Isolated islets of Langerhans from adult male rats were cultured in media containing either basal (1 gm./ml.) or high glucose (3 mg./ml.), and the media were changed every two days for fourteen days. The insulin released in each medium was measured by a back-titration radioimmunoassay. Insulin secretion was fairly constant in the basal glucose medium and a consistent three-to-sevenfold increase was observed in the alternated or continuous presence of high glucose. Addition of an excess of anti-insulin serum did not change significantly the basal or stimulated release of insulin. The electron microscopic appearance of the islets after fourteen days of incubation showed well-preserved islets both with or without antiinsulin serum. Anti-insulin serum did not alter the ultrastructural aspects of the islets in either basal or high glucose media.

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