The deleterious effects of bed rest on human skeletal muscle fibers are exacerbated by hypercortisolemia and ameliorated by dietary supplementation

R. H. Fitts, J. G. Romatowski, J. R. Peters, D. Paddon-Jones, R. R. Wolfe, A. A. Ferrando

Abstract

Prolonged inactivity associated with bed rest in a clinical setting or spaceflight is frequently associated with hypercortisolemia and inadequate caloric intake. Here, we determined the effect of 28 days of bed rest (BR); bed rest plus hypercortisolemia (BRHC); and bed rest plus essential amino acid (AA) and carbohydrate (CHO) supplement (BRAA) on the size and function of single slow- and fast-twitch muscle fibers. Supplementing meals, the BRAA group consumed 16.5 g essential amino acids and 30 g sucrose at 1100, 1600, and 2100 h, and the BRHC subjects received 5 daily doses of 10–15 mg of oral hydrocortisone sodium succinate throughout bed rest. Bed rest induced atrophy and loss of force (mN) and power (μN·FL·s−1) in single fibers was exacerbated by hypercortisolemia where soleus peak force declined by 23% in the type I fiber from a prevalue of 0.78 ± 0.02 to 0.60 ± 0.02 mN post bed rest (compared to a 7% decline with bed rest alone) and 27% in the type II fiber (1.10 ± 0.08 vs. 0.81 ± 0.05 mN). In the BRHC group, peak power dropped by 19, 15, and 11% in the soleus type I, and vastus lateralis (VL) type I and II fibers, respectively. The AA/CHO supplement protected against the bed rest-induced loss of peak force in the type I soleus and peak power in the VL type II fibers. These results provide evidence that an AA/CHO supplement might serve as a successful countermeasure to help preserve muscle function during periods of relative inactivity.

  • isotonic contractile properties
  • peak force and power
  • calcium sensitivity
  • essential amino acids

periods of limb unloading, whether produced by bed rest (BR) or spaceflight, have been shown to induce muscle atrophy and loss of force and power. The latter always exceeds the volume loss and this has been attributed to a selective decline in myofibrillar protein (6). For example, 84 days of BR caused a 17% decrease in knee extensor muscle size and 40% loss in various functional tests (19). Similarly, following 6 mo in space, crew members showed a 6 to 20% reduction in calf plantar flexor volume with 20 to 48% decline in maximal voluntary contraction (6). The 28-day Skylab 2 mission showed greater declines in thigh vs. arm and extensor vs. flexor torque (6).

In the case of microgravity, the loss of muscle volume and torque appears to be exponential with the largest decline in the first month and a new steady state reached by 120 days. This is supported by the observation that 110 days of microgravity reduced ankle plantar flexion strength as much as 237 days (6). Observations following short and prolonged spaceflight suggest that while ankle extensors (plantar flexors) atrophy and lose function faster than ankle flexors, the difference by 237 days between ankle extensors and flexors is less apparent (6). Another consistent observation is that regardless of the type of unloading (BR or spaceflight) considerable variability exists between subjects with some demonstrating more muscle atrophy and functional decline than others (20, 22).

Muscle wasting, whether induced by spaceflight or clinically mandated BR, results from an imbalance between protein synthesis and breakdown (4), and it is also generally associated with elevated plasma concentrations of the stress hormone cortisol (6, 14). With prolonged space flights on MIR (>3 mo), astronauts and cosmonauts showed a 45% drop in whole body protein synthesis rates, and a reduced protein breakdown of a somewhat lesser magnitude (16). This suggests that the primary problem contributing to muscle wasting was the reduced protein synthetic rate. This also appears to be the dominate factor producing muscle atrophy with periods of BR. Ferrando et al. (4) observed a 50% drop in protein synthesis with no change in protein breakdown in the vastus lateralis (VL) of subjects following a 14-day BR. In many space crew members and patients confined to BR, the extent of muscle protein breakdown was exacerbated by an inadequate energy intake, which resulted in a negative caloric balance (14, 16, 17). In patients, the incidence of undernutrition during hospitalization has been estimated to be as high as 50% (1). The importance of proper nutrition in the maintenance of muscle mass was recently demonstrated by Paddon-Jones et al. (14). These investigators showed that the catabolic effects of prolonged bed rest with or without acute hypercortisolemia could be ameliorated by an essential amino acid and carbohydrate (AA/CHO) supplement through the maintenance of muscle protein synthesis. In contrast, the ingestion of a typical mixed clinical meal had minimal or no effect on protein synthesis (14).

