Protein-caloric food restriction affects insulin-like growth factor system in fetal Wistar rat.

We have previously shown that fetuses from protein-caloric undernourished pregnant rats (35% of control diet during the last week of pregnancy) at 21.5 d post coitum exhibit increased beta-cell mass. This alteration is correlated with increased insulinemia and total pancreatic insulin content, a pattern similar to that reported in infants of mild diabetic mothers. In this work, we investigated in undernourished fetuses: 1) whether availability of growth factors such as insulin, GH, and IGFs and their binding proteins (IGFBPs) could be implicated in this alteration, and 2) the beta-cell mitogenic response to IGFs in vitro. The results show that maternal undernutrition increases pancreatic IGF-I expression and islet IGF-I receptor content in undernourished fetuses, whereas hepatic IGF-I expression and serum IGF-I levels were decreased. No changes were observed in serum IGF-II, and its expression was diminished in undernourished pancreases and unchanged in the liver, compared with control fetuses. Serum levels and liver and pancreatic mRNA expression of IGFBP-1 were found to be normal in undernourished fetuses, whereas the serum concentration and abundance of IGFBP-2 mRNA in pancreas were increased. Finally, the beta-cell mitogenic response to IGFs in vitro was significantly increased in undernourished fetal islets, compared with controls. In conclusion, in undernourished fetuses the increased beta-cell mass can be related to the stimulation of replicative beta-cell response due to locally increased pancreatic IGF-I mRNA; this effect is perhaps potentiated or favored by the enhanced islet IGF-I receptor content and pancreatic IGFBP-2 gene expression.

Protein calorie restriction has opposite effects on glucose metabolism and insulin gene expression in fetal and adult rat endocrine pancreas.

We previously demonstrated that fetuses from undernourished pregnant rats exhibited increased beta-cell mass and hyperinsulinemia, whereas keeping food restriction until adult age caused reduced beta-cell mass, hypoinsulinemia, and decreased insulin secretion. Because these alterations can be related to insulin availability, we have now investigated early and long-term effects of protein calorie food restriction on insulin mRNA levels as well as the possible mechanisms that could modulate the endogenous insulin mRNA content. We used fetuses at 21.5 days of gestation proceeding from food-restricted rats during the last week of pregnancy and 70-day-old rats undernourished from day 14 of gestation until adult age and with respective controls. Insulin mRNA levels, glucose transporters, and total glycolysis and mitochondrial oxidative fluxes were evaluated. We additionally analyzed undernutrition effects on signals implicated in glucose-mediated insulin gene expression, especially pancreatic duodenal homeobox-1 (PDX-1), stress-activated protein kinase-2 (p38/SAPK2), and phosphatidylinositol 3-kinase. Undernourished fetuses showed increased insulin mRNA, oxidative glucose metabolism, and p38/SAPK2 levels, whereas undernutrition until adult age provoked a decrease in insulin gene expression, oxidative glucose metabolism, and PDX-1 levels. The results indicate that food restriction caused changes in insulin gene expression and content leading to alterations in glucose-stimulated insulin secretion. The molecular events, increased p38/SAPK2 levels in fetuses and decreased PDX-1 levels in adults, seem to be the responsible for the altered insulin mRNA expression. Moreover, because PDX-1 activation appears to be regulated by glucose-derived metabolite(s), the altered glucose oxidation caused by undernutrition could in some manner affect insulin mRNA expression.

Age-dependent adaptation of the liver thyroid status and recovery of serum levels and hepatic insulin-like growth factor-I expression in neonatal and adult diabetic rats.

