Effects of support stimulation on human soleus fiber characteristics during exposure to "dry" immersion

In this study the model of 7-day dry immersion (DI) was used. 17 male volunteers (23-29 years old) were divided in 2 groups: (i) 7-day DI without support (DI, n=9), (ii) 7-day DI using support stimulation (DIS, n=8). Support stimulator device exerted pressure of 0.2 +/- 0.15 kg/cm2 upon the plantar support zones simulating the walking pattern 6 times a day for 20 minutes of every hour: 10 minutes at a speed of 75 steps/min and 10 minutes at a speed of 120 steps/min. M. soleus biopsy was performed before and immediately after DI. The m. soleus fiber myosin heavy chain (MHC) profile, myofiber cross-sectional area (CSA) and total protein concentration were analyzed in frozen serial sections. In addition, NO-synthase 1 (NOS1) levels indicative of normal muscle cell signaling were analyzed by western blotting in 4 persons in each group. After dry immersion, percentage of muscle fibers containing type I MHC decreased by 6% (p<0.05) in group DI, but was not changed significantly in group DIS. Percentage of the type IIa fibers was significantly altered in none of the groups. Type I fiber CSA decreased by 24.4% (p<0.05) in group DI. No significant changes of type I fiber CSA were found in group DIS. CSA of the type IIa fibers significantly altered in none of the groups. The total protein concentration was found increased by 17.6% in group DI and by 21% in group DIS. The increased total protein content in group DI suggests a diminution of fiber CSA attributed to the loss of non-protein component of fibers. NOS1 decreased by 35.6% in group DI and increased by 58.1% in group DIS. We conclude that 7 days in dry immersion lead to reduction in the type I muscle fiber percentage, loss of the non-protein component and decline in NOS1. These changes were clearly prevented by the support stimulation protocol applied during the DI period.