Alterations in phenotypic and contractile properties of the rat diaphragm: Influence of hypothyroidism

This study examined the influence of experimental hypothyroidism on myosin isoform distribution and contractile function of the costal diaphragm. Adult female Sprague-Dawley rats were randomly assigned to control (n = 12) or hypothyroid groups (n = 13) over a 6-wk treatment period. In comparison to the control group, in the hypothyroid group the relative distribution of type I myosin heavy chain (MHC) was increased 35% (P < 0.05), whereas type IIb MHC decreased 63% (P < 0.05). Similarly, Ca2+- activated myosin adenosinetriphosphatase activity (nmol P-i . mg(-1)min(-1)) in the hypothyroid group was reduced 30% compared with the control group (P < 0.05). Furthermore, significant reductions in diaphragmatic maximal tetanic specific tension (P-o; N . cm(-2); -21%) and maximal shortening velocity (V-max; muscle length/s; -25%) were observed in the hypothyroid group. These data provide the first evidence that hypothyroid produces a fast- (type IIb) to-slow (type I) shift in costal diaphragmatic MHC isoform profile that is highly correlated to the observed decrease in V-max. Finally, the present findings indicate that hypothyroidism does not alter myofibrillar content or noncontractile elements of the diaphragm, thereby suggesting an alternative mechanism(s) to explain the reduction in specific P-o.