Johnson, Brian

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 (Vo) and maximum specific force (Po) 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, Vo was determined at 15 degrees C by use of the “slack” test. Isometric Po was determined at 15 and 26 degrees C. Simple and stepwise regressions were used to evaluate the correlations between Vo, Po, and MHC phenotype transitions and the various developmental ages. The progressive increases in Vo and Po 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 Vo and Po, with changes in MHC-2X and MHC-2B expression also contributing significant increments to total variance. The postnatal increase in Vo most likely relates to differences in the actomyosin adenosinetriphosphatase activity between neonatal and adult fast MHC phenotypes. The increase in Po 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.

In this study, we examined whether exhaustive activation reduces succinate dehydrogenase (SDH) activity in diaphragm muscle fibers. In adult male rats (approximately 300 g), the costal diaphragm was excised and positioned in a chamber perfused with mammalian Ringer solution kept at 26 degrees C and oxygenated with 95% O2–5% CO2. The muscle was stimulated directly at 10 or 75 Hz in trains of 500 ms duration (1/s) for 8 min. An adjacent unstimulated segment of muscle served as control. The two muscle segments were frozen, and serial sections were stained for myofibrillar adenosinetriphosphatase activity after alkaline and acid preincubation to classify type I, IIa, and IIb fibers. The extent of glycogen utilization was also examined histochemically to confirm exhaustive activation of muscle fibers. SDH activity was quantified using a microdensitometric procedure implemented on an image-processing system. Exhaustive activation at both 10 and 75 Hz caused a significant decrease in SDH activity of all fiber types, with the decrease after 10-Hz stimulation being greater than that after 75-Hz stimulation. At both stimulation frequencies, type IIb fibers demonstrated the greatest decrease in SDH activity (36% after 10-Hz and 27% after 75-Hz stimulation), whereas type I and IIa fibers both displayed reductions of approximately 27 and approximately 19% after 10- and 75-Hz stimulation, respectively. The greater reduction of SDH activity in type IIb fibers indicates an inverse relationship between activation-induced reductions in SDH activity and fiber oxidative capacity.(ABSTRACT TRUNCATED AT 250 WORDS)

Abstract The present study sought to examine the effects of aging on the isometric contractile and fatigue properties as well as the myosin heavy chain (MCH) isoform composition of the rat diaphragm muscle. Male Fischer 344 (F344) specific pathogen-free rats 6 and 24 mo old were used in the study. Peak twitch force was approximately 23% lower (p &lt; 0.05) in the senescent diaphragm compared with the young. Time to peak twitch force and one-half relaxation time of twitch force did not differ between groups. There was a significant decrease (15 to 18%, p &lt; 0.05) in the specific force (N/cm2) of the senescent diaphragm at all stimulation frequencies (10 to 100 Hz) examined. In addition, the fatigability of the diaphragm did not significantly differ between the two groups. No significant changes in the distribution of MHC 1 and 2A isoforms were observed with aging. However, the contribution of MHC 2X significantly decreased with senescence (young, 37.5%; senescent, 30.5%), whereas the contribution of MHC 2B in the senescent diaphragm was significantly higher (young, 6.5%; senescent, 15.0%; p &lt; 0.05). We conclude that the age-related decline in diaphragm muscle specific force is caused by intrinsic factors other than changes in MHC composition.