Sex hormones' regulation of rodent physical activity: a review

There is a large body of emerging literature suggesting that physical activity is regulated to a varying extent by biological factors. Available animal data strongly suggests that there is a differential regulation of physical activity by sex and that the majority of this differential regulation is mediated by estrogen/testosterone pathways with females in many animal species having higher daily activity levels than males. The purpose of this manuscript is to review the mechanisms by which estrogen, progesterone, and testosterone affect the regulation of physical daily activity. This review lays the foundation for future investigations in humans as well as discussions about relative disease risk mediated by differential biological regulation of physical activity by sex.

Quantitative trait loci for physical activity traits in mice

The genomic locations and identities of the genes that regulate voluntary physical activity are presently unknown. The purpose of this study was to search for quantitative trait loci (QTL) that are linked with daily mouse running wheel distance, duration, and speed of exercise. F(2) animals (n = 310) derived from high active C57L/J and low active C3H/HeJ inbred strains were phenotyped for 21 days. After phenotyping, genotyping with a fully informative single-nucleotide polymorphism panel with an average intermarker interval of 13.7 cM was used. On all three activity indexes, sex and strain were significant factors, with the F(2) animals similar to the high active C57L/J mice in both daily exercise distance and duration of exercise. In the F(2) cohort, female mice ran significantly farther, longer, and faster than male mice. QTL analysis revealed no sex-specific QTL but at the 5% experimentwise significance level did identify one QTL for duration, one QTL for distance, and two QTL for speed. The QTL for duration (DUR13.1) and distance (DIST13.1) colocalized with the QTL for speed (SPD13.1). Each of these QTL accounted for approximately 6% of the phenotypic variance, whereas SPD9.1 (chromosome 9, 7 cM) accounted for 11.3% of the phenotypic variation. DUR13.1, DIST13.1, SPD13.1, and SPD9.1 were subsequently replicated by haplotype association mapping. The results of this study suggest a genetic basis of voluntary activity in mice and provide a foundation for future candidate gene studies.

Quantitative trait loci associated with maximal exercise endurance in mice

The role of genetics in the determination of maximal exercise endurance is unclear. Six- to nine-week-old F2 mice (n = 99; 60 female, 39 male), derived from an intercross of two inbred strains that had previously been phenotyped as having high maximal exercise endurance (Balb/cJ) and low maximal exercise endurance (DBA/2J), were treadmill tested to estimate exercise endurance. Selective genotyping of the F2 cohort (n = 12 high exercise endurance; n = 12 low exercise endurance) identified a significant quantitative trait locus (QTL) on chromosome X (53.7 cM, DXMit121) in the entire cohort and a suggestive QTL on chromosome 8 (36.1 cM, D8Mit359) in the female mice. Fine mapping with the entire F2 cohort and additional informative markers confirmed and narrowed the QTLs. The chromosome 8 QTL (EE8(F)) is homologous with two suggestive human QTLs and one significant rat QTL previously linked with exercise endurance. No effect of sex (P = 0.33) or body weight (P = 0.79) on exercise endurance was found in the F2 cohort. These data indicate that genetic factors in distinct chromosomal regions may affect maximal exercise endurance in the inbred mouse. Whereas multiple genes are located in the identified QTL that could functionally affect exercise endurance, this study serves as a foundation for further investigations delineating the identity of genetic factors influencing maximum exercise endurance.

Influence of Age of Exposure to a Running Wheel on Activity in Inbred Mice

PURPOSE

It is currently unknown whether the age of wheel exposure influences running wheel activity in mice. The purpose of this study was to determine whether the age at which a running wheel was introduced affected running wheel activity for a subsequent 15-wk period.

METHODS

Twenty female C57Bl/6J mice (age 7 wk) were assigned to one of four experimental groups. Group 1 received a running wheel at 7 wk of age. Thereafter, groups 2, 3, and 4 received running wheels at 10, 13, and 16 wk of age, respectively. Daily running wheel activity (duration, distance, and velocity) was recorded from the time of running wheel exposure until 30 wk of age.

RESULTS

A repeated-measures MANOVA found significant differences between groups for distance (P = 0.02), duration (P = 0.04), and velocity (P = 0.001) during the 15-wk concurrent running period (age 16-30 wk). Post hoc tests revealed significantly greater distance and duration in group 4 compared with group 2 and significantly greater velocity in group 4 compared with each of the other groups. Significant interactions were found between groups over time for distance (P = 0.01) and duration (P = 0.05). No significant difference between groups was observed for body weight over the 24-wk period (P > 0.05).

CONCLUSION

Although differences were found between groups 2 and 4, these data suggest that the age at which physical activity is introduced has little influence on the subsequent level of physical activity in C57Bl/6J mice. However, it appears that introduction of the running wheel at 16 wk of age results in greater within group variance, suggestive of a greater environmental influence on daily running wheel activity.

