Maximum specific force depends on myosin heavy chain content in rat diaphragm muscle fibers

Abstract

In the present study, myosin heavy chain (MHC) content per half sarcomere, an estimate of the number of cross bridges available for force generation, was determined in rat diaphragm muscle (Dia(m)) fibers expressing different MHC isoforms. We hypothesize that fiber-type differences in maximum specific force [force per cross-sectional area (CSA)] reflect the number of cross bridges present per CSA. Studies were performed on single, Triton X-100-permeabilized rat Dia(m) fibers. Maximum specific force was determined by activation of single Dia(m) fibers in the presence of a high-calcium solution (pCa, -log Ca(2+) concentration of 4.0). SDS-PAGE and Western blot analyses were used to determine MHC isoform composition and MHC content per half sarcomere. Differences in maximum specific force across fast MHC isoforms were eliminated when controlled for half-sarcomere MHC content. However, the force produced by slow fibers remained below that of fast fibers when normalized for the number of cross bridges available. On the basis of these results, the lower force produced by slow fibers may be due to less force per cross bridge compared with fast fibers.

Respiratory muscle fiber remodeling in chronic hyperinflation: dysfunction or adaptation?

“…In that analysis, despite consistent reductions in fiber mATPase activities, the ratio of SDH to mATPase was elevated. The significance of this finding is highlighted by the idea that SDH-to-mATPase ratio is considered a marker of fatigue resistance (26). A parallel study by Wijnhoven et al (88) in a group of patients with largely moderate disease found increases in ␤-oxidation capacity and in mitochondrial respiratory enzyme activity that increased as a function of worsening disease severity.…”

“…It is defined as the maximum tetanic force per cross-sectional muscle area. It is relatively independent of muscle length, but is dependent on myosin content, a measure of cross-bridge density (26). It is well known that patients with COPD and chronic hyperinflation exhibit reductions in maximum inspiratory pressures and reduced Pdi max , e.g., Ref.…”

“…Perhaps more importantly, the data so far suggest that metabolic adaptations toward a predominance of slower fiber types with higher aerobic capacity (discussed below) may contribute to any decrease in specific force that is observed. For example, in control rodent diaphragms, type I fibers demonstrate a 50% loss of specific force compared with fast fibers (26), due in part to reductions in myosin content. Ottenheijm et al (62) have shown that essentially all of the loss of single-fiber specific force in human diaphragm from patients with moderate disease can be accounted for by reductions in myosin content, although whether this applies to patients with severe disease is still unclear.…”

Mechanisms underlying myosin heavy chain expression during development of the rat diaphragm muscle

“…A linear relationship between the brightness-area product and the myosin content across rabbit myosin samples was used to determine the myosin content in the individual rat Dia m samples. This method has been previously described and validated (13,14,16). For each Diam sample, MHC content was normalized to the tissue weight.…”

“…Identification of MHC isoforms by migration patterns was verified by Western blot analysis as previously described (16,17). Briefly, rat Dia m bundles were run on SDS-PAGE and transferred to nitrocellulose overnight.…”