The effects of hypothyroidism on single fibers of the rat soleus muscle

Abstract

Skinned single fibers were used to test the hypothesis that skinned fibers from hypothyroid soleus muscle would have a higher sensitivity to calcium compared with control fibers, as indicated by a leftward shift of the pCa-force curve. Control rats (n = 14) received sham injections, while the hypothyroid group (n = 16) received thyroidectomy and a 6-week injection period of the antithyroid drug 6-n-propyl-2-thiouracil. Hypothyroidism caused the type I fiber number to increase significantly (11%) in the soleus. Hypothyroid fibers produced 16% less absolute tension than control fibers. However, cross-sectional areas of control fibers were significantly greater (25%) than those of hypothyroid fibers, so that when force was normalized to cross-sectional area, no differences between groups existed. Calcium requirement for half-maximal force production (pCa50) did not differ, but the slope of the pCa-force curve was different between groups. These data suggest that hypothyroidism did not alter the intrinsic force-generating capacity of the soleus muscle fibers. Thus, alterations in hypothyroid soleus contractile function seen in vitro may be explained by alterations in excitation-contraction coupling and (or) shifts in muscle fiber types.