Validity of a cardiology fellow performance assessment: reliability and associations with standardized examinations and awards

BACKGROUND

Most work on the validity of clinical assessments for measuring learner performance in graduate medical education has occurred at the residency level. Minimal research exists on the validity of clinical assessments for measuring learner performance in advanced subspecialties. We sought to determine validity characteristics of cardiology fellows' assessment scores during subspecialty training, which represents the largest subspecialty of internal medicine. Validity evidence included item content, internal consistency reliability, and associations between faculty-of-fellow clinical assessments and other pertinent variables.

METHODS

This was a retrospective validation study exploring the domains of content, internal structure, and relations to other variables validity evidence for scores on faculty-of-fellow clinical assessments that include the 10-item Mayo Cardiology Fellows Assessment (MCFA-10). Participants included 7 cardiology fellowship classes. The MCFA-10 item content included questions previously validated in the assessment of internal medicine residents. Internal structure evidence was assessed through Cronbach's α. The outcome for relations to other variables evidence was overall mean of faculty-of-fellow assessment score (scale 1-5). Independent variables included common measures of fellow performance.

FINDINGS

Participants included 65 cardiology fellows. The overall mean ± standard deviation faculty-of-fellow assessment score was 4.07 ± 0.18. Content evidence for the MCFA-10 scores was based on published literature and core competencies. Cronbach's α was 0.98, suggesting high internal consistency reliability and offering evidence for internal structure validity. In multivariable analysis to provide relations to other variables evidence, mean assessment scores were independently associated with in-training examination scores (beta = 0.088 per 10-point increase; p = 0.05) and receiving a departmental or institutional award (beta = 0.152; p = 0.001). Assessment scores were not associated with educational conference attendance, compliance with completion of required evaluations, faculty appointment upon completion of training, or performance on the board certification exam. R² for the multivariable model was 0.25.

CONCLUSIONS

These findings provide sound validity evidence establishing item content, internal consistency reliability, and associations with other variables for faculty-of-fellow clinical assessment scores that include MCFA-10 items during cardiology fellowship. Relations to other variables evidence included associations of assessment scores with performance on the in-training examination and receipt of competitive awards. These data support the utility of the MCFA-10 as a measure of performance during cardiology training and could serve as the foundation for future research on the assessment of subspecialty learners.

Predicting Quality of Clinical Performance From Cardiology Fellowship Applications

Variables in cardiology fellowship applications have not been objectively analyzed against applicants' subsequent clinical performance. We investigated possible correlations in a retrospective cohort study of 65 cardiology fellows at the Mayo Clinic (Rochester, Minn) who began 2 years of clinical training from July 2007 through July 2013. Application variables included the strength of comparative statements in recommendation letters and the authors' academic ranks, membership status in the Alpha Omega Alpha Honor Medical Society, awards earned, volunteer activities, United States Medical Licensing Examination (USMLE) scores, advanced degrees, publications, and completion of a residency program ranked in the top 6 in the United States. The outcome was clinical performance as measured by a mean of faculty evaluation scores during clinical training. The overall mean evaluation score was 4.07 ± 0.18 (scale, 1-5). After multivariable analysis, evaluation scores were associated with Alpha Omega Alpha designation (β=0.13; 95% CI, 0.01-0.25; P=0.03), residency program reputation (β=0.13; 95% CI, 0.05-0.21; P=0.004), and strength of comparative statements in recommendation letters (β=0.08; 95% CI, 0.01-0.15; P=0.02), particularly in letters from residency program directors (β=0.05; 95% CI, 0.01-0.08; P=0.009). Objective factors to consider in the cardiology fellowship application include Alpha Omega Alpha membership, residency program reputation, and comparative statements from residency program directors.

HLA class II diversity in HIV-1 uninfected individuals from the placebo arm of the RV144 Thai vaccine efficacy trial

The RV144 HIV vaccine trial in Thailand elicited antibody responses to the envelope of HIV-1, which correlated significantly with the risk of HIV-1 acquisition. Human leukocyte antigen (HLA) class II molecules are essential in antigen presentation to CD4 T cells for activation of B cells to produce antibodies. We genotyped the classical HLA-DRB1, DQB1, and DPB1 genes in 450 individuals from the placebo arm of the RV144 study to determine the background allele and haplotype frequencies of these genes in this cohort. High-resolution 4 and 6-digit class II HLA typing data was generated using sequencing-based methods. The observed diversity for the HLA loci was 33 HLA-DRB1, 15 HLA-DQB1, and 26 HLA-DPB1 alleles. Common alleles with frequencies greater than 10% were DRB1*07:01, DRB1*09:01, DRB1*12:02, DRB1*15:02, DQB1*02:01/02, DQB1*03:01, DQB1*03:03, DQB1*05:01, DQB1*05:02, DPB1*04:01:01, DPB1*05:01:01, and DPB1*13:01:01. We identified 28 rare alleles with frequencies of less than 1% in the Thai individuals. Ambiguity for HLA-DPB1*28:01 in exon 2 was resolved to DPB1*296:01 by next-generation sequencing of all exons. Multi-locus haplotypes including HLA class I and II loci were reported in this study. This is the first comprehensive report of allele and haplotype frequencies of all three HLA class II genes from a Thai population. A high-resolution genotyping method such as next-generation sequencing avoids missing rare alleles and resolves ambiguous calls. The HLA class II genotyping data generated in this study will be beneficial not only for future disease association/vaccine efficacy studies related to the RV144 study, but also for similar studies in other diseases in the Thai population, as well as population genetics and transplantation studies.

Cardiac myosin heavy chain gene regulation by thyroid hormone involves altered histone modifications

The antithetical regulation of cardiac α- and β-myosin heavy chain (MHC) genes by thyroid hormone (T(3)) is not well understood but appears to involve thyroid hormone interaction with its nuclear receptor and MHC promoters as well as cis-acting noncoding regulatory RNA (ncRNA). Both of these phenomena involve epigenetic regulations. This study investigated the extent that altered thyroid state induces histone modifications in the chromatin associated with the cardiac MHC genes. We hypothesized that specific epigenetic events could be identified and linked to cardiac MHC gene switching in response to a hypothyroid or hyperthyroid state. A hypothyroid state was induced in rats by propylthiouracil treatment (PTU), whereas a hyperthyroid (T(3)) was induced by T(3) treatment. The left ventricle was analyzed after 7 days for MHC pre-mRNA expression, and the chromatin was assessed for enrichment in specific histone modifications using chromatin immunoprecipitation quantitative PCR assays. At both the α-MHC promoter and the intergenic region, the enrichment in acetyl histone H3 at K9/14 (H3K9/14ac) and trimethyl histone H3 at K4 (H3K4me3) changed in a similar fashion. They were both decreased with PTU treatment but did not change under T(3), except at a location situated 5' to the antisense intergenic transcription start site. These same marks varied differently on the β-MHC promoter. For example, H3K4me3 enrichment correlated with the β-promoter activity in PTU and T(3) groups, whereas H3K9/14ac was repressed in the T(3) group but did not change under PTU. Histone H3K9me was enriched in chromatin of both the intergenic and α-MHC promoters in the PTU group, whereas histone H4K20me1 was enriched in chromatin of β-MHC promoter in the normal control and T(3) groups. Collectively, these findings provide evidence that specific epigenetic phenomena modulate MHC gene expression in altered thyroid states.

Change of form of septate and gap junctions during development of the insect midgut

In insects, smooth septate junctions join cells derived from the embryonic midgut, and pleated septate junctions are found in all other tissues. Relatively little is known about either type of septate junction or the relationship between them, but they have been treated as two different junctions in the literature. The gap junctions which are associated with these septate junctions also differ. Crystalline gap junctions are found in the midgut, associated with smooth septate junctions, and irregular gap junctions are found in tissues where pleated septate junctions are located. We have examined the development of smooth septate junctions and crystalline gap junctions and the relationship between them, by studying the embryogenesis of the midgut in Manduca sexta (tobacco hornworm). At 56 hr of development (hatching is at 104 hr) pleated septate junctions and irregular gap junctions joined the midgut epithelial cells. At 65 hr, the septate junctions had disappeared, but gap junctions persisted. At 70 hr, smooth septate junctions had replaced the earlier pleated septate junctions and gap junctions associated with these smooth septate junctions were often of the crystalline form. In later embryos, the smooth septate junctions matured and enlarged, while all gap junctions became crystalline in form.

