A comparative analysis of the effects of exercise training on contractile responses in fast- and slow-twitch rat skeletal muscles

Impacts of prolonged chlorpyrifos exposure on locomotion and slow-and fast- twitch skeletal muscles contractility in rats

AIM

Investigate the effect of dietary exposure to chlorpyrifos on locomotion and contraction of soleus andextensor digitorum longus (edl) involved in locomotion. Methods: Rats were fed diets containing 1 or 5 mg kg-1 of chlorpyrifos for six weeks. Locomotion has been assessed weekly using beam walking and beam balance tests. Soleus and edl were removed to study contractile properties, myofibrillar protein content and myosin heavy chain isoforms.

RESULTS

Animals treated with 5 mg kg-1 chlorpyrifos had a decrease body weight. An increase by 28% and 24% in latency time assessed by beam walking test and a decrease by 9% and 13% in the beam balance time was reported after 6 weeks of 1 and 5 chlorpyrifos exposure respectively. The contractile properties in soleus showed an increase in twitch amplitude by 25% and 63% in 1 and 5 doses respectively, without modification in the contraction time and half relaxation time. edl treated with 1 mg kg-1 showed a decrease by 35%, 42% and 22% in twitch amplitude, contraction time and half relaxation time respectively. edl treated with 5 mg kg-1 showed an increase of 23% in twitch amplitude without modification of the other parameters. These changes were associated with modification of myofibrillar protein content in all treated groups. Myosin heavy chain isoforms were altered in both skeletal muscles treated with 1 mg kg-1.

CONCLUSION

Exposure to chlorpyrifos can alter the locomotion and produce physiological changes in a dose and muscle type related manner.

High molecular mass proteomics analyses of left ventricle from rats subjected to differential swimming training

BACKGROUND

Regular exercises are commonly described as an important factor in health improvement, being directly related to contractile force development in cardiac cells.In order to evaluate the links between swimming exercise intensity and cardiac adaptation by using high molecular mass proteomics, isogenic Wistar rats were divided into four groups: one control (CG) and three training groups (TG's), with low, moderate and high intensity of exercises.In order to evaluate the links between swimming exercise intensity and cardiac adaptation by using high molecular mass proteomics, isogenic Wistar rats were divided into four groups: one control (CG) and three training groups (TG's), with low, moderate and high intensity of exercises.

RESULTS

Findings here reported demonstrated clear morphologic alterations, significant cellular injury and increased energy supplies at high exercise intensities. α-MyHC, as well proteins associated with mitochondrial oxidative metabolism were shown to be improved. α-MyHC expression increase 1.2 fold in high intensity training group when compared with control group. α-MyHC was also evaluated by real-time PCR showing a clear expression correlation with protein synthesis data increase in 8.48 fold in high intensity training group. Other myofibrillar protein, troponin , appear only in high intensity group, corroborating the cellular injury data. High molecular masses proteins such as MRS2 and NADH dehydrogenase, involved in metabolic pathways also demonstrate increase expression, respectily 1.5 and 1.3 fold, in response to high intensity exercise.

CONCLUSIONS

High intensity exercise demonstrated an increase expression in some high molecular masses myofibrilar proteins, α-MyHC and troponin. Furthermore this intensity also lead a significant increase of other high molecular masses proteins such as MRS2 and NADH dehydrogenase in comparison to low and moderate intensities. However, high intensity exercise also represented a significant degree of cellular injury, when compared with the individuals submitted to low and moderate intensities.

Effects of 8 wk of voluntary unloaded wheel running on K+ tolerance and excitability of soleus muscles in rat

During intense exercise, efflux of K+ from working muscles increases extracellular K+ ([K+]o) to levels that can compromise muscle excitability and hence cause fatigue. In this context, the reduction in the exercise-induced elevation of [K+]o observed after training in humans is suggested to contribute to the increased performance after training. Although a similar effect could be obtained by an increase in the tolerance of muscle to elevated [K+]o, this possibility has not been investigated. To examine this, isolated soleus muscles from sedentary (sedentary) rats and from rats that had voluntarily covered 13.1 ± 0.7 km/day in an unloaded running wheel for 8 wk (active) were compared. In muscles from active rats, the loss of force induced by exposure to an elevated [K+]o of 9 mM was 42% lower than in muscles from sedentary rats ( P < 0.001). This apparent increase in K+ tolerance in active rats was associated with an increased excitability as evident from a 33% reduction in the electrical current needed to excite individual muscle fibers ( P < 0.0009). Moreover, muscles from active rats had lower Cl− conductance, higher maximal rate of rise of single-fiber action potentials (AP), and higher Na+/K+ pump content. When stimulated intermittently at 6.5 mM K+, muscles from active rats displayed better endurance than muscles from sedentary rats, whereas no difference was found when the muscles were stimulated continuously at 30 or 120 Hz. We conclude that voluntary running increases muscle excitability, leading to improved tolerance to elevated [K+]o.

