The Effect of Na,K-ATPase and Dihydropyridine Receptor Activity on Ca Content and Contractile Properties of in Rats under Three-Day Functional Unloading

Hypokinesia is characterized by muscle atrophy and decreased strength, which can occur at the earliest stages of unloading. To study the relationship between calcium-dependent processes in the fiber and changes in the contractile properties of m. soleus, a 3-day period of unloading was used in male Wistar rats, which were administered infedipine, a dihydropyridine receptor (DHPR) blocker, or ouabain, a cardiac glycoside that binds to the α-subunit of Na,K-ATPase. In two series of experiments, three groups of rats were used (16 individuals in each group): control (C), suspension (3HS), suspension + infedipine (3HS + N), or (for the second series) suspension + ouabain (3HS + Ou). A decrease in weight was found in all suspension groups (HS, 3HS + N, 3HS + Ou) ( < 0.05). The three-day inhibition of DHPR during functional unloading prevented the decrease in force and the increase in the Ca level in the myoplasm of . Administration of infedipine at early stages of unloading prevents the decrease in the maximum force of single and tetanic contraction of the soleus (in contrast to the group of suspension without the drug, where the decrease occurs by 24 and 29% relative to the control, respectively, < 0.05). Administration of ouabain led to a decrease (by 22 and 18%, respectively, < 0.05) in the specific maximum force of single and tetanic contraction of the soleus in animals subjected to suspension (without and with the drug), relative to the intact control. However, the drug significantly affected the passive mechanical properties of : the maximum peak force and the maximum force at the end of the stretch test in muscles with ouabain administration did not differ from the intact control group, whereas in the suspended group (3HS) it decreased (by 33 and 34%, respectively, < 0.05). In summary, the administration of ouabain and infedipine leads to the prevention of myoplasmic calcium accumulation against the background of hindlimb unloading, which is accompanied by the preservation of specific muscle stiffness and strength, a slowdown in the decrease in the cross-sectional area of muscle fibers, and the prevention of a change in the percentage of fibers towards fast-type fibers.