The effect of treatment with thyroxine (T(4)) on the hepatic deiodinase (5'D-I) activity and triiodothyronine (T(3)) content and on insulin-like growth factor-I (IGF-I) secretion and mRNA hepatic expression were studied in neonatal and adult diabetic (D) rats and compared with 4 thyroidectomized (Tx) groups: neonatal and adult Tx rats treated or not with T(4). Serum T(3) and T(4) decreased by 92% in both Tx populations and by 80% to 70% in D adults according to the severity of diabetes: -70 mg/kg body weight (BW) (D(70)) or 50 mg/kg BW (D(50)) of streptozotocin (STZ) injected, whereas only a 30% to 33% decrease was found in D neonates. A similar decrease of liver 5'D-I activity and T(3) concentrations was found in neonatal and adult Tx rats, whereas a significant reduction in those parameters was observed only in adult diabetics, either D(70) or D(50), but not in D neonates. Serum levels and liver mRNA expression of IGF-I determined by ribonuclease protection assay, plasma and pituitary growth hormone (GH), plasma insulin, and glycemia were also measured in both D populations. A decrease in circulating IGF-I, previously reported for Tx adult rats, was also found in both D populations. T(4) treatment recovered IGF-I and liver T(3) in both Tx groups and D neonates, but not in D adults. These results show an age-dependent adaptation of the liver thyroid economy in diabetes, as hepatic 5'D-I does not respond to diabetes in neonates and IGF-I is insensitive to T(4) treatment in adult diabetics and suggest a positive correlation between hepatic T(3) content and IGF-I expression in conditions of diabetes and Tx.

Effects of early undernutrition on the brain insulin-like growth factor-I system.

Undernutrition reduces circulating concentrations of insulin-like growth factor (IGF)-I, but how it affects the brain IGF system, especially during development, is largely unknown. We have studied IGF-I, IGF-II, IGF receptor and IGF binding protein (BP)-2 mRNA expression in the hypothalamus, cerebellum and cerebral cortex of neonatal rats that were food restricted beginning on gestational day 16. One group was refed starting on postnatal day 14. Rats were killed on postnatal day 8 or 22. Undernutrition did not produce an overall reduction in brain weight at either age but, at 22 days, both the cerebellum and hypothalamus weighed significantly less. At 8 days, no change was detected in the central IGF axis in response to undernutrition. However, in 22-day-old undernourished rats, IGF-I and IGF receptor mRNA expression were increased in both the hypothalamus and cerebellum, while IGFBP-2 was decreased, but only in the hypothalamus. Refeeding had no effect on any of these parameters. These results suggest that the hypothalamus and cerebellum respond to malnutrition and the decrease in circulating IGF-I, a peptide fundamental for growth and development, by increasing the local production of both the growth factor and its receptor in attempt to maintain normal development.

Influence of hypothyroidism on circulating concentrations and liver expression of IGF-binding proteins mRNA from neonatal and adult rats.

The aim of this work was to study the influence of the endocrine balance between thyroid hormones, insulin and growth hormone (GH) on the regulation of insulin-like growth factor binding proteins (IGFBPs), complementing a study previously reported for insulin-like growth factors (IGFs) in similar populations. Serum concentrations of IGFBPs-1 to -3 were assayed by Western ligand blot and their mRNA expression in the liver assayed by RNase protection assay in the hypothyroid populations: thyroidectomized and mercapto-1-methylimidazole (MMI)-treated neonates, and thyroidectomized adult rats at different periods after thyroidectomy. Serum concentrations of insulin, GH and IGF-I were increased in thyroidectomized neonates and decreased in the other populations. IGFBPs-1 and -2 increased 79% and 50% respectively in thyroidectomized neonatal rats compared with control at 15 days after thyroidectomy, whereas only IGFBP-2 increased (87%) in MMI-treated neonates, which had low serum insulin and GH compared with control on the same days. In thyroidectomized adult rats, IGFBPs-1 and -2 decreased 60% compared with controls on all days studied. Furthermore, when streptozotocin was administered to thyroidectomized neonates and insulin was given to thyroidectomized adult rats to restore insulin to control values in both groups, a differential regulation was found for IGFBPs-1 and -2. The transcriptionally induced decrease in IGFBP-3 (20-25% compared with control in neonates and 50% in adult rats), however, seemed to be regulated by GH and IGF-I. The similarity of changes in IGFBPs found in hypothyroid, undernourished and streptozotocin-induced diabetic neonatal rats suggests that the regulatory effect of insulin or GH on the IGFBPs requires the reduced biologically active thyroid hormone that is found in these three populations.

