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°C by use of the 'slack' test. Isometric P(o) was determined at 15 and 26°C. Simple and stepwise 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 (r² = - 0.84 and -0.63, respectively) and positively correlated to MHC-2X (r² = 0.78 and 0.57) and MHC-2B (r² = 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.