Gardetto P R, Schluter J M, Fitts R H
Biology Department, Marquette University, Milwaukee, Wisconsin 53233.
J Appl Physiol (1985). 1989 Jun;66(6):2739-49. doi: 10.1152/jappl.1989.66.6.2739.
The purpose of this investigation was to determine how muscle atrophy produced by the hindlimb suspension (HS) model alters the contractile function of slow- and fast-twitch single muscle fibers. After 2 wk of HS, small bundles of fibers were isolated from the soleus and the deep and superficial regions of the lateral and medial heads of the gastrocnemius, respectively. The bundles were placed in skinning solution and stored at -20 degrees C until studied. Single fibers were isolated and suspended between a motor arm and force transducer, the functional properties were studied, and subsequently the fiber type was established by myosin heavy chain (MHC) analysis on 1-D sodium dodecyl sulfate polyacrylamide gel electrophoresis. After HS, slow-twitch fibers of the soleus showed a significant reduction in fiber diameter (68 +/- 2 vs. 41 +/- 1 micron) and peak tension (1.37 +/- 0.01 vs. 0.99 +/- 0.06 kg/cm2), whereas the maximal shortening speed (Vmax) increased [1.49 +/- 0.11 vs. 1.92 +/- 0.14 fiber lengths (FL)/s]. A histogram showed two populations of fibers: one with Vmax values identical to control slow-twitch fibers and a second with significantly elevated Vmax values. This latter group frequently contained both slow and fast MHC protein isoforms. The pCa-force relation of the soleus slow-twitch fibers was shifted to the right; consequently, the free Ca2+ required for the onset of tension and for 50% of peak tension was significantly higher after HS. Slow-twitch fibers isolated from the gastrocnemius after HS showed a significant reduction in diameter (67 +/- 4 vs. 44 +/- 3 microns) and peak tension (1.2 +/- 0.06 vs. 0.96 +/- 0.07 kg/cm2), but Vmax was unaltered (1.70 +/- 0.13 vs. 1.65 +/- 0.18 FL/s). Fast-twitch fibers from the red gastrocnemius showed a significant reduction in diameter (59 +/- 2 vs. 49 +/- 3 microns) but no change in peak tension or Vmax. Fast-twitch fibers from the white superficial region of the medial head of the gastrocnemius were unaffected by HS. Collectively, these data suggest that the effects of HS on fiber function depend on the fiber type and location. Both slow-twitch type I and fast-twitch type IIa fibers atrophied; however, only slow-twitch fibers showed a decline in peak tension, and the increase in Vmax was restricted to a subpopulation of slow-twitch soleus fibers.
本研究的目的是确定后肢悬吊（HS）模型所导致的肌肉萎缩如何改变慢肌和快肌单根肌纤维的收缩功能。HS处理2周后，分别从比目鱼肌以及腓肠肌内侧头和外侧头的深层及浅层区域分离出小束肌纤维。将这些肌束置于去皮溶液中，保存在-20℃直至进行研究。分离出单根肌纤维并悬吊于驱动臂和力传感器之间，研究其功能特性，随后通过一维十二烷基硫酸钠聚丙烯酰胺凝胶电泳上的肌球蛋白重链（MHC）分析确定纤维类型。HS处理后，比目鱼肌的慢肌纤维直径显著减小（68±2微米对41±1微米），峰值张力降低（1.37±0.01千克/平方厘米对0.99±0.06千克/平方厘米），而最大缩短速度（Vmax）增加[1.49±0.11对1.92±0.14肌纤维长度（FL）/秒]。直方图显示有两类纤维群体：一类Vmax值与对照慢肌纤维相同，另一类Vmax值显著升高。后一组纤维通常同时含有慢肌和快肌MHC蛋白亚型。比目鱼肌慢肌纤维的pCa-力关系向右移动；因此，HS处理后产生张力起始和达到50%峰值张力所需的游离Ca2+显著更高。HS处理后从腓肠肌分离出的慢肌纤维直径显著减小（67±4微米对44±3微米），峰值张力降低（(1.2±0.06千克/平方厘米对0.96±0.07千克/平方厘米），但Vmax未改变（1.70±0.13对1.65±0.18 FL/秒）。腓肠肌红色部分的快肌纤维直径显著减小（59±2微米对49±3微米），但峰值张力和Vmax无变化。腓肠肌内侧头白色浅层区域的快肌纤维未受HS影响。总体而言，这些数据表明HS对纤维功能的影响取决于纤维类型和位置。慢肌I型和快肌IIa型纤维均发生萎缩；然而，只有慢肌纤维的峰值张力下降，且Vmax的增加仅限于比目鱼肌慢肌纤维的一个亚群。