Artificial gravity as a countermeasure to microgravity: a pilot study examining the effects on knee extensor and plantar flexor muscle groups

The goal of this project was to examine the effects of artificial gravity (AG) on skeletal muscle strength and key anabolic/catabolic markers known to regulate muscle mass. Two groups of subjects were selected for study: 1) a 21 day-bed rest (BR) group (n = 7) and 2) an AG group (n = 8), which was subjected to 21 days of 6 degrees head-down tilt bed rest plus daily 1-h exposures to AG (2.5 G at the feet). Centrifugation was produced using a short-arm centrifuge with the foot plate approximately 220 cm from the center of rotation. The torque-velocity relationships of the knee extensors and plantar flexors of the ankle were determined pre- and posttreatment. Muscle biopsy samples obtained from the vastus lateralis and soleus muscles were used for a series of gene expression analyses (mRNA abundance) of key factors implicated in the anabolic vs. catabolic state of the muscle. Post/pre torque-velocity determinations revealed greater decrements in knee extensor performance in the BR vs. AG group (P < 0.04). The plantar flexors of the AG subjects actually demonstrated a net gain in the torque-velocity relationship, whereas in the BR group, the responses declined (AG vs. BR, P < 0.001). Muscle fiber cross-sectional area decreased by approximately 20% in the BR group, whereas no losses were evident in the AG group. RT-PCR analyses of muscle biopsy specimens demonstrated that markers of growth and cytoskeletal integrity were higher in the AG group, whereas catabolic markers were elevated in the BR group. Importantly, these patterns were seen in both muscles. We conclude that paradigms of AG have the potential to maintain the functional, biochemical, and structural homeostasis of skeletal muscle in the face of chronic unloading.

Effect of unloading followed by reloading on expression of collagen and related growth factors in rat tendon and muscle

Tendon tissue and the extracellular matrix of skeletal muscle respond to mechanical loading by increased collagen expression and synthesis. This response is likely a secondary effect of a mechanically induced expression of growth factors, including transforming growth factor-beta1 (TGF-beta1) and insulin-like growth factor-I (IGF-I). It is not known whether unloading of tendon tissue can reduce the expression of collagen and collagen-inducing growth factors. Furthermore, the coordinated response of tendon and muscle tissue to disuse, followed by reloading, is unclear. Female Sprague-Dawley rats were subjected to hindlimb suspension (HS) for 7 or 14 days, followed by 2, 4, 8, or 16 days of reload (RL) (n = 8 in each group). Age-matched controls were included for day 0, day 14 HS, and day 16 RL (n = 8). mRNA expression levels for collagen I (COL1A1), collagen III (COL3A1), TGF-beta1, connective tissue growth factor (CTGF), myostatin, and IGF-I isoforms were measured by real-time RT-PCR in Achilles tendon and soleus muscle. The tendon mass was unchanged, while the muscle mass was reduced by 50% after HS (P < 0.05) and returned to control levels during RL. Collagen I and III, TGF-beta1, and CTGF mRNA levels were unaltered by HS, although collagen III tended to decrease in muscle at day 7 HS. IGF-I isoforms were significantly induced in tendon after 7 days of HS (P < 0.001), and mechanogrowth factor increased in muscle at day 14 HS (P < 0.05). Reload increased muscle collagen I and III mRNA (>10-fold) (P < 0.001) and growth factor expression (P < 0.05), while the tendon response was limited to a moderate induction of collagen expression (2-fold) (P < 0.05). Unloading of tendon and muscle tissue did not reduce expression of collagen and collagen-inducing growth factors, indicating that the response to unloading is not opposite that of loading. Furthermore, the tendon response was clearly different and less pronounced than the muscle tissue response.

Combined isometric, concentric, and eccentric resistance exercise prevents unloading-induced muscle atrophy in rats

Previously, we reported that an isometric resistance training program that was effective in stimulating muscle hypertrophy in ambulatory rats could not completely prevent muscle atrophy during unloading (Haddad F, Adams GR, Bodell PW, Baldwin KM. J Appl Physiol 100: 433–441, 2006). These results indicated that preventing muscle atrophy does not appear to be simply a function of providing an anabolic stimulus. The present study was undertaken to determine if resistance training, with increased volume (3-s contractions) and incorporating both static and dynamic components, would be effective in preventing unloading-induced muscle atrophy. Rats were exposed to 5 days of muscle unloading via tail suspension. During that time one leg received electrically stimulated resistance exercise (RE) that included an isometric, concentric, and eccentric phase. The results of this study indicate that this combined-mode RE provided an anabolic stimulus sufficient to maintain the mass and myofibril content of the trained but not the contralateral medial gastrocnemius (MG) muscle. Relative to the contralateral MG, the RE stimulus increased the amount of total RNA (indicative of translational capacity) as well as the mRNA for several anabolic/myogenic markers such as insulin-like growth factor-I, myogenin, myoferlin, and procollagen III-α-1 and decreased that of myostatin, a negative regulator of muscle size. The combined-mode RE protocol also increased the activity of anabolic signaling intermediates such as p70S6 kinase. These results indicate that a combination of static- and dynamic-mode RE of sufficient volume provides an effective stimulus to stimulate anabolic/myogenic mechanisms to counter the initial stages of unloading-induced muscle atrophy.

Expression of collagen and related growth factors in rat tendon and skeletal muscle in response to specific contraction types

Acute exercise induces collagen synthesis in both tendon and muscle, indicating an adaptive response in the connective tissue of the muscle-tendon unit. However, the mechanisms of this adaptation, potentially involving collagen-inducing growth factors (such as transforming growth factor-beta-1 (TGF-beta-1)), as well as enzymes related to collagen processing, are not clear. Furthermore, possible differential effects of specific contraction types on collagen regulation have not been investigated. Female Sprague-Dawley rats were subjected to 4 days of concentric, eccentric or isometric training (n = 7-9 per group) of the medial gastrocnemius, by stimulation of the sciatic nerve. RNA was extracted from medial gastrocnemius and Achilles tendon tissue 24 h after the last training bout, and mRNA levels for collagens I and III, TGF-beta-1, connective tissue growth factor (CTGF), lysyl oxidase (LOX), metalloproteinases (MMP-2 and -9) and their inhibitors (TIMP-1 and 2) were measured by Northern blotting and/or real-time PCR. In tendon, expression of TGF-beta-1 and collagens I and III (but not CTGF) increased in response to all types of training. Similarly, enzymes/factors involved in collagen processing were induced in tendon, especially LOX (up to 37-fold), which could indicate a loading-induced increase in cross-linking of tendon collagen. In skeletal muscle, a similar regulation of gene expression was observed, but in contrast to the tendon response, the effect of eccentric training was significantly greater than the effect of concentric training on the expression of several transcripts. In conclusion, the study supports an involvement of TGF-beta-1 in loading-induced collagen synthesis in the muscle-tendon unit and importantly, it indicates that muscle tissue is more sensitive than tendon to the specific mechanical stimulus.

