Assessment of transcapillary transport of fluorescently labeled insulin in mouse cremaster muscle

Experimental studies have suggested that the transcapillary exchange of insulin from blood to interstitium is a rate‐limiting step for insulin action in skeletal muscle, particularly in conditions of insulin resistance. However, direct information on the dynamics of insulin transport from blood to tissue in muscle is lacking at the moment. In the current study, we determined tissue accumulation of intravenously injected fluorescein isothiocyanate‐labeled insulin (FITC‐insulin; 2.5 μg) in cremaster muscle of anesthetized mice (n=5), prepared for intravital microscopy. Using a two‐photon laser scanning microscope (Nikon EF600N), microvascular fields containing 1–3 capillaries were selected with a 60x objective, and images collected every minute before and after infusion of FITC‐insulin into the femoral vein. FITC‐insulin was observed to rapidly accumulate in the tissue, as evidenced by an increase in fluorescence intensity in extravascular regions of the image within 1 min after tracer administration. After 5 min, extravascular FITC‐insulin intensity was 26 ± 9% of the intravascular intensity. These data suggest that under normal conditions transcapillary exchange of insulin from blood to muscle tissue occurs rapidly. Supported by Dutch Diabetes Research Foundation grant # 2006.00.027