Widrick J J, Bangart J J, Karhanek M, Fitts R H
Department of Biology, Marquette University, Milwaukee, Wisconsin 53201, USA.
J Appl Physiol (1985). 1996 Mar;80(3):981-7. doi: 10.1152/jappl.1996.80.3.981.
This study examined the effectiveness of intermittent weight bearing (IWB) as a countermeasure to non-weight-bearing (NWB)-induced alterations in soleus type I fiber force (in mN), tension (Po; force per fiber cross-sectional area in kN/m-2), and maximal unloaded shortening velocity (Vo, in fiber lengths/s). Adult rats were assigned to one of the following groups: normal weight bearing (WB), 14 days of hindlimb NWB (NWB group), and 14 days of hindlimb NWB with IWB treatments (IWB group). The IWB treatment consisted of four 10-min periods of standing WB each day. Single, chemically permeabilized soleus fiber segments were mounted between a force transducer and position motor and were studied at maximal Ca2+ activation, after which type I fiber myosin heavy-chain composition was confirmed by sodium dodecyl sufate-polyacrylamide gel electrophoresis. NWB resulted in a loss in relative soleus mass (-45%), with type I fibers displaying reductions in diameter (-28%) and peak isometric force (-55%) and an increase in Vo (+33%). In addition, NWB induced a 16% reduction in type I fiber Po, a 41% reduction in type I fiber peak elastic modulus [Eo, defined as (delta force/delta length) x (fiber length/fiber cross-sectional area] and a significant increase in the Po/Eo ratio. In contrast to NWB, IWB reduced the loss of relative soleus mass (by 22%) and attenuated alterations in type I fiber diameter (by 36%), peak force (by 29%), and Vo (by 48%) but had no significant effect on Po, Eo, or Po/Eo. These results indicate that a modest restoration of WB activity during 14 days of NWB is sufficient to attenuate type I fiber atrophy and to partially restore type I peak isometric force and Vo to WB levels. However, the NWB-induced reductions in Po and Eo, which we hypothesize to be due to a decline in the number and stiffness of cross bridges, respectively, are considerably less responsive to this countermeasure treatment.
本研究考察了间歇性负重（IWB）作为一种对策，对非负重（NWB）引起的比目鱼肌I型纤维力（单位为毫牛顿）、张力（Po；单位为千牛顿每平方米的每根纤维横截面积的力）和最大无负荷缩短速度（Vo，单位为纤维长度每秒）变化的有效性。成年大鼠被分为以下几组：正常负重（WB）组、后肢非负重14天（NWB组）以及后肢非负重并进行IWB处理14天（IWB组）。IWB处理包括每天四个10分钟的站立负重时段。将单个化学通透的比目鱼肌纤维段安装在力传感器和位置电机之间，并在最大钙激活状态下进行研究，之后通过十二烷基硫酸钠 - 聚丙烯酰胺凝胶电泳确认I型纤维肌球蛋白重链组成。非负重导致比目鱼肌相对质量损失（-45%），I型纤维直径减小（-28%）、等长峰值力降低（-55%）以及Vo增加（+33%）。此外，非负重导致I型纤维Po降低16%，I型纤维峰值弹性模量[Eo，定义为（力的变化/长度的变化）×（纤维长度/纤维横截面积）]降低41%，且Po/Eo比值显著增加。与非负重相反，间歇性负重减少了比目鱼肌相对质量的损失（减少22%），并减轻了I型纤维直径（减少36%）、峰值力（减少29%）和Vo（减少48%）的变化，但对Po、Eo或Po/Eo没有显著影响。这些结果表明，在14天的非负重期间适度恢复负重活动足以减轻I型纤维萎缩，并将I型纤维等长峰值力和Vo部分恢复到负重水平。然而，我们推测非负重引起的Po和Eo降低分别是由于横桥数量和刚度下降所致，对此对策治疗的反应要小得多。
