CYTOPLASM-TO-MYONUCLEUS RATIOS IN PLANTARIS AND SOLEUS MUSCLE-FIBERS FOLLOWING HINDLIMB SUSPENSION

In multinucleated skeletal muscle fibres the size of the cytoplasmic v olume-to-myonucleus ratio is related to the myosin heavy chain phenoty pe, with the ratio being larger in those fibres expressing the fast my osin heavy chain phenotype. It is unknown however, whether this ratio is modulated during muscle fibre adaptation, such as that which occurs following muscle unloading. In this study the relationship between cr oss sectional area, myonuclear number and myosin type, in single fibre s from the plantaris and soleus muscles of adult rats following 28 day s of hindlimb suspension was examined. Each fibre was cut transversely into two segments; one segment was used for immunohistochemical ident ification of myosin type, the other for determination of cross section al area and myonuclei number. Single fibre analysis revealed significa nt atrophy of both plantaris fast and soleus slow fibres; the mean cro ss sectional area (mu m(2)) of these fibres, 3104 +/- 183 and 2082 +/- 107 (mean +/- SE), being 70 and 45%, respectively, of control means. The decreases in cross sectional area were not accompanied by correspo nding decreases in the number of myonuclei (myonuclei/mm); in plantari s fast fibres the mean myonuclei counts were within the control range (88 +/- 8 (hindlimb suspension), 76 +/- 7 (control)), in soleus slow f ibres the counts were significantly increased (185 +/- 12 (hindlimb su spension), 154 +/- 11 (control)). The changes resulted in a significan t decrease in the cytoplasmic volume-to-myonucleus ratio (mu m(3) x 10 (3)) for both fibre types; the mean ratios of 39 +/- 3 and 12 +/- 1, w ere 60% and 36% of control means for the plantaris fast and soleus slo w fibres, respectively. These results indicate that following hindlimb suspension atrophy of muscle fibres the myonuclei numbers remain cons tant or increase and, hence, the effective cytoplasmic-to-myonucleus r atio is decreased. Further, the described changes are significantly gr eater in soleus slow than plantaris fast fibres.