The circulating half-life of injected insulin-I-125, and the uptake of this radioactive hormone by various tissues, have been compared in the normal and insulin-immune rabbit. In the normal animal the injected radioactive insulin is carried by albumin or an alpha-globulin, while in the immune animal it is carried in complex with antibody. The circulating half-life appears to have a fast and a slow component. In the normal rabbit the fast component was about 4.8 minutes and the slow component about forty-eight minutes, while in the immune animal the former was ten minutes and the latter about ninety minutes.

In the normal rabbit only about 17 per cent of the injected radioactive insulin remained as TCA-precipitable radioactivity (intact insulin) in blood and tissues one hour after injection, while in the immune animal about 75 per cent remained in this form at the end of one hour. It would appear, however, that some biologic effect was derived from the injection of radioactive insulin into immune animals, since some hypoglycemia occurred.

In both normal and immune animals most of the TCAprecipitable radioactivity one hour after injection of radioactive insulin was retained in serum, muscle, kidney and liver. However, the proportion of intact insulin was seventeen times greater in serum, 50 per cent greater in the heart, about 30 per cent greater in liver and muscle, about 16 per cent greater in lung and about the same in kidney of immune as compared with normal rabbits.

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