Contractile properties of adjacent segments of single human muscle fibers

A comparison of the contractile properties of adjacent segments of single h uman muscle fibers may help to explain the interaction among nuclear domain s within the myofiber. Biopsy samples were obtained from the vastus lateral is muscle of 20 healthy untrained women (age 18-79 years). Single fibers (n = 38) were dissected and cut into halves (segments A and B). Segment diame ter and depth were measured using an image analysis system. Maximal force ( Po) was recorded during activation with calcium (pCa 4.5). Maximal unloaded shortening velocity (Vo) was calculated using the slack test. Myosin heavy chain (MyHC) expression was determined using sodium dodecyl sulphate polya crylamide gel electrophoresis (SIDS-PAGE). A significant difference (simila r to7%) in Po was seen between adjacent segments expressing type I MyHC tha t could not be attributed to differences in fiber size. Significant differe nces were observed in Vo even after adjusting for fiber type. A positive co rrelation was seen in Po (concordance coefficient Rho(-)C = 0.803) and Vo ( Rho-C = 0.690) between segments, but concordance was less than perfect in b oth cases. Possible explanations for nonuniformity of contractile propertie s include random variations in physiological systems or variability of prot ein expression among nuclear domains. (C) 2001 John Wiley & Sons, Inc.