Monitoring Total And Free IGF-I In Serum Vs. Transdermal Body Fluid Biocompartments

Abstract

PURPOSE: Examination of hormones in transdermal body fluid (TBF) instead of blood may provide more physiologically relevant information due to closer proximity to tissue and cells. No previous study has determined whether total and free insulin-like growth factor-I (IGF-I) in TBF and serum differ during rest and exercise. METHODS: 10 healthy men (mean±SD, 23±3 years old, 178±3 cm tall, 79±7 kg) with a VO2 peak of 44±3 mL/kg/min had blood obtained by venipuncture and TBF obtained using a patented and proprietary methodology that uses minimally-invasive vacuum pressure in combination with a laser microporation process in the stratum corneum during an exercise and a control condition (i.e. laying supine quietly). Exercise consisted of 60 minutes of cycling at 60% VO2 peak. Post-exercise time points 40, 105, and 225 minutes were pooled for both TBF and serum samples allowing for adequate volumes for analysis. Total and free IGF-I were measured using immuno assays. Repeated-measures ANOVA (biocompartment x condition x time) was used for statistical analysis. Data were log transformed to ensure normal distribution. RESULTS: Total IGF-I concentration in TBF was significantly lower than serum pre-exercise (serum, 483±142 ng/mL; TBF, 100±30 ng/mL), and post-exercise (serum, 432±105 ng/mL; TBF, 127±67 ng/mL). Total IGF-I in TBF significantly increased from baseline in the control condition (pre, 107±39 ng/mL; pool, 198± 91 ng/mL), but not the exercise condition (pre, 100± 30 ng/mL; pool, 127± 67 ng/mL). Free IGF-I concentrations were not significantly different between serum and TBF pre-exercise (serum, 2.4±1.7 ng/mL; TBF, 2.0±.9 ng/mL), but were ∼12% greater in TBF post-exercise (serum, 2.5±2.1 ng/mL; TBF, 2.8±.9 ng/mL). Proportion of free to total IGF-I in TBF increased ∼29% post-exercise, while serum levels remained unchanged. CONCLUSION: This study demonstrates the utility of a minimally invasive method for monitoring total and free IGF-I in TBF during exercise recovery. Our data confirm the working hypothesis that measurement of IGF-I in various biocompartments yield different information. The higher post-exercise free IGF-I in TBF vs. the circulation does suggest that the biologically active form of IGF-I is more proximal to cells/tissues and readily available to mediate exercise-induced responses.