same housing condition, n=5 HCR and 5 LCR on the regular diet, 6 HCR and 6 LCR on the high-fat diet.