Short-term strength training and the expression of myostatin and IGF-I isoforms in rat muscle and tendon: differential effects of specific contraction types

In skeletal muscle, an increased expression of insulin like growth factor-I isoforms IGF-IEa and mechano-growth factor (MGF) combined with downregulation of myostatin is thought to be essential for training-induced hypertrophy. However, the specific effects of different contraction types on regulation of these factors in muscle are still unclear, and in tendon the functions of myostatin, IGF-IEa, and MGF in relation to training are unknown. Female Sprague-Dawley rats were subjected to 4 days of concentric, eccentric, or isometric training ( n = 7–9 per group) of the medial gastrocnemius, by stimulation of the sciatic nerve during general anesthesia. mRNA levels for myostatin, IGF-IEa, and MGF in muscle and Achilles' tendon were measured by real-time RT-PCR. Muscle myostatin mRNA decreased in response to all types of training (2- to 8-fold) ( P < 0.05), but the effect of eccentric training was greater than concentric and isometric training ( P < 0.05). In tendon, myostatin mRNA was detected, but no changes were seen after exercise. IGF-IEa and MGF increased in muscle (up to 15-fold) and tendon (up to 4-fold) in response to training ( P < 0.01). In tendon no difference was seen between training types, but in muscle the effect of eccentric training was greater than concentric training for both IGF-IEa and MGF ( P < 0.05), and for IGF-IEa isometric training had greater effect than concentric ( P < 0.05). The results indicate a possible role for IGF-IEa and MGF in adaptation of tendon to training, and the combined changes in myostatin and IGF-IEa/MGF expression could explain the important effect of eccentric actions for muscle hypertrophy.

Similar acute molecular responses to equivalent volumes of isometric, lengthening, or shortening mode resistance exercise

The present study was undertaken to test the hypothesis that the contraction mode of action [static-isometric (Iso), shortening-concentric (Con), or lengthening-eccentric (Ecc)] used to stress the muscle provides a differential mechanical stimulus eliciting greater or lesser degrees of anabolic response at the initiation of a resistance training program. We performed an acute resistance training study in which different groups of rodents completed four training sessions in either the Iso, Con, or Ecc mode of contraction under conditions of activation and movement specifically designed to elicit equivalent volumes of force accumulation. The results of this experiment indicate that the three modes of contraction produced nearly identical cell signaling, indicative of an anabolic response involving factors such as increased levels of mRNA for IGF-I, procollagen III alpha1, decreased myostatin mRNA, and increased total RNA concentration. The resulting profiles collectively provide evidence that pure mode of muscle action, in and of itself, does not appear to be a primary variable in determining the efficacy of increased loading paradigms with regard to the initiation of selected muscle anabolic responses.

Isometric resistance exercise fails to counteract skeletal muscle atrophy processes during the initial stages of unloading

This study tested the hypothesis that an isometric resistance training paradigm targeting the medial gastrocnemius of adult rodents is effective in preventing muscle atrophy during the early stages of hindlimb unloading by maintaining normal activation of the insulin receptor substrate-1 (IRS-1)/phosphoinositide-3 kinase (PI3K)/Akt signaling pathway. This pathway has been shown to simultaneously create an anabolic response while inhibiting processes upregulating catabolic processes involving expression of key enzymes in the ubiquitination of proteins for degradation. The findings show that during the 5 days of unloading 1) absolute medial gastrocnemius muscle weight reduction occurred by ∼20%, but muscle weight corrected to body weight was not different from normal weight-bearing controls ( P < 0.05); 2) normalized myofibril fraction concentration and content were decreased; and 3) a robust isometric training program, known to induce a hypertrophy response, failed to maintain the myofibril protein content. This response occurred despite fully blunting the increases in the mRNA for of atrogin-1, MURF-1, and myostatin, e.g., sensitive gene markers of an activated catabolic state. Analyses of the IRS-1/PI3K/Akt markers indicated that abundance of IRS-1 and phosphorylation state of Akt and p70S6 kinase were decreased relative to normal control rats, and the resistance training failed to maintain these signaling markers at normal regulatory level. Our findings suggest that to fully prevent muscle atrophy responses affecting the myofibril system during unloading, the volume of mechanical stress must be augmented sufficiently to maintain optimal activity of the IRS-1/PI3K/Akt pathway to provide an effective anabolic stimulus on the muscle.

Regulation of antisense RNA expression during cardiac MHC gene switching in response to pressure overload

Hypertension has been shown to cause cardiac hypertrophy and a shift in myosin heavy chain (MHC) gene expression from the faster alpha- to slower beta-MHC isoform. The expression of the beta- and alpha-MHC pre-mRNAs, mRNAs, as well as the newly discovered antisense beta-RNA were analyzed in three regions of the normal control (NC) and 12-day pressure-overloaded (AbCon) hearts: the left ventricle apex, left ventricle base, and the septum. The RNA analyses in the AbCon heart targeted both the 5' and the 3' ends of each RNA molecule. beta-MHC mRNA expression significantly increased relative to control in all three regions, regardless of the target site (5' or 3' end). In contrast, beta-MHC pre-mRNA expression in the AbCon heart depended on the site of the measurement (5' vs. 3' end). For example, whereas the pre-mRNA did not change when targeted at the 3' end (last intron), it increased significantly in the AbCon heart when measurement targeted the 5' end (2nd intron) of the 25-kb molecule. Analyses of the antisense beta-RNA revealed that its expression in the AbCon heart was significantly decreased relative to control regardless of its measurement site. A negative correlation was observed between the beta-mRNA expression and the antisense beta-RNA (P < 0.05), suggesting an inhibitory role of antisense RNA on the sense beta-MHC gene expression. In contrast, a positive correlation was observed between the antisense beta-RNA and the alpha-MHC pre-mRNA (P < 0.05). This latter observation along with the alpha-MHC gene position relative to that of the beta-antisense suggest that the alpha-MHC sense and beta-antisense transcription are coregulated likely via common intergenic regulatory sequences. Our results suggest that the increased beta-MHC expression in the AbCon heart not only is the result of increased beta-MHC transcription but also involves an antisense beta-RNA regulation scheme. Although the exact mechanism concerning antisense regulation is not clear, it could involve modulation of both transcriptional activity of the beta-MHC gene and posttranscriptional processing.

Comments on classical papers

This essay looks at the historical significance of one APS classic paper that is freely available online: Margaria R, Edwards HT, and Dill DB. The possible mechanisms of contracting and paying the oxygen debt and the role of lactic acid in muscular contraction. Am J Physiol 106: 689–715, 1933 ( http://ajplegacy.physiology.org/cgi/reprint/106/3/689 ).

Pretranslational markers of contractile protein expression in human skeletal muscle: effect of limb unloading plus resistance exercise

Previously, it has been shown that the human ground-based model consisting of unilateral limb suspension (ULLS) induces atrophy and reduced strength of the affected quadriceps muscle group. Resistance exercise (RE) involving concentric-eccentric actions, in the face of ULLS, is effective in ameliorating these deficits. The goal of the present study was to determine whether alterations in contractile protein gene expression, e.g., myosin heavy chain and actin, as studied at the pretranslational level, provide molecular markers concerning the deficits that occur in muscle mass/volume during ULLS, as well as its maintenance in response to ULLS plus RE. Muscle biopsies were obtained from the vastus lateralis muscle of 31 middle-aged men and women before and after 5 wk of ULLS, ULLS plus RE, or RE only. The RE paradigm consisted of 12 sessions of 4 sets of 7 concentric-eccentric knee extensions. Our findings show that there were net deficits in total RNA, total mRNA, and actin and myosin heavy chain mRNA levels of expression after ULLS ( P < 0.05), whereas these alterations were blunted in the two groups receiving RE. Additional observations involving IGF-I and its associated receptor and binding proteins suggest that RE postures the skeletal muscle for signaling processes that favor a greater anabolic state relative to that observed in the ULLS group. Collectively, these findings suggest that molecular markers of contractile protein gene expression serve as useful subcellular indicators for ascertaining the underlying mechanisms regulating alterations in muscle mass in human subjects in response to altered loading states.

