Influence of one-week hindlimb suspension and intermittent high load exercise on rat muscles

Abstract

Hindlimb suspension (HS) is a ground-based simulation model for spaceflight commonly used to study the effects of unloading on skeletal muscle. The purpose of the present study was to determine the effects of a daily short-duration, high-load exercise training program in ameliorating the atrophic response and associated adaptations in the mechanical properties of the soleus (SOL) and medial gastrocnemius (MG) muscles to HS. Male Sprague-Dawley rats (mean body weight, ∼250 g) were assigned randomly to one of three groups: control (CON, n = 7), HS, ( n = 7), and HS plus exercise (HS-EX, n = 8). The exercise consisted of rats climbing up a 1-m grid inclined at ∼85° for eight repetitions with a load equal to 75% body weight attached to their tails repeated four times daily during their active period. Following 7 days, soleus wet weights in the HS and HS-EX rats were 42 and 23% lower than those in the CON rats. The SOL muscle weight to body weight ratio in the HS group was 32% smaller than that in the CON group, whereas it was similar to that of CON in the HS-EX. Maximum tetanic tensions in the HS and HS-EX groups were 44 and 27% less than that in CON, with that in HS-EX being significantly greater than that in HS. SOL contraction time was reduced by 12 and 20% from CON in the HS and HS-EX groups, while the half-relaxation time of the HS group was similar to that of CON and was 24% less than that of CON in the HS-EX group. There were no differences among the three groups in SOL fatigability, absolute maximal shortening velocity, wet to dry weight ratio, tetanic tension to muscle weight ratio, or muscle protein concentration. The MG wet weights were ∼28% less than that of CON in both suspended groups. Relative to body weight, the MGs were 17 and 11% smaller than that of CON in the HS and HS-EX groups. Tetanic tension was 23 and 16% less than that of CON in the HS and HS-EX groups. Consequently, the tetanic tension per muscle weight in the MG was elevated from CON in the HS-EX, but not the HS, group. Contraction time, half-relaxation time, maximal shortening velocity, fatigability, and protein concentration were similar among the three groups, whereas the wet to dry weight ratio of the MG in the HS-EX group was lower than that of CON. These data show that very brief, high-load exercise training can markedly attenuate the detrimental effects of unloading on the SOL, while having a lesser effect on the MG. These studies suggest that high-load exercise can be an effective and efficient means of reducing atrophy during HS.