How an irritating research ‘contaminant’ produced new insights about muscle fatigue
A recently discovered avenue of intracellular communication within striated muscle
Some of the best discoveries in the research laboratory come from exploring factors that cause unexpected assay problems. Early efforts to extract mitochondria from cells produced mitochondria that were ‘contaminated’ with cell membranes. In particular, mitochondria from striated muscle cells frequently had this contamination problem.
The extra membranes posed a dilemma, because for many years it was thought mitochondria operated autonomously within cells. This was because isolated mitochondria in culture did not respond to molecules that mimicked those of the intracellular signaling pathways. The theory was that mitochondria, the energy powerhouses of cells, were regulated simply by substrate availability.
More recent data obtained with intact cells and fluorescent probe molecules discovered mitochondria do respond to local intracellular signals, including some that allow them to strategically increase their rate of ATP production. It is apparent that something different must be happening in the intact cell than in mitochondria cultures.
Mitochondria are tethered to sarcoplasmic reticulum by calcium ion (Ca++) channel proteins
The mitochondria assay contaminant turned out to be an important structural component of muscle fibers. The membranes proved to be tethers tying the sarcoplasmic reticulum to the mitochondria producing a cell structure now named SR-tethered mitochondria.
In other mammalian cells similar tethers tie mitochondria to endoplasmic reticulum. Theories of dynamic interactions between mitochondria and cell regulatory mechanisms began to flourish with the discovery of the tethered structures.
The tethers of SR-tethered mitochondria in cardiac and skeletal muscle include proteins that are critical for the strength of the attachment. The tethers are easily disrupted by controlled protein lysis. Only limited details are known at present about the mechanics of the tethering process. It seems, however, that the Ca++ channels of both organelles are necessary for a stable attachment.
Muscle contains a large population of mitochondria. Mitochondria occupy about 40% of cardiac and 20% of skeletal muscle cell volume. In both types of striated muscle fibers, mitochondria are located near to the Ca++ release units of the sarcoplasmic reticulum that are activated during muscle contraction. Some of the Ca++ released in response to motor neuron stimulation sets in motion the contractile apparatus of muscle fibers and some of it enters the mitochondria.
Ca++ entering mitochondria signals the need for an increased rate of ATP formation by oxidative phosphorylation. The close arrangement of the mitochondria to the sarcoplasmic reticulum Ca++ channels simultaneously fulfills ATP requirements of the contractile filaments and of the Ca++ re-uptake pumps of the sarcoplasmic reticulum. Re-uptake pumps, like those of the sarcoplasmic reticulum, that move Ca++ against its concentration gradient require a large amount of local ATP.
Muscle fatigue is caused by a deficiency of Ca++ for the contractile apparatus
Many studies have led to the conclusion that fatigue of striated muscle after strenuous exercise is associated with reduced Ca++ release from the sarcoplasmic reticulum. With discovery of the close structural contact between mitochondria and the sarcoplasmic reticulum, new theories have emerged to explain the molecular mechanism of muscle fatigue. One such theory is that Ca++ channels of the SR-tethered mitochondria play a role in regulation of the enzymes of the nearby glycogen granule complexes.
The ability of muscles to contract is diminished when glycogen, a storage form of glucose molecules, is reduced to a less than optimum level in the fibers. Originally muscle fatigue was thought to occur because of a shortage of ATP. Surprisingly however, after a rest period during which ATP levels are returned to normal through metabolism of muscle lipids, muscles remain fatigued until glycogen stores are also replenished.
Each glycogen granule in muscle exists in a complex with the set of enzymes needed to alternately synthesize it and break it down into glucose molecules. In muscle, it is likely that glucose is metabolized near the glycogen granule that released it. Muscle fibers, because of their highly ordered structure, probably do not depend upon diffusion to move glucose around in the cell.
Consistent with the idea that glucose is metabolized near its glycogen granule, glycogen is allocated to three physically distant pools within muscle fibers. One pool containing about 12% to 15% of the granules is located directly under the sarcolemma, the cell membrane. The largest pool of glycogen, about 75% of the cell’s total glycogen resides between myofibrils near SR-tethered mitochondria. The fiber’s remaining glycogen is found between myofibrils near the Z-line.
After exercise that produces fatigue, each of the glycogen pools is about 70% to 90% depleted. But, quantitatively the largest amount of remaining glycogen is found near the SR-tethered mitochondria. Possibly it is this pool of glycogen that is used for the energy required to restore normal function of the nearby sarcoplasmic Ca++ channels. Testing such theories will require more detailed study of SR-tethered mitochondria.
SR-tethered mitochondria represent just one of many cases where outstanding discoveries in physiology were made by close examination of experiments that just did not behave as expected. When nature continually produces problems with a scientist’s desire to prove something, it is time to figure out what nature is trying to communicate.
If you are struggling with the mechanics of striated muscle cell contraction, here is a helpful video titled Muscle Contraction Process: Molecular Mechanism. To view the video, click the title.
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