The goal of this study was to assess the effect of chronic hypercortisolemia or AA/CHO supplement during BR on single muscle fiber function. While the former accelerates and the latter protects against BR-induced loss of muscle mass and strength (14), the cellular changes responsible for the limb muscle responses are unknown. The observed impact on muscle mass suggests that at least a portion of the effects of these interventions are likely mediated through differences in fiber atrophy. However, the fiber-type dependence (i.e., effects on slow and fast fiber types) of hypercortisolemia or AA/CHO supplements and whether or not changes in single fiber force and power independent of cell atrophy contribute to the altered muscle strength induced by these modalities are unknown. Given that hypercortisolemia and AA/CHO supplements are known to decrease and increase muscle protein (12, 14), respectively, we hypothesize that independent effects on force and power will exist. These questions will be studied by determining the extent to which hypercortisolemia exacerbates and AA/CHO supplement protects against the bed rest induced atrophy and functional decline of individual slow- and fast-twitch fibers of the soleus and VL muscles.

METHODS

Subjects.

Sixteen healthy males participated in this project. All volunteers provided informed written consent according to the guidelines established by the Institutional Review Boards at the University of Texas Medical Branch and Marquette University. Subjects were randomly assigned to one of three groups: bed rest (BR; n = 5); bed rest plus hypercortisolemia (BRHC; n = 6); and bed rest plus AA/CHO supplement (BRAA; n = 5), (Table 1). Subject eligibility was assessed by a battery of medical screening tests, including medical history, physical examination, electrocardiogram, blood count, plasma electrolytes, blood glucose concentration, and liver and renal function tests. Exclusion criteria included recent injury, the presence of a metabolically unstable medical condition, low hematocrit or hemoglobin, vascular disease, hypertension, or cardiac abnormality.

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Table 1.

Demographic data and changes in outcome variables following 28 days of bed rest

Experimental protocol.

This study was performed as part of a larger project, in which muscle protein fractional synthetic rate and phenylalanine net balance, lean leg mass, and leg extension strength were measured (12, 14). Subjects were admitted to the General Clinical Research Center at The University of Texas Medical Branch for 5-day dietary stabilization before the start of BR. During this period, subjects were sedentary but remained ambulatory. Subjects were placed on a 3-day rotating diet, with daily nutrient intake evenly distributed between three meals (0830, 1300, and 1830 h). The carbohydrate, fat, and protein intake was 59, 27, and 14%, respectively (4). The Harris-Benedict equation was used to estimate daily energy requirements (13), and water was provided ad libitum.

To achieve a stable plasma cortisol concentration of ∼20–25 μg/dl, subjects received 5 daily doses of 10–15 mg of oral hydrocortisone sodium succinate throughout bedrest (days 227: 0800, 1200, 1600, 2000, 2400). On days 1 and 28 of bedrest, a continuous infusion of hydrocortisone sodium succinate (60 μg·kg−1·h−1; Pharmacia), was administered intravenously to induce/maintain hypercortisolemia and reduce potential minor fluctuations in plasma cortisol concentrations during collection of muscle biopsy samples.

Throughout bed rest, subjects in the BRAA group received three daily supplements (1100, 1600, 2100), each containing 16.5 g of essential amino acids and 30 g of sucrose (Table 2). The amino acids and sucrose were dissolved in 250 ml of a noncaloric, noncaffeinated soft drink. The additional daily caloric/nutrient intake (558 kcal) provided by the AA/CHO drinks represented a true dietary supplement and not a caloric replacement or substitution. Subjects in the BR group received only the diet soft drink.

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Table 2.

Composition of amino acid plus carbohydrate supplement

Muscle biopsies.

Following the 5 days of diet stabilization period, a biopsy was obtained from the left soleus and VL muscles using the percutaneous needle biopsy technique. At the conclusion of the 28 days of BR but before reambulation, a post-BR sample was obtained from the same muscles. All biopsies were obtained with subjects in the post-absorptive state. Pre- and post-bed rest muscle samples were divided into several portions. The portion used for the present experiments was immediately placed in cold skinning solution composed of (in mM) 125 K propionate, 20.0 imidazole, pH 7.0, 2.0 EGTA, 4.0 ATP, 1.0 MgCl2, and 50% glycerol (vol/vol) and shipped overnight at 4°C to Marquette University, where it was stored at −20°C. All contractile experiments were conducted within 28 days of muscle sampling.

