Remodelling of the contractile apparatus of striated muscle stimulated electrically in a shortened position

The aim of this study was to examine reorganisation of the contractile apparatus during adaptation to function when the length of a muscle is decreased. The rat soleus muscle was maintained in a shortened position and simultaneously stimulated electrically at a low frequency for 1-45 h. This experimental model decreased the length of the muscle and made the contractile apparatus irregular. The length of the sarcomeres decreased and became variable. The Z-line appeared wavy or fragmented. Foci, within which the sarcomeric organisation was lacking, often appeared within the contractile apparatus. These changes, occurring during the initial hours of stimulation, increased in number during the following hours. Their frequency and intensity depended on the degree of muscle shortening. In muscle moderately shortened during stimulation (up to 20%) the contractile apparatus recovered its normal appearance within 12 h of the experiment. In the muscle considerably shortened (by about 30%) the process of normalisation took much longer, in spite of recovery of sarcomere length. We conclude from these results that these changes are related to the accelerated work-induced reorganisation of the contractile apparatus whenever sarcomere number is reduced and/or when elimination of portions of the contractile apparatus occurs. These anomalies, occurring transiently in normal mature muscle when stimulated electrically in a shortened position, should be considered as adaptive phenomena. They resemble abnormalities appearing in the contractile apparatus of myopathic muscle.