TISSUE UPTAKE OF INSULIN AND INULIN IN RED AND WHITE SKELETAL-MUSCLE INVIVO

By employing a tissue uptake technique, the kinetics of the plasma-interstitial equilibration of radiolabeled insulin, inulin, and albumin were followed in four muscles of differing capillarity in anesthetized rats. The soleus muscle (SOL), and the red portion of the gastrocnemius muscle (RG), as well as the extensor digitorum longus muscle (EDL) and the white portion of the gastrocnemius muscle (WG) were investigated. After constant intravenous tracer infusions and repeated plasma sampling under euglycemic clamp conditions, animals were killed at varying time intervals and the muscles mentioned above were dissected out. The radioactivity of tracer per gram of tissue in each muscle divided by the plasma activity of tracer per milliliter of plasma, i.e., 'the plasma equivalent space' of tracer, thus could be followed as a function of time. From this function the permeability-surface area (PS) of inulin as well as the distribution volumes at time 0 (V0) of inulin and insulin and their equilibrium distribution volumes (V(E)) were assessed. The PS for inulin (in ml.min-1.100 g muscle-1) was 0.52 +/- 0. 10 (means +/- SE) in WG, increasing with more red fibers to 1.37 +/- 0.18 in SOL. Also the inulin interstitial distribution volume (at blood-tissue tracer equilibrium; V(E)) increased in this order (in ml/100 g)from 7.30 +/- 0.91 in WG to 12.93 +/- 0.89 in SOL. The V0 for insulin was found to be approximately fivefold larger than the plasma volume in each muscle sample, indicating a high degree of binding of insulin to structures within the vascular compartment, conceivably to the vascular endothelium. In conclusion, the uptake of insulin into muscle tissue, similar to that of inulin, appears to be correlated positively with the degree of tissue capillarization.