Transcriptional regulation of IGF-I expression in skeletal muscle

The present study investigated the role of transcription in the regulation of insulin-like growth factor (IGF)-I expression in skeletal muscle. RT-PCR was used to determine endogenous expression of IGF-I pre-mRNA and mRNA in control (Con) and functionally overloaded (FO) rat plantaris. The transcriptional activities of five different-length IGF-I promoter fragments controlling transcription of a firefly luciferase (FLuc) reporter gene were tested in vitro by transfection of myoblasts or in vivo during FO by direct gene transfer into the plantaris. Increased endogenous IGF-I gene transcription during 7 days of plantaris FO was evidenced by an approximately 140-160% increase (P < 0.0001) in IGF-I pre-mRNA (a transcriptional marker). IGF-I mRNA expression also increased by approximately 90% (P < 0.0001), and it was correlated (R = 0.93; P < 0.0001) with the pre-mRNA increases. The three longest IGF-I exon 1 promoters induced reporter gene expression in proliferating C2C12 and L6E9 myoblasts. In differentiated L6E9 myotubes, promoter activity increased approximately two- to threefold over myoblasts. Overexpression of calcineurin and MyoD increased the activity of the -852/+192 promoter in C2C12 myotubes by approximately 5- and approximately 18-fold, respectively. However, FO did not induce these exogenous promoter fragments. Nevertheless, the present findings are consistent with the hypothesis that the IGF-I gene is transcriptionally regulated during muscle hypertrophy in vivo as evidenced by the induction of the endogenous IGF-I pre-mRNA during plantaris FO. The exon 1 promoter region of the IGF-I gene is sufficient to direct inducible expression in vitro; however, an in vivo response to FO may require elements outside the -852/+346 region of the exon 1 IGF-I promoter or features inherent to the endogenous IGF-I gene.

Atrophy responses to muscle inactivity. II. Molecular markers of protein deficits

We examined the expression of several molecular markers of protein balance in response to skeletal muscle atrophy induced by spinal cord isolation (SI; i.e., a complete transection of the spinal cord at both a midthoracic and a high sacral level plus complete deafferentation between the two transection sites). This treatment nearly eliminates neuromuscular activity (activation and loading) of the hindlimb muscles while maintaining neuromuscular connectivity. SI was associated with a reduced transcriptional activity (via pre-mRNA analyses) of myosin heavy chain (MHC) and actin. In addition, there was an increased gene expression of enzyme systems impacting protein degradation (calpain-1; plus enzymes associated with polyubquitination processes) that could further contribute to the protein deficits in the SI muscles via degradative pathways. IGF-I receptor and binding protein-5 mRNA expression was induced throughout the 15-day period of SI, whereas IGF-I mRNA was induced at 8 and 15 days. These responses occurred in the absence of an upregulation of translational regulatory proteins (p70 S6 kinase; eukaryotic 4E binding protein 1) to compensate for the decreased protein translational capacity. These data collectively demonstrate that 1). the molecular changes accompanying SI-induced muscle atrophy are not necessarily the reverse of those occurring during muscle hypertrophy, and 2). the rapid and marked atrophy that defines this model of muscle inactivity is likely the result of multifactorial processes affecting transcription, translation, and protein degradation.

Atrophy responses to muscle inactivity. I. Cellular markers of protein deficits

The goal of this study was to use the model of spinal cord isolation (SI), which blocks nearly all neuromuscular activity while leaving the motoneuron muscle-fiber connections intact, to characterize the cellular processes linked to marked muscle atrophy. Rats randomly assigned to normal control and SI groups were studied at 0, 2, 4, 8, and 15 days after SI surgery. The slow soleus muscle atrophied by approximately 50%, with the greatest degree of loss occurring during the first 8 days. Throughout the SI duration, muscle protein concentration was maintained at the control level, whereas myofibrillar protein concentration steadily decreased between 4 and 15 days of SI, and this was associated with a 50% decrease in myosin heavy chain (MHC) normalized to total protein. Actin relative to the total protein was maintained at the control level. Marked reductions occurred in total RNA and DNA content and in total MHC and actin mRNA expressed relative to 18S ribosomal RNA. These findings suggest that two key factors contributing to the muscle atrophy in the SI model are 1). a reduction in ribosomal RNA that is consistent with a reduction in protein translational capacity, and 2). insufficient mRNA substrate for translating key sarcomeric proteins comprising the myofibril fraction, such as MHC and actin. In addition, the marked selective depletion of MHC protein in the muscles of SI rats suggests that this protein is more vulnerable to inactivity than actin protein. This selective MHC loss could be a major contributor for the previously reported loss in the functional integrity of SI muscles. Collectively, these data are consistent with the involvement of pretranslational and translational processes in muscle atrophy due to SI.

Transcriptional regulation of the type I myosin heavy chain gene in denervated rat soleus

Denervation (DEN) of rat soleus is associated with a decreased expression of slow type I myosin heavy chain (MHC) and an increased expression of the faster MHC isoforms. The molecular mechanisms behind these shifts remain unclear. We first investigated endogenous transcriptional activity of the type I MHC gene in normal and denervated soleus muscles via pre-mRNA analysis. Our results suggest that the type I MHC gene is regulated via transcriptional processes in the denervated soleus. Deletion and mutational analysis of the rat type I MHC promoter was then used to identify cis elements or regions of the promoter involved in this response. DEN significantly decreased in vivo activity of the -3,500, -2,500, -914, -408, -299, and -215 bp type I MHC promoters, relative to the alpha-skeletal actin promoter. In contrast, normalized -171 promoter activity was unchanged. Mutation of the betae3 element (-214/-190) in the -215 promoter and deletion of this element (-171 promoter) blunted type I downregulation with DEN. In contrast, betae3 mutation in the -408 promoters was not effective in attenuating the DEN response, suggesting the existence of additional DEN-responsive sites between -408 and -215. Western blotting and gel mobility supershift assays demonstrated decreased expression and DNA binding of transcription enhancer factor 1 (TEF-1) with DEN, suggesting that this decrease may contribute to type I MHC downregulation in denervated muscle.

Functional and cellular adaptations of rodent skeletal muscle to weightlessness

Microgravity perturbs a wide variety of physiological systems. Specifically, skeletal muscle has been shown to undergo dramatic changes even during spaceflights of short durations. Despite a growing body of knowledge, many critical issues related to sarcomeric gene expression and the translation of this information into muscle function remain relatively unexplored. Ultimately, the types and quantities of contractile proteins present in skeletal muscle fibers determine muscle function. As shown in Figure 1, the pool of contractile proteins is determined by 4 important processes: i) transcription of sarcomeric genes; ii) translation of sarcomeric mRNA transcripts; iii) myofibrillar assembly; and iv) myofibrillar disassembly (i.e., protein degradation). Any factor(s) affecting one or more of these systems will ultimately impact skeletal muscle function. With respect to Figure 1, the objective of this paper is to provide a focused summary of the affects of short duration spaceflights on myosin composition of the soleus (SOL) muscle. This will be achieved by describing the affects of spaceflight upon: i) mechanical properties of skeletal muscle that are determined by the types and quantity of myosin; ii) myosin heavy chain (MHC) protein isoform composition; and ii) MHC mRNA isoform content. Data reported in this paper are from spaceflight missions having durations of 6 (PARE-02;2) and 14 days (SLS-2).

Future research directions in seeking countermeasures to weightlessness

The goal of this paper is to briefly review the state of knowledge concerning the adaptive properties of mammalian skeletal muscle in response to varying duration in weightlessness, to identify voids in the understanding of this adaptive process, and to provide some insight for undertaking future research on this important topic.

The role of stem cells in midgut growth and regeneration

The Manduca sexta (L.) [Lepidoptera: Sphingidae] and Heliothis virescens (F.) [Lepidoptera: Noctuidae] midguts consist of a pseudostratified epithelium surrounded by striated muscle and tracheae. This epithelium contains goblet, columnar, and basal stem cells. The stem cells are critically important in that they are capable of massive proliferation and differentiation. This growth results in a fourfold enlargement of the midgut at each larval molt. The stem cells are also responsible for limited cell replacement during repair. While the characteristics of the stem cell population vary over the course of an instar, stem cells collected early in an instar and those collected late can start in vitro cultures. Cultures of larval stem, goblet, and columnar cells survive in vitro for several mo through proliferation and differentiation of the stem cells. One of the two polypeptide differentiation factors which have been identified and characterized from the culture medium has now been shown to be present in midgut in vivo. Thus the ability to examine lepidopteran midgut stem cell growth in vitro and in vivo is proving to be effective in determining the basic features of stem cell action and regulation.

