Selected Contribution: Mechanisms underlying increased force generation by rat diaphragm muscle fibers during development

Abstract

It has been found that maximum specific force (F(max); force per cross-sectional area) of rat diaphragm muscle doubles from birth to 84 days (adult). We hypothesize that this developmental change in F(max) reflects an increase in myosin heavy chain (MHC) content per half-sarcomere (an estimate of the number of cross bridges in parallel) and/or a greater force per cross bridge in fibers expressing fast MHC isoforms compared with slow and neonatal MHC isoforms (MHC(slow) and MHC(neo), respectively). Single Triton 100-X-permeabilized fibers were activated at a pCa of 4.0. MHC isoform expression was determined by SDS-PAGE. MHC content per half-sarcomere was determined by densitometric analysis and comparison to a standard curve of known MHC concentrations. MHC content per half-sarcomere progressively increased during early postnatal development. When normalized for MHC content per half-sarcomere, fibers expressing MHC(slow) and coexpressing MHC(neo) produced less force than fibers expressing fast MHC isoforms. We conclude that lower force per cross bridge in fibers expressing MHC(slow) and MHC(neo) contributes to the lower F(max) seen in early postnatal development.

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

“…It was not possible, however, to design reliable primer pairs for the MHC 2B isoform suitable for real-time RT-PCR using a mimic internal control. This isoform is usually expressed at low levels and rarely is singly expressed (15,37), and thus it is unlikely to produce a measurable impact on conclusions regarding MHC transitions during postnatal development. To document any relative change in MHC 2B mRNA expression across developmental time points, Northern blot analyses of MHC 2B mRNA were conducted, as previously described (13).…”

“…However, during postnatal development of the rat Dia m , MHC content (measured by MHC mass per half-sarcomere volume and fiber cross-sectional area) increases markedly (15). This increase varies across fiber types (classified based on MHC isoform expression).…”

“…In contrast, MHC 2X and MHC 2B protein isoforms are not expressed in the rat Dia m until at least the second postnatal week. Postnatal development of the rat Dia m is also characterized by changes in muscle contractile properties and by significant growth of muscle fibers, most notably those expressing MHC 2X and MHC 2B (15,22,36,38,42,43,45,47). Thus the relative MHC isoform composition changes during postnatal development of the Dia m , and ultimately it determines Dia m function.…”

Changes in actomyosin ATP consumption rate in rat diaphragm muscle fibers during postnatal development

“…This may explain, at least in part, the slower maximum shortening velocity of the rat Dia m during early postnatal development, when MHC Neo and MHC Slow contribute predominantly to the MHC isoform composition of the muscle (16,27,29,30,35). The lower maximum specific force (force normalized for fiber cross-sectional area) and greater fatigue resistance of the neonatal Dia m may also be partially explained by the predominant expression of MHC Neo and MHC Slow isoforms (10,16,29,33,35).In previous studies (14, 27, 31), our laboratory employed a quantitative histochemical procedure to determine the maximum velocity of the actomyosin ATPase reaction (V max ATPase) in muscle fibers expressing different MHC isoforms. In the rat Dia m , we found that fibers expressing MHC 2X and MHC 2B isoforms display significantly higher V max ATPase compared with fibers expressing MHC Slow and MHC 2A isoforms.…”

“…In adult skeletal muscle, several MHC isoforms exist, and the singular expression of these MHC isoforms in muscle fibers forms the basis for histochemical classification of different fiber types (18,24,25,31). In the neonatal rat Dia m , a neonatal isoform of MHC (MHC Neo ) is predominantly expressed, most often in combination with MHC Slow or MHC 2A isoforms (10,16,18,30,33). During the first 3-4 postnatal weeks, a dramatic transition in MHC isoform expression occurs in the rat Dia m , with the complete disappearance of MHC Neo expression by ϳ28 days of age (D-28).…”