Influence of genetic background on daily running-wheel activity differs with aging

In humans, physical activity declines with age. We tested the hypothesis that genetic background and age interact to determine daily wheel-running physical activity patterns in mice. Five female mice from ten inbred strains (A/J, AKR/J, Balb/cJ, CBA/J, C3H/HeJ, C3Heb/FeJ, C57Bl/6J, C57L/J, DBA/2J, and SWR/J) were studied for 26 wk starting at 10 wk of age. All mice were housed in separate cages, each with a running wheel and magnetic sensor. Throughout the 26-wk period, age-related change in daily duration (P < 0.0001), daily distance (P < 0.0001), and average velocity (P = 0.0003) differed between the inbred strains. Unlike the other strains, SWR/J mice increased their running-wheel activity throughout the 6-mo time period. Broad-sense heritability estimations for the strains across the 26-wk period ranged between 0.410 and 0.855 for the three physical activity phenotypes. Furthermore, the broad-sense heritability estimates for daily running-wheel distance differed across time and suggested an interaction between genetic background and age on physical activity in these inbred mice.

Genetic influence on daily wheel running activity level

This project was designed to determine the genetic (between-strain) and environmental (within-strain) variance in daily running wheel activity level in inbred mice. Five male and five female mice, 9.7–15.3 wk old, from each of 13 strains (A/J, AKR/J, BALB/cJ, C3H/HeJ, C57Bl/6J, C57L/J, C3Heb/FeJ, CBA/J, DBA/2J, SWR/J, MRL/MpJ, SPRET/Ei, and CAST/Ei) as well as five female NZB/BinJ mice were housed individually. A running wheel in each cage was interfaced with a magnetic sensor to measure total daily distance and exercise time for each animal every 24 h for 21 consecutive days (3 wk). Average daily distance (km), duration (min), and velocity (m/min) for each strain was then calculated. Significant interstrain differences in average daily distance ( P < 0.001), average daily exercise duration ( P < 0.0001), and average daily exercise velocity ( P < 0.0001) were found, with C57L/J mice running farther and faster than the other strains. Sex was a significant factor in daily running wheel activity, with female mice running an average of 20% farther ( P = 0.01) and 38% faster ( P < 0.0001) than male mice. The male mice ran 15% longer duration on a daily basis ( P = 0.0091). Weight was only associated with exercise velocity in the female mice, but this relationship was not significant when subdivided by strain. Broad-sense heritability estimates on the physical activity differed by sex (for distance, male 31–48% and female 12–22%; for duration, male 44–61% and female 12–21%; for velocity, male 49–66% and female 44–61%). In conclusion, these data indicate that daily running wheel activity level in mice is significantly affected by genetic background and sex.

A wide range of baroreflex stimulation does not alter forearm blood flow

The contribution to the regulation of forearm blood flow (FBF) by different baroreceptor populations has previously only been studied over a limited range of stimuli. Therefore, FBF and R-R interval were recorded during neck suctions and neck pressures ranging from -60 to +40 mmHg. The change in R-R interval (DeltaR-R) during neck suction was significantly increased at each stage when compared to the control ( P<0.05). DeltaR-R did not show any significant change during any of the neck pressure stages ( P>0.05). Suction or pressure applied to the neck did not elicit any significant changes in FBF when compared to the control ( P>0.05). These data show that widening the range of applied stimuli to carotid sinus baroreceptors does not induce a change in FBF. However, the small transient changes reported previously cannot be discounted.

The effects of breathing 5% CO2on human cardiovascular responses and tolerance to orthostatic stress

Breathing carbon dioxide (CO2) is known to induce hypercapnic acidosis and to affect chemoreceptor regulation of the cardiovascular system. However, there is limited information in the literature regarding the effects of breathing CO2 upon tolerance to orthostatic stress where cardiovascular regulation is challenged. The purpose of this study was to investigate the effect of breathing 5% CO2 on presyncopal tolerance to lower body negative pressure (LBNP). Nine subjects (five males and four females; average +/-s.d. age 21.9 +/- 0.9 years, height 172.4 +/- 9.7 cm, mass 70.3 +/- 7.1 kg) volunteered to participate in this study. Orthostatic tolerance was determined by exposing subjects to LBNP until the onset of presyncopal signs and symptoms on two occasions each separated by approximately 1 week. On one occasion investigations were carried out while subjects were breathing room air and on the other while subjects were breathing air containing 5% CO2, inducing hypercapnia and stimulating systemic chemoreceptors. During hypercapnic conditions, as compared with normocapnia, there were significant increases (P < 0.05) in minute ventilation, end-tidal CO2 and estimated arterial P(CO2). Furthermore, under hypercapnic conditions there was an increase in orthostatic tolerance, peak heart rate and time to peak heart rate during LBNP. The LBNP-induced increase in calf circumference was significantly attenuated at -50 mmHg of LBNP in addition to a further 22.3% reduction in stroke volume under hypercapnic conditions. In conclusion, these results suggest that the possible protective element of presyncope was delayed during hypercapnia at the expense of further reductions in stroke volume. This delayed presyncopal response may have been associated with increases in cerebral blood flow (CBF) induced by the increased arterial P(CO2).