In vivo regulation of the beta-myosin heavy chain gene in hypertensive rodent heart

The main goal of this study was to examine the transcriptional activity of different-length beta-myosin heavy chain (beta-MHC) promoters in the hypertensive rodent heart using the direct gene transfer approach. A hypertensive state was induced by abdominal aortic constriction (AbCon) sufficient to elevate mean arterial pressure by approximately 45% relative to control. Results show that beta-MHC promoter activity of all tested wild-type constructs, i.e., -3500, -408, -299, -215, -171, and -71 bp, was significantly increased in AbCon hearts. In the normal control hearts, expression of the -71-bp construct was comparable to that of the promoterless vector, but its induction by AbCon was comparable to that of the other constructs. Additional results, based on mutation analysis and DNA gel mobility shift assays targeting betae1, betae2, GATA, and betae3 elements, show that these previously defined cis-elements in the proximal promoter are indeed involved in maintaining basal promoter activity; however, none of these elements, either individually or collectively, appear to be major players in mediating the hypertension response of the beta-MHC gene. Collectively, these results indicate that three separate regions on the beta-MHC promoter are involved in the induction of the gene in response to hypertension: 1) a distal region between -408 and -3500 bp, 2) a proximal region between -299 and -215 bp, and 3) a basal region within -71 bp of the transcription start site. Future research needs to further characterize these responsive regions to more fully delineate beta-MHC transcriptional regulation in response to pressure overload.

Temporal effects of inactivty on myosin heavy chain gene expression in rat slow muscle

Myosin heavy chain (MHC) mRNA and protein profiles in adult rat soleus and adductor longus were determined after 4, 8, 15, 30, 60, and 90 days of spinal cord isolation (SI). SI results in complete neuromuscular inactivity while leaving the motoneuron-muscle fiber connections intact. From 15 to 90 days, type I MHC mRNA was significantly decreased, whereas type I MHC protein did not significantly decrease until 30 and 60 days in the soleus and adductor longus, respectively. However, in both muscles, slow MHC downregulation was offset by significant upregulation of the faster MHC isoforms, primarily IIx. From 60 to 90 days, type I MHC was almost completely replaced with faster isoforms at the mRNA and protein levels. Thus, chronic inactivity and unloading of slow rat hindlimb muscles shifted the MHC profile from predominately type I to type IIx MHC mRNA and protein.

Effects of different activity and inactivity paradigms on myosin heavy chain gene expression in striated muscle

The goal of this mini-review is to summarize findings concerning the role that different models of muscular activity and inactivity play in altering gene expression of the myosin heavy chain (MHC) family of motor proteins in mammalian cardiac and skeletal muscle. This was done in the context of examining parallel findings concerning the role that thyroid hormone (T(3), 3,5,3'-triiodothyronine) plays in MHC expression. Findings show that both cardiac and skeletal muscles of experimental animals are initially undifferentiated at birth and then undergo a marked level of growth and differentiation in attaining the adult MHC phenotype in a T(3)/activity level-dependent fashion. Cardiac MHC expression in small mammals is highly sensitive to thyroid deficiency, diabetes, energy deprivation, and hypertension; each of these interventions induces upregulation of the beta-MHC isoform, which functions to economize circulatory function in the face of altered energy demand. In skeletal muscle, hyperthyroidism, as well as interventions that unload or reduce the weight-bearing activity of the muscle, causes slow to fast MHC conversions. Fast to slow conversions, however, are seen under hypothyroidism or when the muscles either become chronically overloaded or subjected to intermittent loading as occurs during resistance training and endurance exercise. The regulation of MHC gene expression by T(3) or mechanical stimuli appears to be strongly regulated by transcriptional events, based on recent findings on transgenic models and animals transfected with promoter-reporter constructs. However, the mechanisms by which T(3) and mechanical stimuli exert their control on transcriptional processes appear to be different. Additional findings show that individual skeletal muscle fibers have the genetic machinery to express simultaneously all of the adult MHCs, e.g., slow type I and fast IIa, IIx, and IIb, in unique combinations under certain experimental conditions. This degree of heterogeneity among the individual fibers would ensure a large functional diversity in performing complex movement patterns. Future studies must now focus on 1) the signaling pathways and the underlying mechanisms governing the transcriptional/translational machinery that control this marked degree of plasticity and 2) the morphological organization and functional implications of the muscle fiber's capacity to express such a diversity of motor proteins.

Transcutaneous oximetry and skin surface temperature as objective measures of pressure ulcer risk

OBJECTIVE

To determine if transcutaneous oximetry and skin surface temperature could measure differences in skin perfusion between 2 groups of hospitalized patients: (1) those who made gross or subtle movements in bed, and (2) those who could not move.

DESIGN

A quasi-experimental design was used.

SETTINGS AND PATIENTS

A convenience sample of 38 subjects (17 males, 21 females) was selected from patients admitted to a neurological/neurosurgical critical care and step-down unit in a university health sciences center hospital.

MAIN OUTCOME MEASURES

Prepressure and postpressure oximetry and temperature readings were taken over the sacrum and recorded every minute for 15 minutes. Pressure was applied by positioning subjects on their backs for 2 hours; subjects were observed for gross or subtle movement. Oximetry measurements were taken and recorded every minute.

RESULTS

Paired t tests showed subjects who moved (n = 24) had a statistically significant difference between prepressure and postpressure temperature means over subjects who could not move (n = 14). Postpressure transcutaneous carbon dioxide (TcPCO2) mean levels were lower at all time-points and transcutaneous oxygen (TcPO2) mean levels were higher at most time-points in subjects who moved. Analysis of variance between the 2 groups during pressure showed a statistically significant difference in TcPCO2 levels, but not TcPO2 levels.

CONCLUSIONS

Both methods identified statistically significant differences in moving and nonmoving patients and may prove to be useful tools in assessing pressure ulcer risk. Further research with both methods across various patient populations is needed.

The interaction of space flight and thyroid state on somatic and skeletal muscle growth and myosin heavy chain expression on neonatal rodents

The goal of this research project was to determine how the absence of gravity, when imposed at critical stages (post birth day 7 and 14) in the developmental scheme of the neonatal rodent, impacts body and skeletal muscle growth, as well as expression of the myosin heavy chain (MHC) gene family of motor proteins, which are involved in the regulation of muscle contraction.

In vivo regulation of the beta-myosin heavy chain gene in soleus muscle of suspended and weight-bearing rats

In the weight-bearing hindlimb soleus muscle of the rat, approximately 90% of muscle fibers express the beta-myosin heavy chain (beta-MHC) isoform protein. Hindlimb suspension (HS) causes the MHC isoform population to shift from beta toward the fast MHC isoforms. Our aim was to establish a model to test the hypothesis that this shift in expression is transcriptionally regulated through specific cis elements of the beta-MHC promoter. With the use of a direct gene transfer approach, we determined the activity of different length beta-MHC promoter fragments, linked to a firefly luciferase reporter gene, in soleus muscle of control and HS rats. In weight-bearing rats, the relative luciferase activity of the longest beta-promoter fragment (-3500 bp) was threefold higher than the shorter promoter constructs, which suggests that an enhancer sequence is present in the upstream promoter region. After 1 wk of HS, the reporter activities of the -3500-, -914-, and -408-bp promoter constructs were significantly reduced ( approximately 40%), compared with the control muscles. However, using the -215-bp construct, no differences in promoter activity were observed between HS and control muscles, which indicates that the response to HS in the rodent appears to be regulated within the -408 and -215 bp of the promoter.