Test solutions and single fiber isolation procedures.

The total and free concentrations of each metal, ligand, and metal ligand complex in the relaxing and activating solutions were calculated using an iterative computer program (3). Stability constants used in these calculations (8) were adjusted for the temperature, pH, and ionic strength conditions of the present experiments. Each solution contained (in mM) 7 EGTA, 20 imidazole, 14.5 creatine phosphate, 1 free Mg2+, 4 free MgATP, sufficient KCl and KOH to produce a total ionic strength of 180 mM and a pH of 7.0. In addition, the relaxing and activating solutions had a free [Ca2+] of pCa 9.0 and pCa 4.5, respectively (where pCa = −log Ca2+ concentration). Creatine phosphokinase was not added as we and others have shown that the endogenous levels of this enzyme are adequate (9).

The single fiber isolation and test procedures were exactly as described previously (21, 22) and briefly reviewed here. On the day of an experiment, a single fiber segment was isolated from the muscle bundle and transferred to the stainless steel experimental chamber that contained three solution wells. While submerged under relaxing solution, the fiber was mounted between a force transducer (model 400; Cambridge Technology, Watertown MA) and a direct current position motor (model 300B; Cambridge Technology). The mounting procedure utilized 4-0 monofilament pins and 10-0 suture to fasten the fiber ends securely into small stainless steel troughs extending from the transducer and motor (22). The experimental chamber was attached to the stage of an inverted microscope so that the fiber could be viewed at ×800 during data collection. Sarcomere length was adjusted to 2.5 μm using an eyepiece micrometer and this sarcomere length was maintained throughout the experiement. The length of the fiber (FL) suspended between the transducer and motor was measured with a micrometer that advanced the plate across the field of view. A charge-coupled device camera (model Provideo CVC-140; Sci/Speco) relayed a video of the fiber while it was briefly suspended in air (<5 s). A frame grabber card (model All-in-Wonder 128; ATI) captured a still image of the fiber, and diameter was measured at three points along the fiber using SCION Image software (Scion). Fiber diameter was determined from the average of the three measurements. Fiber cross-sectional area was calculated from fiber diameter under the assumption that the fiber forms a circular cross-section when suspended in air (11).

To initiate contraction, the fiber was rapidly transferred into an adjacent well filled with activating solution. Output from the force transducer and position motor were displayed on a digital oscilloscope before being amplified and interfaced to a personal computer via a LabMaster data-acquisition board. Custom software performed online analysis and stored data to disk. Relaxing and activating solutions were maintained at 15°C during all experiments.

Fibers were subjected to slack tests to measure unloaded maximal shortening velocity (V0), and the force-velocity relation was determined from a series of isotonic releases as described previously (21). P0 was defined as the peak force obtained during the experiment. Vmax was defined as the intercept of the force-velocity curve with the velocity axis. Peak absolute power was calculated from P0, Vmax, and a/P0, the parameter which specifies the curvature of the force-velocity relation. For graphical purposes, composite force-velocity-power relationships were plotted using the mean parameters.

A subgroup of fibers were activated with a series of solutions that had free Ca2+ concentrations ranging from pCa 6.8 to pCa 5.0. These solutions were made by mixing appropriate volumes of the activating (pCa 4.5) and relaxing (pCa 9.0) solutions. Peak force of each contraction was recorded and expressed as a fraction of the peak force obtained during maximal Ca2+ activation. The first and approximately every fourth subsequent contraction was performed at pCa 4.5. Hill plots were used to determine the Ca2+ concentration associated with Ca2+ activation threshold and with half-maximal activation (21). During these experiments, the fibers were subjected to sinusoidal length changes (frequency, 1.5 kHz; amplitude, 0.05% of FL), first in relaxing solution and then at the peak of the Ca2+ activated contraction. Changes in length (Δlength) and force (Δforce) were used to calculate peak elastic modulus or E0 {E0 = [Δforce in activating solution − Δforce in relaxing solution/(Δlength)] (FL/CSA)}, where CSA is cross-sectional area.

Following the contractile measurements, the fiber was removed from the transducer and motor, solubilized in 10 μl of 1% SDS sample buffer, and stored at −80°C. The myosin heavy chain content of each individual fiber was determined by 5% polyacrylamide gel electrophoresis (22).

