Muscle Fiber Composition and Capillary Density in Turner Syndrome
Evidence of increased muscle fiber size related to insulin resistance
- Claus Højbjerg Gravholt1,
- Birgit Nyholm1,
- Bengt Saltin2,
- Ole Schmitz1 and
- Jens Sandahl Christiansen1
- 1Medical Department M (Endocrinology and Diabetes) and Medical Research Laboratories, Åarhus Kommunehospital, Aarhus University Hospital, Aarhus
- 2Copenhagen Muscle Research Center, Rigshospitalet, Copenhagen, Denmark
Abstract
OBJECTIVE—To assess muscle fiber composition and capillary density in Turner syndrome, a condition linked with insulin resistance and increased frequency of type 2 diabetes, and link these findings with insulin sensitivity and physical fitness.
RESEARCH DESIGN AND METHODS—A total of 10 patients with Turner syndrome who were off hormone replacement therapy (aged 32.7 ± 8.9 years) and a control group of 14 normal women (aged 35.6 ± 9.3 years) were studied. None of the participants had diabetes or any family history of type 2 diabetes. An oral glucose tolerance test was performed, and insulin sensitivity was assessed by homeostasis model assessment (HOMA) and a composite whole-body insulin sensitivity index (ISIcomp). Physical fitness was assessed, and a muscle biopsy was obtained.
RESULTS—Women with Turner syndrome were insulin resistant, as seen by a lower ISIcomp (P = 0.003) and increased glucose (P < 0.0005) and insulin (P = 0.01) levels at 120 min. Impaired glucose tolerance was present in most Turner syndrome patients (6 of 10), but not in the control subjects. Women with Turner syndrome had an increased size of type IIa fibers (P = 0.01), whereas the size of their type I and IIa fibers were comparable with the control group. The groups did not differ in percentage of type I, Iia, or IIx fibers, and there was no difference in the capillary density. Significant correlations were found among ISIcomp, the HOMA index (RHOMA), and the mean area of type IIa fibers (ISIcomp: r = −0.632, P = 0.002; RHOMA: r = 0.570, P = 0.006). Furthermore, capillaries/type IIa fibers correlated significantly with ISIcomp (r = −0.618, P = 0.01). There were no significant correlations between Vo2max and muscle fiber composition.
CONCLUSIONS—Healthy women with Turner syndrome are characterized by impaired glucose tolerance, insulin resistance, low physical capacity, and enlarged type IIa muscle fibers, indicating diminished oxygen and substrate supply for metabolic processes. These findings could be indicative of a prediabetic state.
- CV, coefficient of variation
- FFA, free fatty acid
- HOMA, homeostasis model assessment
- HRT, hormone replacement therapy
- ISIcomp, insulin sensitivity index (composite)
- LBM, lean body mass
- OGTT, oral glucose tolerance test
- RHOMA, HOMA index
Footnotes
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Address correspondence and reprint requests to Claus Højbjerg Gravholt, Medical Department M (Endocrinology and Diabetes), Århus Kommunehospital, DK-8000 Aarhus C, Denmark. E-mail: ch.gravholt{at}dadlnet.dk.
Received for publication 18 January 2001 and accepted in revised form 31 May 2001.
J.S.C. has received honoraria from Novo Nordisk, Pharmacia, and Eli Lilly.
A table elsewhere in this issue shows conventional and Système International (SI) units and conversion factors for many substances.
