title: Response of slow and fast muscle to hypothyroidism: maximal shortening velocity and myosin isoforms.
abstract: This study examined both the shortening velocity and myosin isoform distribution of slow- (soleus) and fast-twitch (plantaris) skeletal muscles under hypothyroid conditions. Adult female Sprague-Dawley rats were randomly assigned to one of two groups: control (n = 7) or hypothyroid (n = 7). In both muscles, the relative contents of native slow myosin (SM) and type I myosin heavy chain (MHC) increased in response to the hypothyroid treatment. The effects were such that the hypothyroid soleus muscle expressed only the native SM and type I MHC isoforms while repressing native intermediate myosin and type IIA MHC. In the plantaris, the relative content of native SM and type I MHC isoforms increased from 5 to 13% and from 4 to 10% of the total myosin pool, respectively. Maximal shortening velocity of the soleus and plantaris as measured by the slack test decreased by 32 and 19%, respectively, in response to hypothyroidism. In contrast, maximal shortening velocity as estimated by force-velocity data decreased only in the soleus (-19%). No significant change was observed for the plantaris.
authors.affiliation: Department of Physiology and Biophysics, University of California, Irvine 92717.
authors.forename: V J
authors.initials: VJ
authors.lastname: Caiozzo
authors.forename: R E
authors.initials: RE
authors.lastname: Herrick
authors.forename: K M
authors.initials: KM
authors.lastname: Baldwin
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism; D000818:Animals; D001835:Body Weight; D005260:Female; D007037:Hypothyroidism / Q000201:enzymology / Q000473:pathology / Q000503:physiopathology*; D007527:Isoenzymes / Q000378:metabolism*; D007537:Isometric Contraction; D009119:Muscle Contraction; D009124:Muscle Proteins / Q000378:metabolism; D009132:Muscles / Q000201:enzymology / Q000473:pathology / Q000503:physiopathology*; D009210:Myofibrils / Q000378:metabolism; D009218:Myosins / Q000737:chemistry / Q000378:metabolism*; D009929:Organ Size; D051381:Rats; D011919:Rats, Inbred Strains
chemical_list: D007527:Isoenzymes; D009124:Muscle Proteins; D000251:Adenosine Triphosphatases; D009218:Myosins