Pre- and post-bed rest means between groups were analyzed using a one-way ANOVA, and a post hoc two-tailed t-test. Statistical significance was accepted at P < 0.05. All data are presented as means ± SE.

RESULTS

Hypercortisolemia effects on fiber contractile function..

In this section, the contractile properties of the slow type I and fast type II fibers from the soleus and VL muscles of the BRHC group (pre- and post- bed rest) are presented and where differences exist compared with the effects of bed rest alone. Atrophy induced by BRHC was greater in the slow- and fast-twitch fibers of the soleus then VL (Table 3), and greater then that observed with BR alone (Fig. 1). In fact, BR alone caused no atrophy in type II VL fibers (Fig. 1). In the BRHC group all fibers showed a significant decline in absolute force (mN) with the change ∼2-fold greater in the soleus compared with fibers (Table 3). For the slow type I fibers of both muscles, P0 was reduced more than could be attributable to fiber atrophy such that the relative force (kN/m2) was depressed post-bed rest (Tables 3). For fast fibers, all of the force loss with BRHC was explained by fiber atrophy, such that, the specific force (kN/m2) was unaltered. In comparison, Bed rest alone produced a 7% and 8% decline in peak force in the type I fibers of the soleus (0.98 ± 0.03 to 0.91 ± 0.03 mN) and VL (0.83 ± 0.03 to 0.76 ± 0.03), respectively, and no change in the VL type II fiber P0. Unlike BRHC, bed rest alone had no effect on specific force in any fiber type. The mean pre-bed rest peak fiber force for the type I fibers of both muscles was significantly higher than the pre-bed rest BRHC subjects and this was in large part due to a greater mean fiber size. The mean soleus type I fiber diameter for 101 fibers was 101 ± 2 compared with the pre-bed rest value of 87 ± 1 for the 140 soleus type I fibers in the BRHC study (Table 3).

Fig. 1.

Bar graphs showing the percent decline in the mean fiber diameter for the soleus type I fibers (top plot) and vastus lateralis (VL) type II fibers (bottom plot) for bed rest alone group (BR), bed rest plus amino acid/carbohydrate supplement group (BRAA), and bed rest plus hypercortisolemia group (BRHC) compared with the pre-bed rest mean value. The BRHC value is shown as the percentage of its own pre-bed rest value (Table 3), while the other two groups are compared with their combined (BR and BRAA) pre value, which was 101 μm.

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Table 3.

Effect of hypercortisolemia plus bed rest on soleus and vastus lateralis fiber diameter, peak force, and maximal shortening velocity

BRHC had no effect on V0 in either muscle or fiber type (Table 3). In contrast, while BR alone had no effect on soleus type I fiber V0 (pre, 0.53 ± 0.01 and post, 0.56 ± 0.03 FL/s), it significantly depressed V0 in the VL type I fiber (pre, 0.66 ± 0.02 and post 0.56 ± 0.02 FL/s). This effect was not prevented by the supplement as the post-bed rest V0 for the VL type I fiber in the BRAA group was 0.57 ± 0.02 FL/s a value not significantly different from the post BR group.

The force velocity and power data for the BRHC group are shown in Table 4. It is clear that the primary effect of BRHC was to depress the peak power of the slow type I fibers in the soleus (19%) and VL (15%). The peak power of the VL type II fibers showed an 11% decline, but this drop was not significant. In contrast, the peak power of the VL type II fiber in the BR group showed a significant 17% depression. In the soleus, there were too few fast type II fibers to assess peak power. The reduced power in the type I and type II fibers of the BRHC group can be entirely explained by the ∼20% drop in force at peak power as the velocity at peak power was unaltered (Table 4).

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Table 4.

Effect of hypercortisolemia plus bed rest on the power parameters of soleus and vastus lateralis fibers

The average force-velocity and force-power curves for the soleus and VL slow type I fibers pre- and post-BRHC are shown in Fig. 2. The maximal shortening velocity determined from the Hill plot (Vmax) pre-bed rest was 0.642 ± 0.028 for soleus type I fiber, and 0.611 ± 0.033 for VL type I fiber. This parameter as well as the curvature of the force-velocity relationship (a/P0) was unaltered by either BR or BRHC. Nonetheless, due to the reduced ability to generate force, power was depressed at all loads (Fig. 2).

