ALTERATIONS IN PHENOTYPIC AND CONTRACTILE PROPERTIES OF THE RAT DIAPHRAGM - INFLUENCE OF HYPOTHYROIDISM

This study examined the influence of experimental hypothyroidism on my osin isoform distribution and contractile function of the costal diaph ragm. Adult female Sprague-Dawley rats were randomly assigned to contr ol (n = 12) or hypothyroid groups (n = 13) over a 6-wk treatment perio d. In comparison to the control group, in the hypothyroid group the re lative distribution of type I myosin heavy chain (MHC) was increased 3 5% (P < 0.05), whereas type IIb MHC decreased 63% (P < 0.05). Similarl y, 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 o bserved in the hypothyroid group. These data provide the first evidenc e 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 indic ate that hypothyroidism does not alter myofibrillar content or noncont ractile elements of the diaphragm, thereby suggesting an alternative m echanism(s) to explain the reduction in specific P-o.