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Insulin concentrations in acid/ethanol extracts of the whole rat brain were on the average 25 times higher than plasma insulin levels. Brain insulin was indistinguishable from authentic pancreatic insulin, based on its behavior in radioimmunoassay, radioreceptor assay, and bioassay and its chromatographic pattern on Sephadex G-50 column chromatography. Insulin was found in all regions of the brain examined, but distribution was uneven. Some regions had insulin concentrations as much as 100 times higher than in plasma; levels at least 10 times higher were found in other regions. The role of insulin in the central nervous system is not clear at present but, because both insulin and insulin receptors are abundant in the central nervous system, an extensive physiological regulation of the central nervous system by insulin is proposed.

Insulin has been detected, at levels higher than those in plasma, in a broad range of extrapancreatic tissues in both rats and humans. Rat liver insulin was shown to be indistinguishable from genuine insulin by radioimmunoassay, Sephadex chromatography, bioassay, and antibody neutralization. Liver insulin (like brain insulin) was unchanged in ob/ob mice, in rats treated with streptozotocin, or in fasted rats, despite marked alterations in pancreatic secretion of insulin and in liver content of insulin receptors. Insulin was found in cultured human IM-9 lymphocytes and cultured fibroblasts at concentrations greater than 100 times the levels in the media. IM-9 lymphocyte insulin also was shown to be indistinguishable from genuine insulin, by the same criteria used for liver insulin. The insulin concentration in cultured human cells was unaffected by depletion of insulin from the culture medium or by addition of beef insulin to the medium. The data suggest that a part, if not all, of the extrapancreatic tissue insulin is independent of plasma insulin and may be synthesized by the tissues themselves.