Single Fiber Contractile Function Research Articles

Repeated high-intensity exercise modulates Ca(2+) sensitivity of human skeletal muscle fibers.

The effects of short-term high-intensity exercise on single fiber contractile function in humans are unknown. Therefore, the purposes of this study were: (a) to access the acute effects of repeated high-intensity exercise on human single muscle fiber contractile function; and (b) to examine whether contractile function was affected by alterations in the redox balance. Eleven elite cross-country skiers performed four maximal bouts of 1300 m treadmill skiing with 45 min recovery. Contractile function of chemically skinned single fibers from triceps brachii was examined before the first and following the fourth sprint with respect to Ca(2+) sensitivity and maximal Ca(2+) -activated force. To investigate the oxidative effects of exercise on single fiber contractile function, a subset of fibers was incubated with dithiothreitol (DTT) before analysis. Ca(2+) sensitivity was enhanced by exercise in both MHC I (17%, P < 0.05) and MHC II (15%, P < 0.05) fibers. This potentiation was not present after incubation of fibers with DTT. Specific force of both MHC I and MHC II fibers was unaffected by exercise. In conclusion, repeated high-intensity exercise increased Ca(2+) sensitivity in both MHC I and MHC II fibers. This effect was not observed in a reducing environment indicative of an exercise-induced oxidation of the human contractile apparatus.

Human vastus lateralis and soleus muscles display divergent cellular contractile properties.

The purpose of this study was to investigate potential differences in single-fiber contractile physiology of fibers with the same myosin heavy chain isoform (MHC I and MHC IIa) originating from different muscles. Vastus lateralis (VL) and soleus biopsies were obtained from 27 recreationally active females (31 +/- 1 yr, 59 +/- 1 kg). A total of 943 single fibers (MHC I = 562; MHC IIa = 301) were isolated and examined for diameter, peak tension (Po), shortening velocity (Vo), and power. The soleus had larger (P < 0.05) fibers (MHC I +18%; MHC IIa +19%), higher MHC I Vo (+13%), and higher MHC I Po (+18%) compared with fibers from the VL. In contrast, fibers from the VL had higher (P < 0.05) specific tension (MHC I +18%; MHC IIa +20%), and MHC I normalized power (+25%) compared with the soleus. There was a trend for MHC IIa soleus fibers to have higher Vo [MHC IIa +13% (P = 0.058)], whereas VL MHC IIa fibers showed a trend for higher normalized power compared with soleus fibers [MHC IIa +33% (P = 0.079)]. No differences in absolute power were detected between muscles. These data highlight muscle-specific differences in single-fiber contractile function that should serve as a scientific basis for consideration when extending observations of skeletal muscle tissue from one muscle of interest to other muscles of origin. This is important when examining skeletal muscle adaptation to physical states such as aging, unloading, and training.

Single fiber physiology differs between three human leg muscles

While single fiber contractile function within a given fiber type population is largely similar between muscles, fiber type distribution can vary considerably. Because single fiber contractile function and fiber type distribution both contribute to whole muscle function, we generated composite profiles to compare three leg muscles of interest. Eight recreationally active males (5) and females (3) (23 ± 1 yr, 64 ± 3 kg, 51.8 ± 2.6 ml/kg/min) were biopsied from the vastus lateralis (VL) and soleus (S). To make further comparisons, seven recreationally active males (4) and females (3) (22 ± 1 yr, 68 ± 2 kg, 49.5 ± 2.6 ml/kg/min) were biopsied from the gastrocnemius (G). Single fibers were isolated and investigated for size and function, and subsequently processed for myosin heavy chain (MHC) composition (SDS‐PAGE). A composite value, which is a theoretical representation of the entire muscle, was then calculated for each contractile parameter based on the MHC composition of the studied fibers. The VL had higher specific tension (VL 66% &gt; G and 28% &gt; S), shortening velocity (40% &gt; G; 48% &gt; S), peak power (266% &gt; G; 152% &gt; S), and normalized power (276% &gt; G; 221% &gt; S), when compared to the gastrocnemius and soleus. Additionally, the gastrocnemius had lower peak tension (G 37% &lt; VL; G 31% &lt; S) and lower specific tension (40% &lt; VL; 22% &lt; S) when compared to the vastus lateralis and soleus. These findings highlight the unique intrinsic profile of these select muscles, which should be considered when interpreting how they adapt to exercise training.

