The effects of hypothyroidism on single fibers of the rat soleus muscle.
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.
Norenberg
K M
KM
Department of Exercise and Sports Sciences, University of Florida, Gainesville 32611-8206, USA.
Herb
R A
RA
Dodd
S L
SL
Powers
S K
SK
D000818:Animals; D001835:Body Weight / Q000502:physiology; D002118:Calcium / Q000494:pharmacology; D005260:Female; D007037:Hypothyroidism / Q000378:metabolism / Q000503:physiopathology*; D007537:Isometric Contraction / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology; D018485:Muscle Fibers, Skeletal / Q000502:physiology* / Q000648:ultrastructure; D018482:Muscle, Skeletal / Q000503:physiopathology*; D009929:Organ Size / Q000502:physiology; D010101:Oxygen Consumption / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D014284:Triiodothyronine / Q000097:blood
D014284:Triiodothyronine; D002118:Calcium