Evaluation of A Muscle Damage Marker Titin N‐fragment in Human Skin Interstitial Fluid Using Intradermal Microdialysis

Abstract

Unfamiliar exercises and/or muscle contractions during intense exercise, especially eccentric exercise, may cause muscle damage. Consequently, muscle protein creatine kinase (CK) and myoglobin (Mb) leaks into the bloodstream and titin N‐fragment is subsequently excreted through urine. Currently, blood (e.g., CK and Mb) and urine (e.g., titin N‐fragment) markers are generally used to monitor muscle damage in individuals who engage in intense training. However, these muscle damage markers in blood and urine have several limitations: 1) they cannot identify local damaged area, 2) several days are required to assess muscle damage thus immediate feedback is not possible, 3) assessment of blood markers requires invasive techniques, which may increase the risk of infection and/or can be painful to some individuals. Therefore, developing a new muscle damage marker without the above limitations is practically important.Interstitial fluid exists between cells. Previous studies reported that glucose concentrations of the interstitial fluid correlate well with those in the blood. Based on the information, minimally invasive wearable devices that can continuously predict blood glucose level by measuring skin interstitial glucose concentrations have been developed and now are available. However, it remains to be determined if muscle damage markers such as CK and titin N‐fragment can be detectable from skin interstitial fluid. Given that CK and titin N‐fragment leak into interstitial fluid before entering lymphatic vessels and blood, concentrations of CK and titin N‐fragment may appear earlier in the skin interstitial fluid relative to blood and/or urine. Furthermore, muscle damage marker concentrations in interstitial fluid appear to be specifically elevated in the muscle‐damaged area. However, these possibilities have never been assessed in human skin interstitial fluid.To assess these possibilities, muscle damage markers in blood, urine, and skin interstitial fluid were assessed in 12 healthy young adults (5 women, age: 26 ± 3 years; height: 168 ± 10 cm; body mass: 64 ± 10 kg) who performed 30 maximal isokinetic (60°/s) eccentric elbow flexor contractions using an isokinetic dynamometer (the non‐dominant arm). Intradermal microdialysis probes were implanted near the brachialis muscle in both the exercising and non‐exercising arms (i.e., 60‐70 % distal to the length between the acromial process and epicondylus lateralis humeri). Urinary and dialysate titin N fragment concentrations were assessed at baseline and 3 hours, 1, 4, and 7 days after the exercise. Serum and dialysate CK activity, serum and dialysate Mb concentrations, MVC torque, ROM, and muscle soreness were also assessed.After the eccentric exercise, titin N‐fragment in urine significantly increased at 4 and 7 days postexercise. Similarly, titin N‐fragment in dialysate significantly increased at 4 and 7 days postexercise. However, titin N‐fragment in dialysate did not increase in the non‐exercising arm at any postexercise periods. We conclude that the skin interstitial fluid can be used to assess a muscle damage marker, titin‐N fragment, and may reflect the site where muscle damage occurs.