Fig. 2.

Composite force-velocity-power relationships plotted using the mean parameters for P0, Vmax, and a/P0 for the type I soleus (top plot) and type I vastus lateralis (VL; bottom plot) fibers for pre-bed rest and BRHC condition.

Figure 3 plots the pre-BRHC (x-axes) and post-BRHC (y-axes) values for fiber diameter, P0 (mN), P0 (kN/m2), and peak power for soleus type I fibers of the individual subjects. The graph reveals the subject variability in response to BRHC. For example, subjects 10 and 13 showed little or no change in fiber diameter, force and power, whereas subjects 1 and 5 showed relatively large declines in these values. Unlike BR alone where specific force was unaltered and the slope of the linear regression line describing the pre- and post-bed rest mean values for individual subjects was 1.02 or not different from the line of identity (22), four of the six BRHC subjects showed a significant depression in specific force and the linear regression line of 0.57 (which excluded subject 7, who was an outlier) was significantly less than one.

Fig. 3.

Relationship between pre-bed rest (x-axis) and post-bed rest (y-axis) for type I fiber diameter (A, in μm), absolute peak isometric force (B, in mN), normalized peak force (C, in kN/m2), and peak power (D, μN·FL·s−1) for the individual subjects in the BRHC group. Symbols represent means ± SE values for a particular subject (with the number adjacent to symbol indicating subject's identification number). Line of identity denoted by solid diagonal line. Subject sample sizes were as follows (pre-bed rest, post-bed rest, respectively): AC, subject 1 = 14, 10; subject 5 = 31, 18; subject 7 = 19, 31; subject 9 = 25, 20; subject 10 = 25, 15; and subject 13 = 25, 14. D, subject 1 = 11, 9; subject 5 = 16, 14; subject 7 = 2, 10; subject 9 = 13, 3; subject 10 = 9, 6; and subject 13 = 10, 3.

The peak elastic modulus (E0) of the slow type I and fast type II fibers was not significantly affected by BRHC (Table 5). This suggests that BRHC had no effect of the number of cross-bridges per CSA. The P0/E0 ratio was also unaltered except for the VL type II fibers where the post-BRHC ratio was depressed (Table 5). Both pre- and post-BRHC the P0/E0 ratio was higher in the fast type II fiber type.

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Table 5.

Effect of hypercortisolemia plus bed rest on Ktr, E0, and P0/E0 ratio of soleus and VL fibers

Fiber Ca2+ sensitivity was determined from the Hill-plot analysis. The free Ca2+ required to initiate contraction (activation threshold), and for one-half maximal activation (pCa50) was not affected by either BR or BRHC. Additionally, the slope of the force-pCa relationship (Hill plot) below (n2) and above (n1) one-half maximal activation was also unaltered (Fig. 4, and Table 6). An exception was the VL type I fiber where bed rest alone caused a small but significant 2.6% increase in the pCa (lower free Ca2+) required for activation (pCa pre, 7.25 ± 0.04 and post, 7.44 ± 0.05) and a reduced n2 (pre, 1.98 ± 0.06 and post, 1.70 ± 0.06).

Fig. 4.

Mean pCa-force relationships for type I soleus (top plot) and type I and type II VL (bottom plot) fibers. Values are means for the number of fibers shown in Table 4.

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Table 6.

Effect of hypercortisolemia plus bed rest on activation threshold, pCa50, and slope of the Hill plot for forces below and above half-maximal activation

Effect of dietary supplementation.

Importantly, the AA/CHO supplement prevented the bed rest-induced loss in type I fiber peak force (post BRAA group averaged 0.99 ± 0.05 mN) in the soleus (Fig. 5). However, the supplement had no effect on the peak force of either slow or fast fibers in the VL (Fig. 5).

Fig. 5.

Bar graphs showing the percent decline in the mean fiber force (in mN) for the soleus type I fibers (top plot), and VL type I (middle plot) and type II (bottom plot) fibers for bed rest alone group (BR), BRAA, and BRHC compared with the pre-bed rest mean value. The BRHC value is shown as a percentage of its own pre-bed rest value (Table 3), while the other two groups are compared with their combined (BR and BRAA) pre value, which was 0.98 mN.

