Medler, Scott. Comparative trends in shortening velocity and force production in skeletal muscles. Am J Physiol Regulatory Integrative Comp Physiol 283: R368-R378, 2002. First published March 22, 2002 10.1152/ajpregu.00689. 2001.-Skeletal muscles are diverse in their properties, with specific contractile characteristics being matched to particular functions. In this study, published values of contractile properties for Ͼ130 diverse skeletal muscles were analyzed to detect common elements that account for variability in shortening velocity and force production. Body mass was found to be a significant predictor of shortening velocity in terrestrial and flying animals, with smaller animals possessing faster muscles. Although previous studies of terrestrial mammals revealed similar trends, the current study indicates that this pattern is more universal than previously appreciated. In contrast, shortening velocity in muscles used for swimming and nonlocomotory functions is not significantly affected by body size. Although force production is more uniform than shortening velocity, a significant correlation with shortening velocity was detected in muscles used for locomotion, with faster muscles tending to produce more force. Overall, the contractile properties of skeletal muscles are conserved among phylogenic groups, but have been significantly influenced by other factors such as body size and mode of locomotion. skeletal muscle; scaling; shortening velocity; tetanic tension SKELETAL MUSCLES ARE DIVERSE in their contractile properties, with significant differences existing among and within various animal species. As such, skeletal muscles are striking examples of biological structures adapted for a specific function. Yet skeletal muscles are also highly conserved in terms of the molecular mechanisms responsible for producing muscle contraction (91, 99). Over the last 10 years, a considerable amount of data has been collected on the contractile properties of skeletal muscles from a diverse group of animals. In this study, the contractile properties of Ͼ130 skeletal muscles were analyzed to determine what broad trends could be observed that might give insight into the principles of skeletal muscle design. These muscles represent several distinct phylogenic lines, have varied functional demands, and come from animals spanning more than eight orders of magnitude in body mass. Although there have been several excellent reviews summarizing skeletal muscle properties, most focused on terrestrial mammals and none subjected the data to any type of statistical analysis (22,54,91,95). In the present study, maximum shortening velocity (V max ) and maximum tetanic tension (P o ) were analyzed with respect to body mass, taxonomic group, mode of locomotion, and compared with one another.The wide range of shortening velocities is one of the most prominent features that distinguishes muscle fiber types, with values in the present study ranging from ϳ0.5 muscle lengths (L)/s to nearly 40 L/s. V max is often used to define different ...

Gene looping juxtaposes the promoter and terminator regions of RNA polymerase II-transcribed genes in yeast and mammalian cells. Here we report an activator-dependent interaction of transcription initiation and termination factors during gene looping in budding yeast. Chromatin analysis revealed that MET16, INO1, and GAL1p-BUD3 are in a stable looped configuration during activated transcription. Looping was nearly abolished in the absence of transcription activators Met28, Ino2, and Gal4 of MET16, INO1, and GAL1p-BUD3 genes, respectively. The activator-independent increase in transcription was not accompanied by loop formation, thereby suggesting an essential role for activators in gene looping. The activators did not facilitate loop formation directly because they did not exhibit an interaction with the 3 end of the genes. Instead, activators physically interacted with the general transcription factor TFIIB when the genes were activated and in a looped configuration. TFIIB cross-linked to both the promoter and the terminator regions during the transcriptionally activated state of a gene. The presence of TFIIB on the terminator was dependent on the Rna15 component of CF1 3 end processing complex. Coimmunoprecipitation revealed a physical interaction of Rna15 with TFIIB. We propose that the activators facilitate gene looping through their interaction with TFIIB during transcriptional activation of genes.Transcription of protein encoding genes by RNA polymerase (RNAP) 2 II involves several distinct steps including the assembly of preinitiation complex, initiation, elongation, termination, and reinitiation (1, 2). Transcription starts with the recruitment of RNAP II and the general transcription factors TFIID, TFIIB, TFIIA, TFIIF, TFIIE, and TFIIH onto the promoter to form a preinitiation complex. RNAP II and general transcription factors are sufficient for accurate basal level transcription (2, 3). The response to activators requires additional cofactors that bring about stimulation of transcription by modifying chromatin structure in the promoter region and facilitating recruitment of RNAP II and general transcription factors to the exposed promoter (3-5). Once the gene is activated, the amount of transcripts produced is determined primarily by the number of reinitiation events (6). Despite the remarkable progress made in understanding the molecular mechanisms that govern initiation of transcription in eukaryotes, relatively little is known about the processes that mediate reinitiation.The studies with RNAP I and III have implicated proper termination as a prerequisite for reinitiation of transcription (7,8). During RNAP I and RNAP III-mediated transcription, termination factors help the polymerase to pause at the terminator region. This is followed by the release of the polymerase from the terminator. In RNAP I transcription, PTRF (Pol I and transcription release factor) facilitates release of the paused polymerase from the terminator region, whereas in RNAP III transcription, factor La performs an analogous fun...

