CONTRACTILE PROPERTIES OF THE DEVELOPING DIAPHRAGM CORRELATE WITH MYOSIN HEAVY-CHAIN PHENOTYPE

The objective of this study was to determine the relationship between developmental transitions in myosin heavy chain (MHC) composition and changes in maximum unloaded shortening velocity (V-o) and maximum spec ific force (P-o) of the rat diaphragm muscle. The diaphragm was excise d at postnatal days 0, 3, 7, 14, 21, and 28 and in adults. MHC isoform expression was determined by sodium dodecyl sulfate-polyacrylamide ge l electrophoresis and laser densitometry. In muscle fiber bundles, V-o was determined at 15 degrees C by use of the ''slack'' test. Isometri c P-o was determined at 15 and 26 degrees C. Simple and step wise regr essions were used to evaluate the correlations between V-o, P-o, and M HC phenotype transitions and the various developmental ages. The progr essive increases in V-o and P-o with age were found to be inversely co rrelated to MHC-neonatal isoform expression (r(2) = -0.84 and -0.63, r espectively) and positively correlated to MHC-2X (r(2) = 0.78 and 0.57 ) and MHC-2B (r(2) = 0.51 and 0.40) isoform expression (P < 0.001). Ch anges in MHC-neonatal isoform expression contributed to most of the de velopmental variance in V-o and P-o, with changes in MHC-2X and MHC-2B expression also contributing significant increments to total variance . The postnatal increase in V-o most likely relates to differences in the actomyosin adenosinetriphosphatase activity between neonatal and a dult fast MHC phenotypes. The increase in P-o may reflect inherent dif ferences in myofibrillar density, cross-bridge cycling kinetics, and/o r the force produced per cross bridge among fibers composed of the dif ferent MHC isoforms.