Despite the higher peak force, the soleus type I fiber force elicited at peak power was not effected by the AA/CHO supplement, and as a consequence neither was peak power (Fig. 6A). In this muscle and fiber type, both the BR and the BRAA groups showed a 5% decline in peak power compared with pre-bed rest, which was less than the 19% decline observed in the BRHC group. The effect of bed rest on the VL type I fiber peak power was also unaltered by the AA/CHO supplement (Fig. 6B). In contrast, the AA/CHO supplement did protect the peak power of the VL type II fibers (Fig. 6C), such that, the post-bed rest value (63.95 ± 3.89 μN·FL·s−1) was not significantly different from pre-bed rest (69.47 ± 3.46 μN·FL·s−1). In contrast, the peak power of the VL type II fibers from the BR group (57.59 ± 2.53 μN·FL·s−1) was significantly depressed. The protection was due to a higher velocity at peak power in the BRAA compared with the BR group (0.346 ± 0.015 vs 0.311 ± 0.014 FL/s) as the forces at peak power were identical between groups (0.18 ± 0.01 mN). However, only in the BR group was the force at peak power significantly lower (P < 0.05) than the pre-bed rest condition of 0.21 ± 0.01 mN.

Fig. 6.

Composite force-power relationships plotted using the mean parameters for P0, Vmax, and a/P0 for the type I soleus (top plot) and type I VL (middle plot) and type II VL (bottom plot) fibers for pre-bed rest, BR, and BRAA conditions.

Similar to BR and BRHC, the VL type II fibers of the BRAA group showed no pre- to post- bed rest change in Ca2+ sensitivity. In contrast, for the VL type I fiber, BRAA prevented the bed rest induced increase in activation threshold (BRAA post, pCa 7.18 ± 0.06) and decrease in n2 (BRAA post, 2.03 ± 0.10). The supplement also significantly increased n1 from the pre value of 1.32 ± 0.04 to 1.63 ± 0.09 after bed rest.

DISCUSSION

This is the first study to assess the effects of bed rest plus hypercortisolemia on single fiber function, and also the first to evaluate the cellular properties of both ankle (soleus) and knee (VL) extensors. The most important findings were the following: 1) The bed rest-induced atrophy and functional loss in soleus type I fibers were exacerbated by hypercortisolemia; 2) With BRHC atrophy and loss of force (mN) in single fibers were 2 to 3-fold greater in the soleus compared with the VL; and 3) The AA/CHO supplementation protected against the decline in soleus type I fiber force, and prevented the bed rest-induced decline in the peak power of the VL type II fibers. It is difficult to determine whether or not hypercortisolemia altered the soleus type II fiber response to bed rest as the BR group showed too few soleus type II fibers for analysis. However, the soleus type II fiber atrophy and loss of force in the BRHC group was greater than and the same, respectively, as that observed following a 3-wk spaceflight (20). Since the effect of spaceflight on fiber diameter and function are known to be greater than bed rest (20, 22), this suggests that hypercortisolemia did exacerbate the effects of bed rest alone in this population of fibers.

In this study, the administration of cortisol was designed to mimic the plasma levels reached during hospitalization associated with injury or illness, and the plasma cortisol concentrations frequently observed with prolonged space flight (16). The primary effect of excess cortisol is to increase muscle protein catabolism such that breakdown exceeds synthesis resulting in a loss of lean body mass (5). In a companion study of these same subjects, Paddon-Jones et al. (14) observed that a hypercortisolemic challenge contributed to a reduction in post-absorptive and post-meal net muscle protein synthesis. The reduced muscle protein balance compared with the eucortisolemia condition likely caused the greater atrophy and loss of force in the soleus fibers of the BRHC group. In addition, while bedrest alone had no effect on fiber force per cross-sectional area, this parameter was depressed by 7% in the soleus and VL type I fibers of the BRHC subjects. This suggests that the loss of contractile protein exceeded cell atrophy in the BRHC but not the BR subjects, and implies that the inhibition of net muscle protein synthesis (in part by accelerating protein degradation) by hypercortisolemia selectively affects myofibrillar proteins.

