To investigate possible differences in the distribution and metabolism of insulin in diabetics and normals, insulin labeled with radioactive iodine (I131) was given intravenously to 40 diabetics of varying type and severity, 15 healthy controls, and 11 other nondiabetics with various illnesses. The labeled insulin was given in tracer amounts (50 to 100 microcuries) and test doses varied between 0.5 and 5.0 units of insulin. The dose was diluted in normal physiologic saline solution and injected slowly into fasting subjects over a two-minute period. In most cases therapeutic insulin had been withheld from diabetics for 24 to 96 hours. Plasma samples were drawn at predetermined times, and the amount of protein-bound radioactivity was determined by precipitation with trichloroacetic acid (TCA).

Since labeled insulin is virtually completely precipitated with TCA, the protein-bound radioactivity was assumed to represent undegraded labeled insulin, whereas the TCA-soluble radioactivity was assumed to be comprised of the degradation products of the labeled hormone. Total urinary radioactivity, of which an average of 95 per cent is TCA-soluble, was also measured during the test period. In some subjects surface measurement of radioactivity over various organ areas, such as the thyroid, liver, kidney, and muscle masses, was made.

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