The basal calcium level in fibers of the rat soleus muscle under gravitational unloading: The mechanisms of its increase and the role in calpain activation

Abstract

The first data on increase in the basal level of cal� cium in immobilized muscles appeared in 1970 (3). Nasledov and coworkers hypothesized that calcium ions accumulated under the conditions of gravita� tional unloading (1). In that study, the authors used the model of antiorthostatic suspension. Using an isolated rat m. soleus, the mechanical response has been dem� onstrated to attenuate rapidly after a series of tetanic contractions in calciumfree Ringer's solution. How� ever, after 14�day unloading in the standard model of antiorthostatic suspension, there was no attenuation in tests performed on muscles of the suspended ani� mals. When, in the second experimental series, Vera� pamil (a calcium channel blocker) was added to the calciumfree medium, attenuation of the mechanical response was observed in the suspended animals. It was suggested that an increased resistance to muscle fatigue in calciumfree Ringer's solution was a result of intracellular accumulation of calcium ions, which freely came out of the muscle through the channels, and Verapamil inhibited this process. Using the calcium probe Fluo�3, a significant increase in the concentration of calcium ions of the mouse m. soleus was later demonstrated as soon as the second day of suspension (8). An excessive amount of calcium ions might occur under microgravity in the myoplasm of the muscle fiber either from the extracellular space through exter� nal calcium channels or from the sarcoplasm reticu� lum because of "leakage" of Ryanodine channels. There is evidence suggesting that Ryanodine channels are involved in increasing the intracellular calcium (12). In addition, a higher expression of dihydropyri� dine receptors was already observed upon shortterm gravitational unloading (9). It was subsequently dem� onstrated that the use of Nifedipine, a selective blocker of calcium channels, against the background of 14�day functional unloading led to a significant diminishing the level of calcium ions in the myoplasm of m. soleus fibers, and it largely prevented the trans� formation of the myosin phenotype (6, 12). At the same time, peroral Nifedipine administration may ini� tiate multiple systemic effects. Because of this, only a decrease in the basal level of calcium ions in the mus� cle fiber in response to a direct Nifedipine action against the background of gravitational unloading can serve as valid evidence of the involvement of slow Ltype calcium channels into accumulation of these ions in fibers of postural muscles. An increase in the calcium ion concentration in the myoplasm of muscular fibers at rest may stimulate the activity of calpains, calciumdependent proteases. Both µ� and mcalpains, which are activated by microand millimolar calcium concentrations, respectively, are present in all tissues of vertebrate ani� mals. These enzymes play the key role in the degrada� tion of a series of cytoskeletal proteins (2). The calpain activity has been demonstrated to grow up signifi� cantly even upon a shortterm gravitational unloading (4, 5), and this was accompanied by a significant decrease in the concentration of desmin, one of the key cytoskeletal muscle proteins. At the same time, the importance for increasing the calpainstimulating activity of calcium ions in the muscle fibers during gravitational unloading, may be understood after application of calcium chelating agents with an intra� cellular activity under these conditions.