[123I]Insulin metabolism in normal rats and humans: external detection by a scintillation camera.

[123I]Insulin was injected intravenously into rats and the distribution and kinetics of radioactivity were analyzed by external detection with a scintillation camera connected to a computer. When injected alone, [123I]insulin was rapidly taken up by the liver and to a smaller extent by the kidneys. After reaching a maximum at 3 to 5 minutes after injection, liver radioactivity rapidly declined and free iodide appeared in the plasma. After previous saturation of the insulin receptor compartment, [123I]insulin was concentrated by the kidneys only and the rate of appearance of free iodide was markedly decreased. The results demonstrate the potential usefulness of this noninvasive technique to visualize insulin interaction with the liver and kidneys and to study the rate of insulin degradation by each organ in vivo. Preliminary experiments in man demonstrate its feasibility and low radiotoxicity.