DIET INDUCED OBESITY MAY AFFECT THE FORCE-VELOCITY RELATIONSHIP IN RAT SOLEUS

INTRODUCTION Obesity is associated with chronic, low-grade inflammation that has been shown to affect several musculoskeletal tissues [1].  Previously, we observed that diet induced obesity (DIO) using a high-fat, high sugar diet results in molecular and morphological alterations in muscles, including an increase in intramuscular fat. However, it remains unclear if these alterations affect muscle function, as few studies have characterized the functional properties of muscles in obese individuals [2]. Arguably, one of the most important functional properties of skeletal muscle is the force-velocity relationship (FVR).  When muscles are shortening at increasing velocities, force decays exponentially as the proportion of attached cross bridges and the average force per cross-bridge decreases. As fat infiltration reduces the contractile material and structure of obese muscles, the functional properties of muscles, specifically the FVR may be affected. Therefore, the purpose of this study was to characterize the FVR in rat soleus muscles obtained from obese and normal rats. We hypothesized that soleus muscles of obese rats would produce lower absolute and relative forces at any given shortening velocity compared with muscles from control animals. METHODS Outbred, individually housed male Sprague-Dawley rats, aged 10-12 weeks, were randomized to a high fat, high sugar diet (DIO, 40% fat 45% sucrose, n=9) or a standard chow diet (n=5, chow, 12% fat, 0% sucrose) for 12 weeks. Prior to surgery, animals were sedated, weighed, and body composition was quantified using dual energy X-ray absorptiometry. The right soleus muscle was exposed and a custom cuff-type electrode was implanted on the tibial nerve. The soleus tendon was isolated from the Achilles with the calcaneus attached and fixed to a motor along the muscle’s natural alignment. The muscle was then stretched to its optimum length and electrically stimulated at 35Hz at 2.5x the motor unit threshold [3]. An isometric force reading was acquired over 2.5 s stimulation. The soleus was then stretched past its optimum length and shortened at increasing velocities, a force reading was collected at optimal length using Windaq Software. Data were collected at 1000Hz. Forces during shortening were measured at the same length and time point as the isometric reference force. This process was repeated for shortening velocities increasing from 2 mm/s to 70 mm/s. Animals were sacrificed and soleus muscles were harvested and weighed. Data were processed using a custom Matlab ® zero-phase filter program. Instantaneous forces were normalized to the peak isometric force for each animal. Comparisons were made using a Student’s t-tests, α=0.05. RESULTS On average, DIO animals had higher body mass and body fat compared to the control rats (p<0.001). Soleus mass was similar between DIO animals and chow (p=0.321), as was peak active isometric force (DIO: 2.48±0.10 N, chow: 2.08±0.33 p=0.183). FVR relationships were statistically different at shortening velocities between 3 and 35 mm/s (DIO > Chow at each given velocity; p<0.05, Fig. 1). DISCUSSION The results opposed the expected outcomes of this study and, therefore, the hypothesis was not satisfied. These findings could be due to a higher proportion of fast twitch fibers in the DIO or longer fascicle lengths in the DIO rats, but these speculations remain to be tested. Another possible explanation involves a potential increase in sensitivity to stimulation in the DIO soleus muscle resulting in increased force. Future work will examine other structural levels and muscle contractile proteins to understand these preliminary findings

THE EFFECTS OF DIET INDUCED OBESITY ON THE FORCE-LENGTH RELATIONSHIP IN RAT SOLEUS

INTRODUCTION Obesity is associated with chronic inflammation, which has been shown to affect the integrity of musculoskeletal tissues [1]. Previous data from our group suggests that obesity can result in intramuscular fat deposition [1]. It is unclear if this structural alteration has functional consequences, as the implications of obesity on muscle mechanics are not well understood. Therefore, the purpose of this study was to quantify the active force produced by soleus muscles of obese and non-obese rats at a range of muscle lengths. As the inclusion of fat into the muscle fibers will leave less room for contractile proteins, we hypothesized that obese rats will produce lower forces normalized to muscle mass at every length than non-obese control rats.   METHODS Fourteen rats were randomly allocated to a 12-week diet: either an obesity-inducing high fat high sucrose diet (DIO, 40% fat, 45% sucrose, n=8) or a standard chow diet (chow, 12% fat 0% sucrose, n=6). Prior to surgery, body composition was evaluated using dual energy X-ray absorptiometry. Custom-made tibial nerve cuffs were surgically attached to the right tibial nerve of each animal. The soleus was exposed, mechanically isolated, and clamped to a force transducer. The muscle was then stretched to a predetermined length and electrically stimulated at 3 times the motor unit threshold (50Hz) and the force output was measured [3]. Force tracings were digitized using WINDAQ® software. Passive, active, and total forces produced by the soleus were normalized to the maximum in vivo length of each animal. Forces were averaged into 5% length intervals within each animal. Students t-tests or a two-way ANOVA were conducted between groups, and a Bonferroni correction was used as needed, α=0.05. RESULTS DIO rats had increased body mass (DIO 816.4 ± 30.1g, chow 645.0 ± 28.3g; p<0.05) and body fat (DIO 39.2 ± 1.3%, chow 21.8 ± 2.1%; p<0.05) compared to chow-fed rats. Soleus mass (DIO: 0.28 ± 0.01 g, chow: 0.26 ± 0.11 g, p=0.32), was similar between the two groups. Absolute peak isometric force was similar between the two groups (DIO: 2.58 ± 0.10 N, chow: 2.18 ± 0.34 N, p=0.23). Active isometric force normalized to soleus mass was significantly higher in DIO group rats at every muscle length (Figure 1, p<0.05). DISCUSSION AND CONCLUSIONS On average, DIO rats produced more active force at a given normalized length and soleus mass than chow rats, a finding that refutes our original hypothesis. Since optimal length occurs at the same relative muscle length for both groups, and since the decline in force from maximum is similar between groups, it appears that fascicle length, and an associated shift in the force-length relationship cannot explain our results. Results of differences in the force-velocity relationship (not shown here) suggest that the DIO rats may have a higher proportion of fast twitch fibres, but the relative force among slow and fast fibres is similar, and thus also should not affect these results. The results suggest that the force per cross-sectional area is higher in muscles from obese compared to lean rats, a finding that defies explanation at this time and needs thorough investigation in the future. Histology and tests looking at fibre and cell level muscle structures may provide more insight