Shortening induced effects on force (re)development in pig urinary smooth muscle

INTRODUCTION

When muscle is allowed to shorten during an active contraction, the maximum force that redevelops after shortening is smaller than the isometric force at the same muscle length without prior shortening. We studied the course of force redevelopment after shortening in smooth muscle to unravel the mechanism responsible for this deactivation.

METHOD

In a first series of measurements the shortening velocity was varied resulting in different shortening amplitudes. In a second series, the duration of stimulation before shortening (shortening delay) was varied. In a third series, the stimulation was interrupted for a certain duration immediately after shortening. Force, muscle length and stimulation were continuously recorded. Time constants were calculated to describe the rate of force development before and after shortening.

RESULTS

With increasing shortening amplitude and with increasing shortening delay, force redevelopment decreased. Redevelopment increased with an increase in the interruption time. After stimulus interruption force redeveloped mono-exponentially with a time constant similar to that of isometric contractions (approximately 3s). Without the interruption of stimulation, the redevelopment of force immediately after shortening was best described by two time constants; one similar to and one about 3-5 times faster than the isometric time constant.

DISCUSSION

Force (re)development is caused by a cascade of events leading to the cycling of cross-bridges. In smooth muscle, isometric force development is described by a time constant of about 3s. Force redevelopment immediately after shortening involves a second process which takes place at a faster rate (time constant about 1s). We assume that this process is faster due to the immediate availability of cytoplasmic calcium released during active shortening. Deactivation presumably is caused by disorganization of filaments during shortening.

Functional roles of the gamma subunit of the skeletal muscle DHP-receptor

In excitation-contraction coupling (EC coupling) of skeletal muscle, large and rapid changes of the myoplasmic Ca2+ concentration mediate the activation and termination of force. The L-type Ca2+ channel (dihydropyridine receptor, DHP receptor) is a central component of the EC coupling process. Its predominant role is to provide the Ca2+ release channels of the sarcoplasmic reticulum (SR) with the sensitivity to cell membrane voltage. The DHP receptor consists of five different proteins (alpha1S, beta1, gamma1, delta and alpha2) whose tasks and functional characteristics are still incompletely understood. This short review summarizes progress made in studying the physiology of the gamma1 subunit, a membrane polypeptide that is highly specific for skeletal muscle. The focus is on recent results obtained from muscle of gamma1-deficient mice.

Altered inactivation of Ca2+ current and Ca2+ release in mouse muscle fibers deficient in the DHP receptor gamma1 subunit

Functional impacts of the skeletal muscle-specific Ca2+ channel subunit gamma1 have previously been studied using coexpression with the cardiac alpha1C polypeptide in nonmuscle cells and primary-cultured myotubes of gamma1-deficient mice. Data from single adult muscle fibers of gamma-/- mice are not yet available. In the present study, we performed voltage clamp experiments on enzymatically isolated mature muscle fibers of the m. interosseus obtained from gamma+/+ and gamma-/- mice. We measured L-type Ca2+ inward currents and intracellular Ca2+ transients during 100-ms step depolarizations from a holding potential of -80 mV. Ratiometric Ca2+ transients were analyzed with a removal model fit approach to calculate the flux of Ca2+ from the sarcoplasmic reticulum. Ca2+ current density, Ca2+ release flux, and the voltage dependence of activation of both Ca2+ current and Ca2+ release were not significantly different. By varying the holding potential and recording Ca2+ current and Ca2+ release flux induced by 100-ms test depolarizations to +20 mV, we studied quasi-steady-state properties of slow voltage-dependent inactivation. For the Ca2+ current, these experiments showed a right-shifted voltage dependence of inactivation. Importantly, we could demonstrate that a very similar shift occurred also in the inactivation curve of Ca2+ release. Voltages of half maximal inactivation were altered by 16 (current) and 14 mV (release), respectively. Muscle fiber bundles, activated by elevated potassium concentration (120 mM), developed about threefold larger contracture force in gamma-/- compared with gamma+/+. This difference was independent of the presence of extracellular Ca2+ and likely results from the lower sensitivity to voltage-dependent inactivation of Ca2+ release. These results demonstrate a specific alteration of voltage-dependent inactivation of both Ca2+ entry and Ca2+ release by the gamma1 subunit of the dihydropyridine receptor in mature muscle fibers of the mouse.