Work E-mail: email@example.com
Everyone has felt the effect of fatigue in our muscle when we do exercise. For more than a century scientists have tried to elucidate the mechanisms of muscle fatigue. For a long time, changes in metabolites, such as ATP, H + and lactate, were thought to reduce force production by the contractile components and thus be responsible for the decrease in force during fatigue. Surprisingly, recent studies have now demonstrated that neither ATP, H + nor lactate may be responsible for the decrease in force. Even more exciting is the fact that lactate and acidic pH may in fact protect muscle against the fatigue process! As a consequence of these new findings, we must now study how muscle contractility is regulated not only during fatigue but throughout the entire exercise process. This constitutes one of the major research projects in this laboratory.
Mechanisms of Muscle Fatigue In Skeletal Muscle: Role Of Ions & Ion Channels
Muscle fatigue is now considered a protective mechanism that prevents large depletion of energy reserve and excessive increase in intracellular Ca 2+, two situations that can lead to fiber damage and death. It is now well established that the decrease in force during fatigue is associated with decreases in Ca 2+ release by the sarcoplasmic reticulum. What remain to be elucidated are the mechanisms that lead to the decrease in Ca 2+ release. There is now evidence that changes in membrane excitability is a major cause for the decrease in Ca 2+ release.
Role of Na +, K + and H +
During fatigue, there are large increases in extracellular K + and intracellular Na + and H + concentrations. It has long been though that these changes in Na + and K + concentrations are in part responsible for the decrease in membrane excitability and thus force while a decrease in intracellular pH lowers force production by the contractile components during fatigue (see Reference # 1 below). However, new and interesting studies have now demonstrated that the changes in K + (Reference # 2) Na +, and H + concentrations can also prevent fatigue! In a recent review (Reference #3), questions were raised as to i) under what physiological conditions are Na +, K + and H + protecting against fatigue? ii) under what physiological conditions are Na +, K + and H + contributing to fatigue? and iii) what are the intracellular signaling pathways regulating the ion effects? The hypothesis being tested is that the ions protect against fatigue at the onset of exercise in order to optimize muscle performance and start depressing membrane excitability and force when there is an energy deficit or excessive increase in intracellular Ca 2+. To test this hypothesis we are determining how the activity of different K + and Cl - channels change during exercise and fatigue and how it modulates Na +, K + and H + effects on muscle contractility.
Role of the K ATP channel
Among the several ion channels expressed in skeletal muscle, this laboratory has largely focused on the ATP-sensitive K + channel or KATP channel. This channel is interesting because its activity is regulated by the energy state of the muscle fibers, where a decrease in energy reserve increases the activity of the channel. It therefore behaves as an energy sensor. Being an ion channel, it is also an effector, which links the energy state of the fiber to the electrical activity of the cell membrane.
Our studies have now clearly demonstrated that the KATP channel is crucial in preventing fiber damage during treadmill running and fatigue elicited in vitro (References #4-5). We are in the process of elucidating two major mechanisms of action for the channel. The first mechanism involves a reduction in action potential amplitude (Reference #6). As a consequence of lower action potential amplitude, less Ca 2+ is released by the sarcoplasmic reticulum and less force is developed by the contractile component (Reference #6). We are now testing the hypothesis that the decrease in Ca 2+ release and force is important to reduce the activity of two major ATPases, the myosin ATPase and the Ca 2+-ATPase, in order to prevent large and damaging ATP depletion. The second mechanism is to prevent large and excessive membrane depolarization. In the absence of K ATP channel activity, the excessive depolarization activates Ca 2+ channels that results in excessive increase in intracellular Ca 2+ and resting tension (References #5-7). We hypothesized that the large increase in Ca 2+ is responsible for an excessive production of reactive oxygen species and the activation of Ca 2+-sensitive calpain, both of which causing fiber damage.
One pertinent question about the KATP channel is whether they contribute to the decrease in force. According to our studies, using channel opener, the answer is yes. However, when the K ATP channel activity is abolished, by pharmaceutical or genetic means, the rate of fatigue does not slow down as expected, but in fact it increases. Cifelli et al., (Reference #7) showed for the first time that preventing the large increase in intracellular Ca 2+ results in slower rate of fatigue when K ATP channel activity is abolished. However, the difference with control is small, possibly because the excessive membrane depolarization inactivates a large number of Na + channels. We are now testing the hypothesis that preventing the excessive membrane depolarization will not only prevent the excessive increase in intracellular Ca 2+, but will also result in much slower fatigue rate as less Na + channels are inactivated.
Together, our studies on the K ATP channel will demonstrate an important fatigue mechanism which prevents how muscle fibers protect themselves from large and damaging ATP depletion and increases in intracellular Ca 2+.
Hyperkalemic periodic paralysis (HyperKPP)
HyperKPP is a rare channelopathy that affects membrane excitability. It is primarily characterized by myotonic discharges between and during periods of paralysis that mainly affect limb muscles. The paralysis completely incapacitates patients because they are unable to move. Patients are often confined to bed for a period of time varying from a few hours to days, with extreme cases persisting for a year. Past the age of 30, myotonic discharge and paralysis may cease, but patients suffer of a debilitating myopathy such that walking becomes a difficult task; in some cases, patients become wheelchair bound. The disease is caused by missense mutations of the SCNA4 gene that encodes for the voltage-sensitive Na + channel, NaV1.4. Using a mouse model with the M1592V mutation, Hayward et al. (Reference #8) demonstrated that HyperKPP muscles have a great sensitivity to an increased K + concentration where the decrease in force is much higher when compared to normal muscles.
This laboratory now has the mouse model which will allow us to study HyperKPP. One objective of our studies will be to determine how the defect in the NaV1.4 channel results in the myotonic discharges and the paralysis. An interesting phenotype of the disease is the lack of paralysis during moderate exercise despite the large increase in plasma K + concentration. Therefore, a second objective of our studies will be to determine how the activity of different ion channels changes during exercise and whether these changes prevent the paralysis when plasma K + concentration increases. Finally the third objective is to find new treatments to treat patients suffering of HyperKPP.