SUMMARYSkeletal muscles are diverse in their contractile properties, with many of these differences being directly related to the assemblages of myofibrillar isoforms characteristic of different fibers. Crustacean muscles are similar to other muscles in this respect, although the majority of information about differences in muscle organization comes from vertebrate species. In the present study, we examined the correlation between myofibrillar protein isoforms and the patterns of myofibrillar gene expression in fast, slow-phasic(S1) and slow-tonic (S2) fibers of the American lobster Homarus americanus. SDS-PAGE and western blotting were used to identify isoform assemblages of myosin heavy chain (MHC), P75, troponin T(TnT) and troponin I (TnI). RT-PCR was used to monitor expression of fast and slow (S1) MHC, P75 and actin in different fiber types, and the MHC and actin levels were quantified by real-time PCR. Fast and slow fibers from the claw closers predominantly expressed fast and S1 MHC,respectively, but also lower levels of the alternate MHC. By contrast, fast fibers from the deep abdominal muscle expressed fast MHC exclusively. In addition, slow muscles expressed significantly higher levels of actin than fast fibers. A distal bundle of fibers in the cutter claw closer muscle was found to be composed of a mixture of S1 and S2 fibers,many of which possessed a mixture of S1 and S2 MHC isoforms. This pattern supports the idea that S1 and S2fibers represent extremes in a continuum of slow muscle phenotype. Overall,these patterns demonstrate that crustacean skeletal muscles cannot be strictly categorized into discrete fiber types, but a muscle's properties probably represent a point on a continuum of fiber types. This trend may result from differences in innervation pattern, as each muscle is controlled by a unique combination of phasic, tonic or both phasic and tonic motor nerves. In this respect, future studies examining how muscle phenotype correlates with innervation pattern may help account for variation in crustacean fiber types.

Skeletal muscles are highly plastic tissues capable dramatic remodeling in response to use, disuse, disease, and other factors. Growing evidence suggests that adipose tissues exert significant effects on the basic fiber‐type composition of skeletal muscles. In the current study, we investigated the long‐term effects of a high‐fat diet and subsequent obesity on the muscle fiber types in C57 BLK/6J mice. Litters of mice were randomly assigned to either a high‐fat diet or a control group at the time of weaning, and were maintained on this diet for approximately 1 year. Single fibers were harvested from the soleus and plantaris muscles, and fiber types were determined using SDS‐PAGE. The high‐fat diet mice were significantly heavier than the control mice (39.17 ± 2.7 g vs. 56.87 ± 3.4 g; P < 0.0003), but muscle masses were not different. In male mice, the high‐fat diet was associated with a significantly lower proportion of slow, type I fibers in the soleus muscle (40.4 ± 3.5% vs. 29.33 ± 2.6%; P < 0.0165). Moreover, the proportion of type I fibers in the soleus of male mice was inversely proportional to the relative fatness of the male mice (P < 0.003; r2 = 0.65), but no association was observed in female mice. In male mice, the decline in type I fibers was correlated with an increase in type I/IIA hybrid fibers, suggesting that the type I fibers were transformed primarily into these hybrids. The reported trends indicate that type I fibers are most susceptible to the effects of obesity, and that these fiber‐type changes can be sex specific.

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