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SUMMARY Various methods of tissue culture were studied in an attempt to grow human foetal pancreas under conditions favourable for insulin release. Simple dicing of pancreas was superior to plasma clot adhesion or collagenase digestion for the preparation of tissue for culture. The culture medium described by Kahri (1966) (containing 25% heated postnatal calf serum) was the most suitable of four tested for the study of insulin release from the tissue culture. In this medium insulin could be measured quantitatively by radioimmunoassay and insulin degradation occurred at the rate of 20–25%/24 h. In other media the pancreas grew less well or insulin degradation was much greater. Human foetal pancreas grown under optimal conditions released insulin for up to 34 days. Insulin released into the culture medium did not appear to inhibit the further release of insulin. In some experiments the total amount of insulin released into the culture medium was several-fold greater than that in the pancreas originally seeded. In acute incubation experiments barium and theophylline stimulated insulin release from pancreas cultures. It proved impossible to identify the cells from which insulin was released but they did not appear to be in the monolayer which was composed of fibroblasts.

Insulin release was studied in vitro using pieces of pancreas from rabbits of between 24 days gestational age and 6 weeks postnatal age. When allowance was made for the fraction of pancreas which was endocrine, 16-5mM-glucose caused increasing stimulation of insulin release as development advanced and 3-3 mM-glucose caused a similar rate of secretion at all ages. Secretion was not significantly influenced by insulin destruction in the incubation medium. Glucagon (5 mug/ml) did not stimulate insulin secretion from 24-day foetal pancreas but did so postnatally. Theophylline (1 mmol/1) stimulated insulin release at all ages and was equipotent on 24-day foetal pancreas in 3-3 or 16-5 mM-glucose. The stimulation of insulin release from 24-day foetal pancreas by 1 mM-theophylline occurred in the absence of extracellular glucose, pyruvate, fumarate and glutamate and in the presence of mannoheptulose and 2-deoxyglucose (each 3 mg/ml). Adrenaline (1 mumol/1) and diazoxide (250 mug/ml) abolished or attenuated the stimulation of insulin release by glucose, leucine plus arginine or theophylline from 24-day foetal, 1 day and 6 weeks postnatal pancreas. The stimulation of insulin release from 6-week-old pancreas by 1mM-barium was blocked by adrenaline and diazoxide but the effect became less with increasing immaturity. The experimental results illustrate some of the ways in which insulin secretion by the rabbit beta cell changes as a function of development and draw attention to the importance of glucose and cyclic adenosine monophosphate in this process.