BRHC but not bed rest alone caused a greater atrophy and loss of force and power in type I fibers of the soleus compared with the VL. It is not clear why cortisol would have a preferential affect on one muscle over another. However, a comparison of the results of Yamashita-Goto et al. (23) with Trappe et al. (19) suggests that longer periods of bed rest (>2 mo) may elicit preferential atrophy of the soleus without cortisol injections. At 84 days of bed rest, the latter group observed 15, 47, and 28% decline in the fiber size, force (mN) and force/CSA of VL type I fibers, while after 4 mo of bed rest, the former found a 36, 76, and 42% decrease for these properties in soleus type I fibers. The differences could be attributed to: the greater duration of bed rest in the Yamashita-Goto et al. (23) study; a greater susceptibility of the soleus compared with the VL to bed rest induced atrophy; and/or a faster time course of the fiber atrophy and functional loss in the soleus compared with the VL. The latter was shown to be the case for ankle plantar and dorsal flexors during microgravity-induced loss of muscle strength (6).

While the data suggests that the type I fibers from both the soleus and VL of the BRHC subjects selectively lost contractile protein, the remaining fibrils appeared to function normally. The rate of cross-bridge binding as reflected by Ktr (Table 5) and the pCa-force relationship (Table 6 and Fig. 4) were unaltered. The slope of the Hill plot for the pCa-force relationship below one-half maximal force (n2) is thought to reflect the degree of cooperativity; a parameter influenced by both thin and thick filament proteins, and it was not affected by the bed rest plus hypercortisolemia condition.

Previously, we observed 3 wk of bed rest to significantly increase soleus type I fiber V0, a change associated with and perhaps caused by a selective loss in the actin (thin) relative to the myosin (thick) filament (22). Here, soleus type I fiber V0 was unaltered by any of the bed rest conditions suggesting that the filament spacing was normal. Bed rest has been shown to either have no effect or depress V0 in both slow and fast fibers of the VL (10, 19). In agreement, we observed a significant decline in VL type I fiber V0 following BR an effect not prevented by the AA/CHO supplement. The apparent slowing of this fiber type by bed rest in the VL was also reflected in the Ca2+ sensitivity as the activation at lower free Ca2+ and reduced n2 post bed rest are characteristic of slower fibers. In contrast, the AA/CHO supplement increased the cooperativity of force development compared with bed rest alone. The mechanism for this effect is unknown, but could be related to the supplements stimulation of myofibrillar protein synthesis (14).

AA/CHO supplementation prevented the bed rest-induced loss of force in the slow type I fibers of the soleus, but not the slow fibers of the VL. This result is surprising because the study by Paddon-Jones et al. (12), which focused on the same subjects, showed the supplement to preserve lean leg mass and ameliorate the loss of single-leg, one-repetition maximum (1RM) leg extension strength compared with the BR group. Therefore, one would expect the protective effects of the supplement to preserve contractile protein and force in all fibers and leg muscles. Paddon-Jones et al. (12) showed that the protective effect of the AA/CHO supplement was associated with higher mixed muscle protein synthetic rates and not mediated by changes in plasma cortisol which were unaltered by the supplement. It seems that the protective effect of the 1RM leg extension was at least in part caused by the AA/CHO amelioration of the bed rest-induced decline in the peak power of the fast type II fibers of the VL. Fiber V0 for the VL type II fiber in the AA/CHO supplement group was higher than the pre-bed rest value and significantly higher (P < 0.1) than the post bed rest control group. This increase in type II fiber velocity was entirely responsible for the increased peak power in this group.

Paddon-Jones et al. (12) used stable isotope methodology and DEXA to infer that the BR subjects were in a mild catabolic state throughout the 28 days of bed rest. The AA/CHO supplement preserved muscle protein synthesis and muscle mass. While the molecular mechanisms for the AA/CHO protection of single fiber function observed in this study are unknown, they are most likely linked to the ability of the supplement to stimulate muscle protein synthesis. Thus it seems reasonable to speculate that this supplement would reduce the deleterious changes associated with space flight as well as bed rest. Since the microgravity-induced loss of fiber function is primarily caused by inhibition of muscle protein synthesis leading to a selective loss of myofibrillar protein, fiber atrophy and a loss of fiber force and power (6, 20), an AA/CHO supplement that stimulates muscle protein synthesis should prove to be an effective countermeasure. This hypothesis needs to be tested in controlled studies aboard the International Space Station.

GRANTS

This research was supported by the National Space Biomedical Research Institute Grant NCC9-59-207 Project No. NPFR00403 and in part by the National Aeronautics and Space Administration Grant NCC9-116 (to R. H. Fitts).

Acknowledgments

We thank Carla Langenthal for assistance with some of the single fiber studies.

Footnotes

  • The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

REFERENCES

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