Relationship Between Insulin Concentrations in Plasma and Pancreas of Foetal and Weanling Rats

Abstract

During the third part of pregnancy in the rat the concentrations of plasma insulin and tissue glycogen in the foetus increase progressively. These levels and the release of insulin by pancreas incubated in vitro were significantly higher than the values found in adult non-pregnant rats. After birth the correlation between plasma insulin concentration and the stores of glycogen was also evident. In the first day of life, the concentrations of plasma insulin, glycogen in liver, striated muscle and kidney decreased significantly; these values decrease even more during the first 15 days of lactation. After weaning started (20 days post partum), rapid increases of insulin and glycogen were observed, parallel to the slow growth of the newborn rat during the first 15 days of life and the more rapid rate of growth after 20 days. Similarly, the insulin content of the pancreas increased more significantly during the period in which pancreatic weight and plasma insulin concentrations increased more slowly. These results show that when plasma insulin concentrations increase, body growth and stores of glycogen are higher, suggesting an anabolic role of insulin in the foetal and newborn rat. 15, 30, 45, 60 and 90 min.The incubation medium, Krebs-Ringer bicarbonate buffer, was supplemented with glucose (0-6 mg/ml), gelatine (0-6 mg/ml), sodium fumarate, sodium glutamate and sodium pyruvate (5 mM). The incubations were done in a Dubnoff metabolic shaker at 37°C in a mixture of 02 and C02 (95 % : 5 %).

The Synthesis and Release of Insulin in Fetal, Nursing and Young Adult Rats: Studies in Vivo and in Vitro

“…The relatively high rate of biosynthesis, accompanied by a relatively low pancreatic insulin content and by relatively high levels of circulating hormone, characteristic of the rat fetus, are consistent with the observations of other workers (5,13,63) and suggest that the insulin turnover is faster in the fetus than in the suckling animal. On the other hand, if one calculates the ratio of the amount of insulin secreted by pancreatic islets isolated from animals of different ages and incubated in buffer containing glucose at concentrations of 30 and 300 mgjlOO ml, one observes (Table 1) a rather symmetrical developmental pattern which indicates that relatively more insulin is secreted at low glucose concentration at an age when glucose tolerance is impaired.…”

“…During this period, the islets release relatively little insulin and, as noted also by other workers, the fasting serum IRI level is low (2,13,14,26,30) and so is the insulinogenic response (49,5 5,64).…”

“…Indeed, in the adult (9,10,52,68). as well as in the weaning (7,8,13,14,17,31,46,56,71,72) rat, a high fat diet increases serum lipids and decreases glucose tolerance, hepatic glucokinase activity, insulin secretion, and insulin sensitivity, whereas hyperlipemia and relative insulin resistance are characteristic of conditions as different as pregnancy, fasting, diabetes, and obesity (29,43,62). Thus, the availability of lipids in the newborn rat in which gluconeogenesis is the major source of glucose, may have a glucose-and, hence, a protein-sparing effect.…”

“…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 insulin (IRI) reaches values that are higher than those found in the dam (2,13,26,42,63). A significant decrease in fetal serum IRI occurs on the last day of intrauterine life (14), at the time when the level of glucagon and, consequently, the glucagon to insulin ratio, increase markedly (14, 15, 3 l ) , possibly explaining why the mammalian neonate does not tolerate a glucose load as well as the adult animal (49,64).…”

Insulin Secretion and Metabolism during the Perinatal Period in the Rat

“…By contrast, if the feto-placental unit was removed from the dam, maintaining fetal INTRODUCTION Fetal hyperinsulinism has been documented during the last days of gestation in the rat. After birth, plasma immunoreactive insulin (IRI)' concentration decreases towards values similar to those found in adult rats (1)(2)(3). Little is known about the mechanism whereby hyperinsulinemia is brought about at the end of gestation and whereby it subsides after birth.…”