Contractile function and myonuclear organization in single fibers from monozygotic female twins discordant for hormone replacement therapy

• Ageing is associated with a decline in muscle mass and strength leading to increased physical dependency in old age. Post-menopausal women experience a greater decline than men of similar age due to a dramatic decrease in sex hormones production. We recruited six monozygous female twin pairs (55 – 59 years old) discordant for postmenopausal hormone replacement therapy (HRT use = 7.8 ± 4.3 years) to investigate the association of HRT with the cytoplasmic domain supported by individual myonuclei (myonuclear domain size, MND) together with specific force at the single fiber level. MyHC isoform content of the fibers was determined using silver-stained SDS-PAGE. HRT use was associated with a significantly smaller (~27%; p < 0.05) mean MND size in muscle fibers expressing the type I but not the IIa MyHC isoform. An increase in specific force was recorded in the HRT user group both in muscle fibers expressing type I (~27%; p < 0.05) and type IIa (~23%; p < 0.05) MyHC isoforms. These positive effects on specific force were fiber-type dependent, i.e., in fast-twitch muscle fibers the increased specific force was primarily caused by an increased force per cross-bridge while slow-twitch fibers relied on both an increase in both number and force per cross-bridge. HRT use had no effect on fiber cross-sectional area (CSA), velocity of unloaded shortening (V0) and relative proportion of MyHC isoforms. In conclusion, HRT has significant positive effects on both regulation of muscle contraction and myonuclei organization in menopausal women, but the response is fiber-type specific.

Ämnesord

MEDICIN OCH HÄLSOVETENSKAP  -- Medicinska och farmaceutiska grundvetenskaper -- Fysiologi (hsv//swe)

MEDICAL AND HEALTH SCIENCES  -- Basic Medicine -- Physiology (hsv//eng)

Nyckelord

Skinned fibers

contractility

myonuclear domain size

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