MHC polymorphism in rodent plantaris muscle: effects of mechanical overload and hypothyroidism

In a previous study, it was shown that a combined treatment of hyperthyroidism and hindlimb suspension effectively converted the slow-twitch soleus muscle to a fast-twitch muscle. The objective of this study was to test the hypothesis that hypothyroidism [absence of triiodothyronine (-T(3))] and mechanical overload (OV) would convert the plantaris (Plan) muscle from a fast- to a slow-twitch muscle. Single-fiber analyses demonstrated that the normal rodent Plan muscle was composed of approximately 13 different fiber types as defined by myosin heavy chain (MHC) isoform content. The largest proportion of fibers ( approximately 35%) coexpressed the fast type IIX and IIB MHC isoforms (i.e., type IIX/IIB fibers). In this context, the combined intervention of -T(3) and OV produced a significant reduction in the relative proportion of the fast type IIB MHC isoform and a concomitant increase in the slow type I MHC isoform. These transitions were manifested by a large decrease in the proportion of type IIX/IIB fibers and a large increase in fibers coexpressing all four MHC protein isoforms. The mechanical consequences of these transitions, however, were modest, producing a 15% decrease in maximal shortening velocity. The findings of this study demonstrate that -T(3) + OV does produce a partial shift toward a slower phenotype; however, the high degree of polymorphism found in the Plan muscle represents a unique design that appears to minimize the functional consequences of these significant MHC transitions.

Single-fiber myosin heavy chain polymorphism during postnatal development: modulation by hypothyroidism

The primary objective of this study was to follow the developmental time course of myosin heavy chain (MHC) isoform transitions in single fibers of the rodent plantaris muscle. Hypothyroidism was used in conjunction with single-fiber analyses to better describe a possible linkage between the neonatal and fast type IIB MHC isoforms during development. In contrast to the general concept that developmental MHC isoform transitions give rise to muscle fibers that express only a single MHC isoform, the single-fiber analyses revealed a very high degree of MHC polymorphism throughout postnatal development. In the adult state, MHC polymorphism was so pervasive that the rodent plantaris muscles contained approximately 12-15 different pools of fibers (i.e., fiber types). The degree of polymorphism observed at the single-fiber level made it difficult to determine specific developmental schemes analogous to those observed previously for the rodent soleus muscle. However, hypothyroidism was useful in that it confirmed a possible link between the developmental regulation of the neonatal and fast type IIB MHC isoforms.

Effects of spaceflight and thyroid deficiency on hindlimb development. I. Muscle mass and IGF-I expression

Thyroid deficiency (TD) in neonatal rats causes reduced growth of skeletal muscle that is disproportionately greater than that for other tissues (G. R. Adams, S. A. McCue, M. Zeng, and K. M. Baldwin. Am. J. Physiol. Regulatory Integrative Comp. Physiol. 276: R954-R961, 1999). TD depresses plasma insulin-like growth factor I (IGF-I) levels, suggesting a mechanism for this effect. We hypothesized that TD and exposure to spaceflight (SF) would interact to reduce skeletal muscle growth via a reduction in IGF-I levels. Neonatal rats were flown in space for 16 days. There was a similar, nonadditive reduction in the growth of the body ( approximately 50%) and muscle weight (fast muscles, approximately 60%) with either TD or SF. In the soleus muscle, either SF or TD alone resulted in growth reductions that were augmented by SF-TD interactions. There were strong correlations between 1) muscle mass and muscle IGF-I levels and 2) circulating IGF-I and body weight. These results indicate that either hypothyroidism or exposure to SF will limit the somatic and muscle-specific growth of neonatal rats. The impact of these perturbations on skeletal muscle growth is relatively greater than the effect on somatic growth. The mechanisms by which either TD or SF impact growth appear to have a common pathway involving the control of plasma and muscle IGF-I concentrations.

Effects of spaceflight and thyroid deficiency on rat hindlimb development. II. Expression of MHC isoforms

Both slow-twitch and fast-twitch muscles are undifferentiated after birth as to their contractile protein phenotype. Thus we examined the separate and combined effects of spaceflight (SF) and thyroid deficiency (TD) on myosin heavy chain (MHC) gene expression (protein and mRNA) in muscles of neonatal rats (7 and 14 days of age at launch) exposed to SF for 16 days. Spaceflight markedly reduced expression of the slow, type I MHC gene by approximately 55%, whereas it augmented expression of the fast IIx and IIb MHCs in antigravity skeletal muscles. In fast muscles, SF caused subtle increases in the fast IIb MHC relative to the other adult MHCs. In contrast, TD prevented the normal expression of the fast MHC phenotype, particularly the IIb MHC, whereas TD maintained expression of the embryonic/neonatal MHC isoforms; this response occurred independently of gravity. Collectively, these results suggest that normal expression of the type I MHC gene requires signals associated with weight-bearing activity, whereas normal expression of the IIb MHC requires an intact thyroid state acting independently of the weight-bearing activities typically encountered during neonatal development of laboratory rodents. Finally, MHC expression in developing muscles is chiefly regulated by pretranslational processes based on the tight relationship between the MHC protein and mRNA data.

Role of denervation in modulating IIb MHC gene expression in response to T(3) plus unloading state

Previously, we have reported that the combination of hindlimb suspension (HS) and thyroid hormone [triiodothyronine (T(3))] treatment induces the de novo expression of the fast IIb myosin heavy chain (MHC) gene in the soleus. Thus we tested the hypotheses that the induction of IIb MHC expression with HS + T(3) is prevented with denervation and that this IIb induction is regulated by transcriptional processes. Adult female rats were subjected to 2 wk of combined HS + T(3) in which one side of the lower leg was simultaneously denervated. HS + T(3) caused decreased expression of the slow type I MHC and concomitant increases in both the fast type IIx and IIb MHC isoforms in the intact soleus muscle. Denervation prevented the endogenous expression of the IIb MHC gene at both the protein and mRNA levels. Although HS + T(3) intervention was able to markedly increase the expression of the 2.6-kb IIb MHC promoter-reporter construct using direct gene transfer, this induction, however, was not inhibited by denervation. These findings collectively suggest that normal innervation is essential for inducing the unique expression of the IIb MHC in a slow muscle in response to HS + T(3); however, in the denervated muscle, there is a discordance between the regulation of the endogenous IIb MHC gene relative to the exogenous IIb MHC promoter-reporter construct.

Research in the exercise sciences: where do we go from here?

The goal of this article is to provide a perspective on how research involving the acute and chronic effects of exercise (referred to as "exercise sciences") on the structure and function of organs systems will evolve in the next century. Within the last 30 years, exercise-related research has rapidly transitioned from an organ to a subcellular/molecular focus. Thus future research will continue to be heavily influenced by molecular biology tools, fueled by both emerging technologies (e.g., "gene-chip microarrays") designed to dissect gene function on a macro scale as well as by the completion of the human genome project in which the approximately 80,000 genes comprising humans will be completely sequenced. These successes will drive the emerging fields of functional genomics (the dissecting of a gene's identity and function) and proteomics (the study of the properties of proteins). Funding levels at the National Institutes of Health will likely increase in order to expand these emerging fields as well as provide avenues for translating fundamental knowledge into solving the complexities of a number of degenerative diseases influenced heavily by activity/inactivity factors such as cardiopulmonary disease, diabetes, obesity, and the debilitating disorders associated with aging. Thus there are many challenges facing future exercise scientists who must harness the new technologies and take an aggressive stance in bringing this important field to the forefront.

Time course of changes in markers of myogenesis in overloaded rat skeletal muscles

During the process of compensatory muscle hypertrophy, satellite cells are thought to proliferate, differentiate, and then fuse with existing myofibers. We hypothesized that early in this process changes occur in the expression of cellular markers indicative of the onset of myogenic processes. The plantaris muscles of rats were overloaded via the unilateral ablation of synergists. Groups of rats were killed at time points from 6 h to 12 days. Changes in muscle gene expression (mRNA) of cyclin D1, p21, myogenin, MyoD, and insulin-like growth factor I (IGF-I, mRNA and peptide) were measured. Cyclin D1 (a cell cycle marker) was increased after 24 h of overloading and corresponded with changes in muscle DNA content. In contrast, p21 and myogenin, markers of cellular differentiation, were increased after just 12 h. Muscle IGF-I peptide levels were also increased at early time points. The results of this study indicate that myogenic processes are activated in response to increased loading at very early time points (e.g., 12 h) and that IGF-I may be modulating this response. Furthermore, these findings suggest that some cells may have been differentiating very early in the adaptation process before events leading to cellular proliferation have been initiated.

