Browsing by Author "Otis, Jeffrey Scott"
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- Metabolic profile of myosin heavy chain-based fiber types in the rat soleus after spinal cord transectionOtis, Jeffrey Scott (Virginia Tech, 2000-10-27)Fully differentiated muscle fibers can undergo considerable phenotypic changes in order to adjust to changing conditions of the physiological environment. It is generally accepted that the electrical impulses a muscle receives play a role in modulating the quantities of metabolic proteins (glycolytic and oxidative enzymes) and types of contractile proteins (myosin heavy chain, MHC) that are expressed. Research has shown that decreased neuromuscular activation following spinal cord transection (ST) results in adaptations in the physiological characteristics of paralyzed muscles, including atrophy and an accompanying loss of force production, and transformations of contractile and metabolic proteins toward a more fatigable state. However, it remains unclear whether or not a strong interdependence of energy metabolism and MHC isoform composition persists. Therefore, the goal of this study was to identify and quantify relative myosin heavy chain (MHC) isoform expression and metabolic enzyme profile adaptations at multiple time points (1, 3 and 6 months) in soleus fibers of rats following spinal cord transection (ST). To accomplish this, female Sprague-Dawley rats (~150 g, n = 15) were subjected to complete transection of the spinal cord at a mid-thoracic level. Age and weight-matched, non-operated rats served as controls (n = 15). The soleus was processed for quantitative single fiber histochemical analyses for succinate dehydrogenase (SDH, oxidative marker) and a-glycerophosphate dehydrogenase (GPD, glycolytic marker) activities (~30 fibers/muscle) and immunohistochemical analysis for MHC isoform composition. The total number of soleus fibers analyzed was ~900. Oxidative capacity was increased in muscle fibers at all time points after ST. Specifically, SDH activity was significantly higher than controls by 142, 127 and 206% at 1, 3 and 6 months post-ST, respectively. ISDH, a measure of total oxidative power, also increased in muscle fibers at all time points after ST. For example, 6 months after ST ISDH activity was 93% higher than controls (91.8-3.8 vs. 47.6-0.9 OD x 10-3, respectively). Glycolytic capacity peaked one month after ST. Thereafter, glycolytic capacity of all fibers steadily declined. For example, by 6 months, GPD activity had declined by 76% compared to 1 month GPD activities (3.3-0.2 vs. 13.7-1.4 OD x 10-3, respectively). These data suggest that the increases in glycolytic capacity are transient as fibers transition toward a faster MHC phenotype and then return towards control levels as fibers of a given type become phenotypically stable. The GPD/SDH ratio, an index of metabolic substrate utilization, peaked at one month after ST (394-41) and significantly decreased at 3 months (224-10) and at 6 months (95-7) after ST. Therefore, a shift occurred such that a greater dependence on oxidative metabolism was apparent. These data suggest that the oxidative capacities of soleus muscle fibers are not compromised after ST. In fact, as the fibers transitioned toward faster MHC isoforms, the GPD/SDH ratio was maintained or decreased, suggesting a reliance on oxidative metabolism regardless of MHC isoform composition. This might imply a dissociation between the contractile and metabolic characteristics of paralyzed soleus muscle fibers. However, these data are consistent with previous data and suggest that the increased fatigability observed after chronic reductions in neuromuscular activity are not due to compromised capacities for ATP synthesis.
- Skeletal muscle adaptations in cachectic, tumor-bearing ratsOtis, Jeffrey Scott (Virginia Tech, 2003-04-04)Cancer cachexia is a debilitating, paraneoplastic syndrome commonly associated with late stage malignancy. It is estimated that ~25% of cancer-related deaths are due directly to complications arising from cachexia (Barton, 2001). Cachexia manifests as severe body wasting, primarily due to the loss of skeletal muscle mass. This study tested the hypothesis that muscle atrophy associated with cancer cachexia could be attenuated by using a unilateral, functional overload (FO) model applied concurrently with tumor development. To accomplish this, Morris hepatoma MH-7777 cells were implanted in adult female, Buffalo rats (n = 12) and allowed to incubate for 6 weeks. FO surgeries (n = 12) were performed five days prior to MH-7777 cell implantation. Over the course of six weeks, healthy, age, sex and strain-matched, vehicle-injected rats (n = 12) gained ~5% of body weight compared to tumor-bearing rats that lost ~6% of body weight when adjusted for tumor mass. Tumor-bearing animals experienced significant atrophy to gastrocnemius, tibialis anterior, extensor digitorum longus, plantaris and diaphragm muscles. FO successfully reversed plantaris muscle atrophy in cachectic, tumor-bearing rats (n=5). FO plantaris masses were ~24% larger than contralateral controls. However, this hypertrophic response was not as great as FO plantaris muscles from healthy, sham-operated controls (~44% larger than contralateral controls, n=5). FO plantaris muscles from tumor-bearing rats had ~1.5 fold increase in myonuclei/fiber ratios compared those of sham-operated, tumor-bearing controls (n = 6). Therefore, cancer cachexia did not prevent myonuclear accretion necessary for skeletal muscle hypertrophy. Little data exists on adaptations to myosin heavy chain (MHC) isoforms in cachectic skeletal muscle. Plantaris muscles from tumor-bearing rats displayed decreased percentages of MHC type I compared to plantaris muscles from vehicle-injected controls (7% vs. 3%, respectively). However, FO plantaris muscles from tumor-bearing rats had an increased percentage of MHC type I and decreased percentage of MHC type IIb compared to sham-operated tumor-bearing rats, adaptations commonly seen in trained muscles. Therefore, cancer cachexia did not prevent the capability of skeletal muscle to respond normally to hypertrophic stimuli. This study also attempted to characterize a mechanism responsible for the hypertrophic response, increased myonuclei/fiber ratio and transition toward a slower MHC profile in FO plantaris muscles from tumor-bearing rats. Recently, the Ca2+/calmodulin-dependent protein phosphatase, calcineurin, has been suggested as a critical factor regulating skeletal muscle growth and fiber-type dependent gene expression (Chin, 1998; Wu, 2000; Olson, 2000; Otis, 2001). The protein content of the catalytic subunit (CaNa) and the regulatory subunit (CaNb) of calcineurin were unchanged in plantaris muscles from tumor-bearing animals compared to healthy controls. Furthermore, total and specific (normalized to CaNa protein content) calcineurin phosphatase activity were not altered in any group. Therefore, calcineurin activity did not appear to be associated with the regulation of the morphological and physiological response of hypertrophying plantaris muscles in cachectic, tumor-bearing rats. Overall, this study indicated that atrophied plantaris muscles from tumor-bearing animals have a reduced capacity to hypertrophy potentially due to a decreased myonuclei/fiber ratio. Furthermore, it is unlikely that changes to mass and MHC isoform expression are associated with calcineurin phosphatase activity.