The dose-response characteristics of three skeletal muscles, three adipose tissue beds, and heart muscle to single i.v. injection of insulin were compared in vivo. Comparisons were made at 8 dose levels spanning the entire range for response by all tissues and for the integrated whole body response as reflected in the rate of disappearance of 3H-2-deoxyglucose from plasma. The insulin-sensitive tissues varied widely with respect to the magnitude of the maximal response and the sensitivity to insulin as judged by the effective dose 50% (ED 50). Among the muscles, a slow-twitch oxidative muscle, soleus, was more sensitive than the fast-twitch glycolytic muscle, extensor digitorum longus (EDL), while a mixed muscle, quadriceps femoris, displayed even lower sensitivity. Heart muscle sensitivity was comparable to EDL. Among the adipose sites, the rank order of sensitivity was subcutaneous > epididymal ≫ omental. The threshold for a detectable response to insulin was 0.013 U/kg rat.

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