In vivo regulation of beta-MHC gene in rodent heart: role of T3 and evidence for an upstream enhancer

Cardiac beta-myosin heavy chain (beta-MHC) gene expression is mainly regulated through transcriptional processes. Although these results are based primarily on in vitro cell culture models, relatively little information is available concerning the interaction of key regulatory factors thought to modulate MHC expression in the intact rodent heart. Using a direct gene transfer approach, we studied the in vivo transcriptional activity of different-length beta-MHC promoter fragments in normal control and in altered thyroid states. The test beta-MHC promoter was fused to a firefly luciferase reporter gene, whereas the control alpha-MHC promoter was fused to the Renilla luciferase reporter gene and was used to account for variations in transfection efficiency. Absolute reporter gene activities showed that beta- and alpha-MHC genes were individually and reciprocally regulated by thyroid hormone. The beta-to-alpha ratios of reporter gene expression demonstrated an almost threefold larger beta-MHC gene expression in the longest than in the shorter promoter fragments in normal control animals, implying the existence of an upstream enhancer. A mutation in the putative thyroid response element of the -408-bp beta-MHC promoter construct caused transcriptional activity to drop to null. When studied in the -3, 500-bp beta-MHC promoter, construct activity was reduced ( approximately 100-fold) while thyroid hormone responsiveness was retained. These findings suggest that, even though the bulk of the thyroid hormone responsiveness of the gene is contained within the first 215 bp of the beta-MHC promoter sequence, the exact mechanism of triiodothyronine (T3) action remains to be elucidated.

Time course of myosin heavy chain transitions in neonatal rats: importance of innervation and thyroid state

During the postnatal period, rat limb muscles adapt to weight bearing via the replacement of embryonic (Emb) and neonatal (Neo) myosin heavy chains (MHCs) by the adult isoforms. Our aim was to characterize this transition in terms of the six MHC isoforms expressed in skeletal muscle and to determine the importance of innervation and thyroid hormone status on the attainment of the adult MHC phenotype. Neonatal rats were made hypothyroid via propylthiouracil (PTU) injection. In normal and PTU subgroups, leg muscles were unilaterally denervated at 15 days of age. The MHC profiles of plantaris (PLN) and soleus (Sol) muscles were determined at 7, 14, 23, and 30 days postpartum. At day 7, the Sol MHC profile was 55% type I, 30% Emb, and 10% Neo; in the PLN, the pattern was 60% Neo and 25% Emb. By day 30 the Sol and PLN had essentially attained an adult MHC profile in the controls. PTU augmented slow MHC expression in the Sol, whereas in the PLN it markedly repressed IIb MHC by retaining neonatal MHC expression. Denervation blunted the upregulation of IIb in the PLN and of Type I in the Sol and shifted the pattern to greater expression of IIa and IIx MHCs in both muscles. In contrast to previous observations, these findings collectively suggest that both an intact thyroid and innervation state are obligatory for the attainment of the adult MHC phenotype, particularly in fast-twitch muscles.

Differential response of fast hindlimb extensor and flexor muscles to exercise in adult spinalized cats

Adult cats were spinal transected (T12-13) and maintained for approximately 6 months. Spinal cats were either not trained (N-T) or trained for 30 min/day to either step on a treadmill (Stp-T) or stand (Std-T). Spinalization resulted in a decrease in the mass and maximum tension potential of the medial gastrocnemius (MG), a fast ankle extensor. These adaptations were ameliorated in Std-T but not Stp-T cats. The maximum rate of shortening was elevated by 18 (ns), 34, and 19 (ns)% in the N-T, Std-T, and Stp-T cats, respectively, a finding consistent with a shift in the percentage of fast fibers, a decrease in the percentage of fibers expressing only type I myosin heavy chain, and an increase in myofibrillar adenosine triphosphatase activity. The shift toward a faster fiber type profile in the tibialis anterior (TA), a fast ankle flexor, was of a lesser magnitude than in the MG. There were no significant effects on the contractile properties of the TA in any group of spinal cats. The greater preservation of muscle mass, shift toward faster physiological and biochemical properties, and fatigability in the MG of Std-T than Stp-T cats suggest that factors other than the level of activation and force generation must play a role in muscle homeostasis. From a clinical perspective, the results indicate that muscles innervated by motor neurons below the level of a complete spinal cord lesion are affected differentially by specific neuromuscular activity patterns.

Interaction of hyperthyroidism and hindlimb suspension on skeletal myosin heavy chain expression

We examined the novel interaction of hyperthyroidism and hindlimb suspension on the pattern of myosin heavy chain (MHC) expression (mRNA and protein) in skeletal muscles. Female Sprague-Dawley rats were assigned to four groups: 1) normal control (Con); 2) thyroid hormone treated [150 micrograms 3,5,3'-triiodothyronine (T3). kg-1. day-1] (T3); 3) hindlimb suspension (HS); or 4) T3-treated and HS (T3 + HS). Results show for the first time the novel observation that the combination T3 + HS induces a rapid and sustained, marked (80-90%) downregulation of type I MHC gene expression that is mirrored temporally by concomitant marked upregulation of type IIb MHC gene expression, as evidenced by the de novo synthesis of type IIb MHC protein in the soleus. The fast type IIx MHC isoform showed a differential response among the experimental groups, generally increasing with the separate and combined treatments in both the soleus and vastus intermedius muscles while decreasing in the plantaris muscles. The fast type IIa MHC was the least responsive to suspension of the MHCs and reflected its greatest responsiveness to T3 treatment while also undergoing differential adaptations in slow vs. fast muscle (increases vs. decreases, respectively). These results confirm previous findings that all four adult MHC genes are sensitive to T3 and suspension in a muscle-specific manner. In addition, we show that T3 + HS can interact synergistically to create novel adaptations in MHC expression that could not be observed when each factor was imposed separately.

Novel transitions in MHC isoforms: separate and combined effects of thyroid hormone and mechanical unloading

Single-fiber (n = 3,818 fibers) electrophoretic analyses were used to delineate the separate and combined effects of hyperthyroidism (T3) and hindlimb suspension (HS) on the myosin heavy chain (MHC) isoform composition (1-, 2-, and 4-wk time points) of the rat soleus muscle. The key findings of this study are as follows. First, T3 and HS both altered the distribution of MHC isoforms at the single-fiber level; however, the populations of fibers produced by these two interventions were clearly different from one another. Second, T3 + HS rapidly converted the soleus into a fast muscle, producing large increases in the relative contents of the fast type IIx and IIb MHC isoforms which were primarily expressed in several populations of hybrid fibers (e.g., types I/IIa/IIx, I/IIx/IIb, I/IIa/IIx/IIb). Finally, T3 + HS produced unique populations of hybrid fibers that did not adhere to the Ileft arrow over right arrow IIaleft arrow over right arrow IIxleft arrow over right arrow IIb sequential scheme of MHC plasticity. Collectively, the findings of this study demonstrate that the intervention of T3 + HS is a powerful model for manipulating and studying MHC isoform plasticity in slow skeletal muscle.

Pressure ulcer risk following critical traumatic injury

Previously healthy, critically injured survivors of serious trauma are at risk for developing pressure ulcers. A prospective, descriptive, clinical research study was designed to identify the factors associated with the incidence, number, stage, and location of pressure ulcers and to assess the predictive validity of the Braden Scale for Predicting Pressure Sore Risk in this population. A sample of 36 patients was assessed biweekly throughout hospitalization and showed a pressure ulcer incidence of 30.6%. The small sample size in this study precludes generalization of results. However, several findings have potential implications for clinical practice if they can be replicated in larger studies. Total Braden Scale scores were lower for all assessment points in all subjects who developed pressure ulcers. Subscale scores varied in predictive ability from 100% to 0%. Logistic regression using the first two assessment points produced a model that combined mobility and moisture subscale scores. This model correctly predicted 77.78% of those subjects who developed pressure ulcers.