Effects of endurance exercise-training on single-fiber contractile properties of insulin-treated streptozotocin-induced diabetic rats

The purpose of this study was to characterize the contractile properties of individual skinned muscle fibers from insulin-treated streptozotocin-induced diabetic rats after an endurance exercise training program. We hypothesized that single-fiber contractile function would decrease in the diabetic sedentary rats and that endurance exercise would preserve the function. In the study, 28 rats were assigned to either a nondiabetic sedentary, a nondiabetic exercise, a diabetic sedentary, or a diabetic exercise group. Rats in the diabetic groups received subcutaneous intermediate-lasting insulin daily. The exercise-trained rats ran on a treadmill at a moderate intensity for 60 min, five times per week. After 12 wk, the extensor digitorum longus and soleus muscles were dissected. Single-fiber diameter, Ca(2+)-activated peak force, specific tension, activation threshold, and pCa(50) as well as the myosin heavy chain isoform expression (MHC) were determined. We found that in MHC type II fibers from extensor digitorum longus muscle, diameters were significantly smaller from diabetic sedentary rats compared with nondiabetic sedentary rats (P < 0.001). Among the nondiabetic rats, fiber diameters were smaller with exercise (P = 0.038). The absolute force-generating capacity of single fibers was lower in muscles from diabetic rats. There was greater specific tension (force normalized to cross-sectional area) by fibers from the rats that followed an endurance exercise program compared with sedentary. From the results, we conclude that alterations in the properties of contractile proteins are not implicated in the decrease in strength associated with diabetes and that endurance-exercise training does not prevent or increase muscle weakness in diabetic rats.

Human unilateral lower limb suspension as a model for spaceflight effects on skeletal muscle

To the Editor: Widrick et al. ([18][1]) have published data from a 12-day unilateral lower limb suspension (ULLS) study conducted in 1994 using the methods originally published by Berg et al. in 1991 ([3][2]). Analyses included the morphology and contractile behavior of skinned skeletal muscle

ALTERATIONS IN SINGLE TYPE I SKELETAL MUSCLE FIBER CONTRACTILE FUNCTION WITH INACTIVITY AND THERAPEUTIC EXERCISE IN AGED ANIMALS

The benefits of therapeutic interventions during periods of inactivity are unknown in the older population. Therefore, the purpose of this study was to determine the contractile function of single muscle fibers from the soleus and gastrocnemius muscles of 36-month old Fisher 344/BN rats following two weeks of inactivity and two weeks of inactivity plus intermittent treadmill walking. It was hypothesized that the contractile function of single fibers would adapt to inactivity and that treadmill walking for 15 minutes, 4X/day, would be an effective exercise. In order to test this hypothesis there were three experimental groups (8 animals per group): cage control, two weeks of hindlimb unweighting to mimic inactivity, and two weeks of hindlimb unweighting plus intermittent treadmill walking. Contractile function was determined in 180 single permeabilized fiber preparations. Peak force, peak specific force (Po), maximal shortening velocity (Vo, tested by the slack test and Vmax, extrapolated from the force-velocity relationship) and peak power (calculated from the force-velocity relationship) were determined in myosin heavy chain type I fibers (determined by SDS PAGE). In type I fibers from the soleus, inactivity-induced a decline in fiber peak force (31%), Po (10%), and peak power, whereas Vo and Vmax increased. In type I fibers from the gastrocnemius, there was a decline in fiber peak force (50%), Po (24%) and peak power with inactivity. In contrast, Vo and Vmax of type I fibers from the gastrocnemius did not change with inactivity. Attenuation of the inactivity-induced decline in single fiber contractile function was greater in the type I fibers from the gastrocnemius muscle compared to the soleus muscle with treadmill walking. Although the contractile function of single type I fibers from aged animals adapt to altered activity patterns, the extent of change with inactivity and intermittent treadmill walking is dependent on the specific muscle and the contractile property examined. Underlying mechanisms responsible for the contractile property-specific adaptation and the benefits of treadmill walking are unknown. Supported by the AHA-Minnesota Affiliate and Foundation for Physical Therapy.