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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, 16·5 mM-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 µg/ml) did not stimulate insulin secretion from 24-day foetal pancreas but did so postnatally. Theophylline (1 mmol/l) 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 µmol/l) and diazoxide (250 µg/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 1 mM-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 β cell changes as a function of development and draw attention to the importance of glucose and cyclic adenosine monophosphate in this process.