ABSTRACT
We investigated the effects of habitual exercise and diet restriction on the hepatic fat accumulation in Zucker fatty rats. Male 6-week-old Zucker fatty rats were divided into obese (Ob), diet restriction (DR), and diet restriction + exercise (DR + Ex) groups. Male Zucker lean rats (L) were used as a control group. The rats in the L and Ob groups were maintained on ad libitum diets. The rats in the DR and DR + Ex groups were fed a 30% restricted diet. The rats in the DR + Ex group exercised voluntarily on a wheel ergometer. After 6 weeks of intervention, the serum free fatty acid and leptin levels in the Ob group were significantly higher than those in the L group. In the Ob group, the hepatic triglyceride content was higher than that in the L group and hepatocyte fat infiltration was observed on haematoxylin and eosin staining. These changes were suppressed by DR + Ex, but not by the DR intervention. These results suggest that habitual exercise inhibits fat accumulation in the liver of Zucker fatty rats.
ABSTRACT
The objective of this study was to investigate whether isometric resistance exercise (IRE) can attenuate musculoskeletal atrophy during unloading and accelerate its recovery during reloading. Twenty-six female Fischer 344 rats, aged 16 weeks, had their hindlimbs suspended for 3 weeks (unloading) ; 12 of these rats were allowed subsequent cage activity (reloading) for 3 weeks with or without IRE. IRE (stationary support on a cylindrical grid inclined 60 or 80 degrees) was done for 30 min/day, 6 days/week, with an additional load of 30% or 50% body mass attached to the tail during the unloading and reloading periods. The tibial bone and hindlimb skeletal muscles from four experimental and two age-matched control groups were evaluated with dual-energy X-ray absorptiometry, mechanical testing, and muscle mass measurement. Bone mineral density (BMD) was measured in the whole tibia and in 7 regions divided equally along the long axis of the epiphysis from proximal (R1) to distal (R7) . After unloading, fat-free dry mass (FFDM), bone mineral content (BMC), and BMD of the whole tibia decreased by 8%, 10%, and 6%, respectively. FFDM and BMC, but not BMD, returned to the levels of age-matched controls during reloading. Unloading-induced decreases in BMD were observed in the regions from the proximal epiphysis to the diaphysis (R1 to R4) and the distal epiphysis (R7) . The rate of decrease in BMD was regionally specific and was particularly pronounced (12%) in the most proximal region (R1) . These findings indicate regional variations in responses of BMD to skeletal unloading. The BMD in R2 to R4 remained less than that in age-matched control after reloading. No significant changes were observed in maximum breaking load, energy, and deformation after unloading and reloading. Hindlimb-unloading induced loss of mass in the soleus (38%), plantaris (14%), gastrocnemius (25%), tibialis anterior (8%), extensor digitorum longus ( 8%), and rectos lemons (17%) muscles, but the mass of muscles, except for the soleus muscle, recovered during reloading. IRE ameliorated the loss of mass in the soleus and gastrocnemius muscles during unloading but did not promote the recovery of mass in any muscles during reloading. Moreover, IRE showed no effect on bone responses after unloading and reloading. This lack of beneficial effects of IRE on osteopenia may be due, in part, to insufficient exerciseinduced load. We concluded that 1) regional analysis of BMD can be used to assess local bone metabolism, 2) the response of BMD to altered loading conditions does not necessarily depend on the response of muscle mass, 3) recovery from osteopenia progresses more slowly than that from sarcopenia, and a longer time than the unloading period is required to restore BMD. Further studies are needed to develop more effective countermeasures against osteopenia and sarcopenia.
ABSTRACT
The effects of chronic overload by synergist ablation (SA) on fiber type, myosin heavy chain (MyoHC) isoforms, and contractile properties in the rat plantaris muscle were studied. Plantaris muscle was overloaded by bilateral removal of its synergist of gastrocnemius muscle in Fischer 344 female rats (aged 8 weeks) . Muscles of sham-operated legs served as controls.<BR>The overloaded plantaris was 20% heavier than control 3 weeks after SA. Overload by SA resulted in remarkable increases in cross-sectional areas of muscle fibers of all types ; however, the percentage increase was less in type II b fibers. Twitch contraction and half-relaxation time before and after tetanus did not change, but fatigue resistance increased significantly with overload. The percentage of area occupied by type II b fibers, which have lower oxidative capacity, decreased while the percentage of area occupied by other type II fibers, but not type I fibers, increased. Analysis of MyoHC isoforms with gradient sodium dodecylsulfate polyacrylamide gel electrophoresis revealed a decrease in II b isoforms and an increase in II d isoforms.<BR>These results suggest that chronic overload produces changes in muscle fiber type and MyoHC isoform compositions which partly account for increased fatigue resistance in overloaded muscles.