A modified plasma radioimmunoassay has been developed that uses a cellulose slurry to separate bound from free insulin. This material, which is commercially available, shelf stable, and simple to prepare, makes it possible to run the entire method in test tubes and to use a low specific activity insulin. Details concerning conditions and materials that determined sensitivity, accuracy, and precision are reported. The method is sensitive to about 10 μU/ml. of plasma and has a reproducibility of approximately ± 15 per cent. This method is proposed for use by those who desire to measure rises in plasma insulin by a simple, rapid and reasonably accurate technic.

Insulin concentration in each sample of dog urine, carotid artery plasma, or renal vein plasma was estimated following the infusion of bovine insulin-I-125, unlabeled bovine insulin, or glucose. Insulin was estimated by immunoassay and by TCA insoluble radioactivity. Whenever insulin-I-125 and unlabeled insulin were infused together both methods were used on each sample. In agreement with the conclusions of others, who have used less direct technics, it was found that the use of plasma TCA insoluble radioactivity to determine radio-insulin degradation in vivo results in an underestimate of insulin removal, and hence an overestimate of insulin half life. This finding was shown to apply not only to the whole animal but also to the kidney. All forms of insulin were removed by the kidney but the radioactive estimate of kidney insulin removal was less than that by immunoassay. Data from experiments in which radioactive insulin was bound to guinea pig anti-insulin serum prior to infusion and from experiments in which ureteral ligation was performed prior to infusion indicated that the kidney removed insulin primarily by glomerular filtration but that direct absorption also played a role.

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