Physiological Adaptations to Progressive Endurance Exercise Training in Adult and Aged Rats: Insights from the Molecular Transducers of Physical Activity Consortium (MoTrPAC)

Abstract

While regular physical activity is a cornerstone of health, wellness, and vitality, the impact of endurance exercise training on molecular signaling within and across tissues remains to be delineated. The Molecular Transducers of Physical Activity Consortium (MoTrPAC) was established to characterize molecular networks underlying the adaptive response to exercise. Here, we describe the endurance exercise training studies undertaken by the Preclinical Animal Sites Studies component of MoTrPAC, in which we sought to develop and implement a standardized endurance exercise protocol in a large cohort of rats. To this end, Adult (6-mo) and Aged (18-mo) female (n = 151) and male (n = 143) Fischer 344 rats were subjected to progressive treadmill training (5 d/wk, ∼70%-75% VO2max) for 1, 2, 4, or 8 wk; sedentary rats were studied as the control group. A total of 18 solid tissues, as well as blood, plasma, and feces, were collected to establish a publicly accessible biorepository and for extensive omics-based analyses by MoTrPAC. Treadmill training was highly effective, with robust improvements in skeletal muscle citrate synthase activity in as little as 1-2 wk and improvements in maximum run speed and maximal oxygen uptake by 4-8 wk. For body mass and composition, notable age- and sex-dependent responses were observed. This work in mature, treadmill-trained rats represents the most comprehensive and publicly accessible tissue biorepository, to date, and provides an unprecedented resource for studying temporal-, sex-, and age-specific responses to endurance exercise training in a preclinical rat model.

Keywords: aging; biorepository; body composition; citrate synthase; maximal oxygen uptake; skeletal muscle; training; treadmill.

Graphical Abstract

Figure 1.

MoTrPAC PASS1B Study Overview and Design. (A) Overview of cohort intake, testing, and progressive endurance training protocol in male and female Adult and Aged F344 rats. Schematic displays pretraining acclimation and familiarization protocol for all rat cohorts. Note, postexercise blood lactate concentration was also measured on the first of each training week. Also, in the 18 mo cohort, an additional SED control group (for both sexes) was age-matched to 1 W training group; this group was included to account for potential aging effects. (B) Overview of the timeline of events on the day of sacrifice. This figure was created with BioRender.com (www.biorender.com) and confirmation of publication and licensing rights was obtained.

Figure 2.

Dissection. (A) Tissue dissection workflow. Time elapsed from tissue collection (gastrocnemius, white adipose, liver, vena cava, lung, heart) or death to freezing for, (B) Adult females, (C) Adult males, (D) Aged females, and (E) Aged males. Boxes are mean ± 2 SD (calculated from the log-transformed times and back-transformed).

Figure 3.

VO₂max and MRS. Pre- and post-training measures of absolute VO₂max, VO₂max relative to total body mass, and MRS in Adult females (A)–(C), Adult males (D)–(F), Aged females (G)–(I), and Aged males (J)–(L). Each arrow or point represents a single rat, and they span from pre- to post-training values. Arrows are colored according to the direction of change from pre to post, and individual rats are arranged in ascending order by their pretraining value within each group. P-values were obtained from two-sided one sample t-tests of the (post–pre) differences, and they were Holm adjusted within each combination of age and sex.

Figure 4.

Delta VO₂max. Change in absolute (A) and relative (B) VO₂max from pre- to post-training. Horizontal lines represent the mean of each group. A colored line indicates that the mean of the group was significantly different from zero, while a black line indicates that the mean was not significantly different from zero. Exact P-values are shown in Figure 3.

Figure 5.

Body composition. Pre- and post-training measures of body composition (body mass, lean mass, and fat mass) in Adult females (A)–(C), Adult males (D)–(F), Aged females (G)–(I), and Aged males (J)–(L). Each arrow or point represents a single rat, and they span from pre- to post-training values. Arrows are colored according to the direction of change from pre to post, and individual rats are arranged in ascending order by their pretraining value within each group. P-values were obtained from two-sided one sample t-tests of the (post–pre) differences, and they were Holm adjusted within each combination of age and sex.

Figure 6.

Delta body composition. Change in body mass (A), lean mass (B), and fat mass (C) from pre- to post-training. Horizontal lines represent the mean of each group. A colored line indicates that the mean of the group was significantly different from zero, while a black line indicates that the mean was not significantly different from zero. Exact P-values are shown in Figure 5.

Figure 7.

Mean fiber type %. Mean percentage of each fiber type (I, IIa, IIb, and IIx), determined by MHC expression, in the LG, MG, PL, and SOL of Adult females (A)–(D), Adult males (E)–(H), Aged females (I)–(L), and Aged males (M)–(P). Each donut chart summarizes measurements taken from 6 rats. Superscript numbers denote a significant difference (two-sided, two-sample t-test; Holm P < .05) between the 8 W and SED means for a particular fiber type ratio (described on the right of the figure and in the “Fiber-Type-Specific Measures: Fiber type distribution” Methods).

Figure 8.

Fiber-type-specific CSA. Mean CSA of each fiber type for the LG, MG, PL, and SOL muscles from Adult females (A)–(D), Adult males (E)–(H), Aged females (I)–(L), and Aged males (M)–(P). Boxes are 95% confidence intervals for the mean CSA of each group. For each muscle and fiber type, the 8 W trained group was compared to SED, and P-values were Holm-adjusted across all fiber types for a given combination of age, sex, and muscle. Brackets indicate a statistically significant difference between groups (Holm P < .05).

Figure 9.

Citrate synthase activity by muscle. Citrate synthase activity in the LG, MG, PL, and SOL muscles of Adult females (A)–(D), Adult males (E)–(H), Aged females (I)–(L), and Aged males (M)–(P). Each trained group was compared against the SED group using the Dunnett test. Brackets indicate a significant change in CS from SED to trained (Dunnett P < .05).

Figure 10.

Glycogen by muscle. Glycogen concentration in the LG, MG, PL, and SOL muscles of Adult females (A)–(D), Adult males (E)–(H), Aged females (I)–(L), and Aged males (M)–(P). Each trained group was compared against the SED group using the Dunnett test. Brackets indicate a significant change in glycogen from SED to trained (Dunnett P < .05).

Figure 11.

Systemic hormones. Levels of plasma insulin, glucagon, corticosterone, and leptin in Adult females (A)–(D), Adult males (E)–(H), Aged females (I)–(L), and Aged males (M)–(P). Each trained group was compared against the SED group using the Dunnett test. Brackets indicate a significant change in these hormones from SED to trained (Dunnett P < .05). Measurements were performed in all Adult rats, and only in the -omics cohort of Aged rats.

Figure 12.

Clinical metabolites. Levels of plasma glucose, lactate, NEFA, glycerol, and total ketones in Adult females (A)–(E), Adult males (F)–(J), Aged females (K)–(O), and Aged males (P)–(T). Each trained group was compared against the SED group using the Dunnett test. Brackets indicate a significant change in these metabolites from SED to trained (Dunnett P < .05). Measurements were performed in all Adult rats, and only in the -omics cohort of Aged rats.