ExtractThe secretion of insulin in vivo and its synthesis and release by pancreatic islets in vitro were studied in rats at various stages of development. An oral glucose load caused a rapid and significant increase in the serum insulin level of pregnant rats and a slower response in their 21-day-old fetuses. Circulating free fatty acids (FFA) decreased in the dams, but did not change in the fetuses. A relative glucose intolerance was found in pups 1 hr, 5 days, and 10 days after birth. This was accompanied by delayed insulin response and by little or no decrease of circulating FFA. After weaning, the glucose tolerance and the insulin and FFA responses gradually improved. The oral administration of a 10-amino acid mixture caused a greater insulin release in newborn and nursing rats than in 21-day-old fetuses or inyoung adult rats. Total serum lipids were higher in I-, 5-, lo-, and 20-day-old rats than in fetuses and adult animals, whether fed or fasted, but 18 hr of fasting decreased total lipids and phospholipids in all age groups. Thus, the age of relative hyperlipidemia corresponded with that of relative glucose intolerance. The secretion of insulin in vitro by pancreatic islets of 5-, lo-, 20-, 30-, and 45-day-old rats increased significantly when the concentration of glucose in the incubation medium was increased from 30 to 300 mg/ml. The islets of 30-and 45-day-old rats secreted the largest amount of insulin. The insulin content of pancreatic islets was higher and the percentage of insulin secreted in relation to the hormonal content was smaller in the islets of nursing rats. Islets of fetal, nursing, and adult rats incorporated L -[ 4 ,~-~~] l e u c i n e into proinsulin and insulin, but the conversion of proinsulin to insulin was minimal in the 1-day-old rat. This incorporation was greater when the glucose concentration of the medium or the incubation time were increased. Thus, birth appears to be associated with a temporary decrease in the ability to release insulin and to dispose of an exogenous glucose load. Speculation large masses of insulin-responsive tissues. Gluconeogenesis continues to be a major source of glucose throughout the nursing age, when the animal consumes a diet rich in protein and fat and low in carbohydrate. Although the fetal pancreas can synthesize insulin, the mechanism for its release and hence the ability to dispose of a glucose load do not reach full maturity until weaning, when the dietary intake of carbohydrate increases and the level of serum lipids decreases toward I the values characteristic of the adult animal. 1The synthesis and storage of insulin in the pancreatic B cells of the rat appear to start during the 1 l t h day of gestation (36, 59). However, the precise time at which the insulin-releasing mechanism begins to function has not been determined and may vary with the nature of the stimulus. Thus, the fetal pancreas, even though insensitive to glucose (4, SO), may respond to other substances (see Reference 37) so that, toward the end of pregnancy, the fetal serum ...

The lipid and fatty acid composition and the effect of epinephrine and of norepinephrine upon acetate-I-14 C incorparation into liver lipids were studied in the nurse shark. The total lipid content varied between 3.5 and 12.8% of the fresh weight of the organ, an amount significantly lower than that found in the liver of other elasmobranchs. The triglycerides represented about 60% and the phospholipids about 10% of the total extractable lipids, whereas in mammals, these fractions represent 30% and 65% of the total lipid, respectively. Of the total fatty acid content, over 40% was represented by C: 20 and C: 22 fatty acids; 36% was represented by C:18, C:20 and C:22 polyunsaturated fatty acids. Epinephrine and norepinephrine caused I significant decrease in the total uptake of acetate-I-14 C in the liver lipids, but their effeets on the different lipid fraotions were variable. The results are eonsistent with the nations that liver is an important organ for the storage of fat and that the catecholamines have a lipolytie role in elasmabraneh fishes.

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