Effects of isometric training on skeletal myosin heavy chain expression

This study tested the hypothesis that an isometric resistance-training program induces upregulation of slow myosin heavy chain (MHC) expression in a fast-twitch skeletal muscle. Thus we studied the effects of two resistance-training programs on rodent medial gastrocnemius (MG) muscle that were designed to elicit repetitive isometric contractions (10-12 per set; 4 sets per session) of different duration (8 vs. 5 s) and activation frequency (100 vs. 60 Hz) per contraction during each training session (total of 6 and 12 sessions). Results showed that both training paradigms produced significant increases in muscle weight ( approximately 11-13%) after completion of training (P < 0.05). Significant transformations in MHC expression occurred and involved specifically a decrease in the relative expression of the fast type IIb MHC and concomitant increased expression of the fast type IIx MHC. These adaptations were observed in both the "white" and "red" regions of the MG, and they occurred at both the mRNA and protein levels. These adaptations were detected after only six training sessions. Neither of the training programs produced any change in the relative expression of either the slow type I MHC or the moderately fast type IIa MHC, which can be upregulated in the red MG by chronic functional overload. These findings show that the isometric protocols used in this investigation were not sufficient to induce the hypothesized changes in the myosin heavy chain isoform expression in rodent skeletal muscle.

Thyroid receptor plasticity in striated muscle types: effects of altered thyroid state

This study examined nuclear thyroid receptor (TR) maximum binding capacity (Bmax), dissociation constant (Kd), and TR isoform (alpha1, alpha2, beta1) mRNA expression in rodent cardiac, "fast-twitch white," "fast-twitch red," and "slow-twitch red" muscle types as a function of thyroid state. These analyses were performed in the context of slow-twitch type I myosin heavy-chain (MHC) expression, a 3,5,3'-triiodothyronine (T3)-regulated gene that displays varying responsiveness to T3 in the above tissues. Nuclear T3 binding analyses show that the skeletal muscle types express more TRs per unit DNA than cardiac muscle, whereas the latter has a lower Kd than the former. Altered thyroid state had little effect on either cardiac Bmax or Kd, whereas hypothyroidism increased Bmax in the skeletal muscle types without affecting its Kd. Cardiac muscle demonstrated the greatest mRNA signal of TR-beta1 compared with the other muscle types, whereas the TR-alpha1 mRNA signals were more abundant in the skeletal muscle types, especially fast-twitch red. Hyperthyroidism increased the ratio of beta1 to alpha1 and decreased the ratio of alpha2- to alpha1+beta1-mRNA signal across the muscle types, whereas hypothyroidism caused the opposite effects. The nuclear T3 affinity correlated significantly with the TR-beta1 mRNA expression but not with TR-alpha1 mRNA expression. Collectively, these findings suggest that, despite a divergent pattern of TR mRNA expression in the different muscle types, these patterns follow similar qualitative changes under altered thyroid state. Furthermore, TR expression pattern cannot account for the quantitative and qualitative changes in type I MHC expression that occur in the different muscle types.

Cyclical passive stretch influences the mechanical properties of the inactive cat soleus

The effects of cyclical, passive manipulation (PM, 30 min day(-1), 5 days week(-1) for 6 months) mimicking the length excursions observed during stepping on the mechanical and associated biochemical properties of the inactive cat soleus muscle were determined in five cats. Inactivity was produced via spinal cord isolation (SI), i.e. complete spinal cord transections at low thoracic and high sacral levels and bilateral dorsal rhizotomy between the transection sites. Passive manipulation was administered to one leg of each SI cat. Compared with normal controls, SI resulted in approximately 70% decrease in weight, an 80% decrease in maximum tetanic tension (Po) and an approximately 100% increase in maximum rate of shortening (Vmax) and myosin adenosine triphosphatase (mATPase) activity of the soleus. The passive manipulation regime partially ameliorated these effects. When compared with the control SI soleus, the SI-PM soleus weight and maximum tetanic tension were 12 and 21% higher, respectively, and the Vmax and mATPase activity 21 and 12% (p > 0.05) lower, respectively. Thus, inactivity resulted in a smaller and faster muscle, whereas passive manipulation for only 30 min a day tended to maintain these properties closer to normal control values. The results suggest a potential therapeutic effect of short bouts of cyclical, passive manipulation on otherwise inactive skeletal muscles.

The "Space Cycle" Self Powered Human Centrifuge: a proposed countermeasure for prolonged human spaceflight

The Self Powered Human Centrifuge, or Space Cycle, is a countermeasure to the adverse physiologic effects of prolonged human exposure to spaceflight microgravity. This unique device simultaneously provides exercise, impact loading and gravity analogous acceleration to emulate conditions on Earth. One or two crewmembers pedal themselves about a shaft mounted to the space craft located "above" their heads. This creates a short arm centrifuge with a head-to-toe acceleration orientation. The potential advantages of the Space Cycle include: a) reversal of cephalad fluid shift, minimizing post flight orthostatic intolerance; b) pedaling to maintain muscular and cardiovascular fitness; and c) enhancement of skeletal homeostasis by impact loading with a pedal-crank mounted cam and frame mounted resistive device. Other anticipated advantages include generation of usable electricity, physiologic monitoring and a means of mass measurement. Motion sickness is controlled with restraints and virtual reality headsets. The device is compatible with International Space Station dimensional constraints.

Training effects on soleus of cats spinal cord transected (T12-13) as adults

Adult spinal cord transected (T12-13) cats were trained for 30 min/day, 5 days/week to either step on a treadmill (Stp-T) or stand (Std-T) for approximately 5 months. Training ameliorated soleus atrophy and enhanced maximum force capability compared to nontrained (N-T) spinal cats, with Stp-T being significantly different from N-T. Isometric twitch speed and maximum rate of shortening were unaffected by training; the soleus of all spinal groups was significantly faster than control. There was an elevation in myosin adenosine triphosphatase activity and a shift toward faster myosin heavy chain and fiber type compositions in N-T and Std-T, but not Stp-T cats. Thus, rhythmical activity involving muscle length and force changes (stepping) was more effective than a similar amount of a more static activity (standing). This specificity related to the type of training should be considered when developing rehabilitative strategies following spinal cord injury.

Analysis of myosin heavy chain mRNA expression by RT-PCR

An assay was developed for rapid and sensitive analysis of myosin heavy chain (MHC) mRNA expression in rodent skeletal muscle. Only 2 microg of total RNA were necessary for the simultaneous analysis of relative mRNA expression of six different MHC genes. We designed synthetic DNA fragments as internal standards, which contained the relevant primer sequences for the adult MHC mRNAs type I, IIa, IIx, IIb as well as the embryonic and neonatal MHC mRNAs. A known amount of the synthetic fragment was added to each polymerase chain reaction (PCR) and yielded a product of different size than the amplified MHC mRNA fragment. The ratio of amplified MHC fragment to synthetic fragment allowed us to calculate percentages of the gene expression of the different MHC genes in a given muscle sample. Comparison with the traditional Northern blot analysis demonstrated that our reverse transcriptase-PCR-based assay was reliable, fast, and quantitative over a wide range of relative MHC mRNA expression in a spectrum of adult and neonatal rat skeletal muscles. Furthermore, the high sensitivity of the assay made it very useful when only small quantities of tissue were available. Statistical analysis of the signals for each MHC isoform across the analyzed samples showed a highly significant correlation between the PCR and the Northern signals as Pearson correlation coefficients ranged between 0.77 and 0.96 (P < 0.005). This assay has potential use in analyzing small muscle samples such as biopsies and samples from pre- and/or neonatal stages of development.