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.Recently, we demonstrated the presence of insulint in the brain in concentrations higher than in plasma (1). This brain insulin was indistinguishable from genuine insulin by multiple criteria, and its tissue concentration did not vary at all with extreme changes in plasma insulin (2). In the present study we show that insulin immunoreactivity is present in essentially all tissues of humans and rats as well as in cultured lymphocytes and fibroblasts at concentrations that are 2-100 times those present in the plasma or culture medium. The material is similar to (or identical with) genuine insulin by radioimmunoassay, gel filtration, bioassay, and antibody neutralization. The concentration of cellular insulin in vio changes little or not at all in response to extremes of hyperinsulinemia and hypoinsulinemia.
MATERIALS AND METHODSTissue Preparations. Male Sprague-Dawley rats (250-300 g) fed ad lib were decapitated between 1400 and 1600 hr; "fasted rats" had been denied food for 50-72 hr before they were killed. Rats injected with streptozotocin (65 mg/kg intravenously) and control rats (injected with isotonic saline) were sacrificed 1 month after treatment. Obese (ob/ob) mice and their thin littermates (mixtures of ob/+ and +/+) of the C57BL65 strain were killed at 8-10 weeks of age. Organs and plasma were collected and stored as reported (1, 2).Human tissue samples, obtained either during operations or at autopsy within 3 hr after death, were frozen on dry ice prior to extraction. Whole blood (500 ml) from a fasting (overnight) normal human volunteer was separated into mononuclear and granulocytic fractions by a modification (3) of the method of The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact. 572Boyum (4). The cells were washed in phosphate-buffered salin...