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 specific force (P-o) of the rat diaphragm muscle. The diaphragm was excised at postnatal days 0, 3, 7, 14, 21, and 28 and in adults. MHC isoform expression was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and laser densitometry. In muscle fiber bundles, V-o was determined at 15 degrees C by use of the ''slack'' test. Isometric P-o was determined at 15 and 26 degrees C. Simple and step wise regressions were used to evaluate the correlations between V-o, P-o, and MHC phenotype transitions and the various developmental ages. The progressive increases in V-o and P-o with age were found to be inversely correlated to MHC-neonatal isoform expression (r(2) = -0.84 and -0.63, respectively) 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). Changes in MHC-neonatal isoform expression contributed to most of the developmental 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 adult fast MHC phenotypes. The increase in P-o may reflect inherent differences in myofibrillar density, cross-bridge cycling kinetics, and/or the force produced per cross bridge among fibers composed of the different MHC isoforms.