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Interstitial IGF-I in exercising skeletal muscle in women

¹ Pediatric Endocrinology Unit, Department of Woman and Child Health, Karolinska Institute, Astrid Lindgren Children’s Hospital, SE-171 76 Stockholm, Sweden, ² Department of Physiology and Pharmacology, Karolinska Institute, SE-171 77 Stockholm, Sweden, ³ Department of Laboratory Medicine, Karolinska Institute, SE-141 86 Huddinge, Sweden and ⁴ Department of Clinical Science, Intervention and Technology, Karolinska Institute, SE-141 86 Stockholm, Sweden

(Correspondence should be addressed to U Berg; Email: ulrika.berg{at}ki.se)

Objective: To study interstitial IGF-I concentrations in resting and exercising skeletal muscle in relation to the circulating components of the IGF–IGF binding protein (IGFBP) system.

Design and methods: Seven women performed endurance exercise with 1 leg (Ex-leg) for 1 h. The resting leg (Rest-leg) served as a control. IGF-I was determined in microdialysate (MD) and was compared with veno-arterial (v-a) concentrations of circulating IGF–IGFBP components.

Results: Median (range) basal MD-IGF-I was 0.87 (0.4–1.5) µg/l or 0.4 (0.2)% of total-IGF-I (t-IGF-I) determined in arterial serum and in the same concentration range as free dissociable IGF-I (f-IGF-I). Rest-leg MD-IGF-I decreased, reaching significance after exercise. Ex-leg MD-IGF-I was unchanged during exercise and declined after exercise at the level of significance (P = 0.05). There was a release of f-IGF-I from the Ex-leg into the circulation at the end of and shortly after exercise. A small but significant increase in circulating IGFBP-1 was detected at the end of exercise and IGFBP-1 increased further after exercise. Although interleukin-6 (IL-6) has been associated with IGFBP-3 proteolysis, the circulating molecular forms of IGFBP-3 remained unchanged in spite of an IL-6 release from the muscle compartment.

Conclusions: Circulating IGFBP-1 is related to interstitial IGF-I in resting muscle although the temporal relationship may not be simple. Further studies should explore the role of local release of IGF-I and its impact on IGF-I activity during contraction.

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