Regulation of IGF-I and -II by insulin in primary cultures of fetal rat hepatocytes.

During perinatal development, insulin and nutrients, rather than GH, regulate the IGF system. A selective primary culture of fetal rat hepatocytes has been established in our laboratory to elucidate the molecular mechanism of action of the above regulatory factors on IGF-I and -II gene expression during the late fetal period of the rat. In this model we have previously reported a regulatory role for glucose on IGF-I and -II synthesis and secretion. In the same experimental model, we now report that doses of insulin (0.1-5 microM) within the physiological range in rat fetuses during the last stages of gestation evoke an increase of IGF-I and -II mRNA abundance. Insulin regulated in a parallel manner IGF peptide secretion, and an excellent correlation was observed between IGF-I and -II mRNA and IGF-I and -II peptide levels in the conditioned media in response to the hormone. Finally, the insulin-induced rise in IGF-I and -II mRNA was not mediated by stimulation of gene transcription but by increased transcript stability. The results support the hypothesis that insulin plays a major role in IGF regulation at immature stages of development.

Different role of insulin in GLUT-1 and -4 regulation in heart and skeletal muscle during perinatal hypothyroidism.

Two groups of hypothyroid rats were used; one group was given 2-mercapto-1-methylimidazole (MMI) treatment in the drinking water of the mothers and was killed at 2 and 4 days of life, and the other group was given similar MMI treatment and then was thyroidectomized at 5 days of life and killed at 8 or 20 days. Serum insulin, growth hormone (GH), and insulin-like growth factor I (IGF-I) were decreased in MMI-treated rats but increased in MMI-treated plus thyroidectomized rats. No significant reduction of thyroid hormones was observed in 2-day-old MMI rats. Protein and mRNA expression of GLUT-1 increased, and those of GLUT-4 decreased, in the heart in all populations independent of changes in insulin, GH, and IGF-I levels. However, GLUT-4 protein and mRNA expression in quadriceps and gastrocnemius skeletal muscles decreased at 4 days and increased at 8 and 20 days of life in parallel with insulin, GH, and IGF-I levels. GLUT-1 in the skeletal muscles seemed regulated posttranscriptionally and presented a decrease of mRNA expression in all stages studied. A differential sensitivity to insulin regulation of GLUT-1 and GLUT-4 glucose transporters seems to be one of the causes for the tissue-specific regulation of these glucose transporters in heart and skeletal muscles during the perinatal period.

Glucose uptake and glucose transporter proteins in skeletal muscle from undernourished rats.

Undernutrition in rats impairs secretion of insulin but maintains glucose normotolerance, because muscle tissue presents an increased insulin-induced glucose uptake. We studied glucose transporters in gastrocnemius muscles from food-restricted and control anesthetized rats under basal and euglycemic hyperinsulinemic conditions. Muscle membranes were prepared by subcellular fractionation in sucrose gradients. Insulin-induced glucose uptake, estimated by a 2-deoxyglucose technique, was increased 4- and 12-fold in control and food-restricted rats, respectively. Muscle insulin receptor was increased, but phosphotyrosine-associated phosphatidylinositol 3-kinase activity stimulated by insulin was lower in undernourished rats, whereas insulin receptor substrate-1 content remained unaltered. The main glucose transporter in the muscle, GLUT-4, was severely reduced albeit more efficiently translocated in response to insulin in food-deprived rats. GLUT-1, GLUT-3, and GLUT-5, minor isoforms in skeletal muscle, were found increased in food-deprived rats. The rise in these minor glucose carriers, as well as the improvement in GLUT-4 recruitment, is probably insufficient to account for the insulin-induced increase in the uptake of glucose in undernourished rats, thereby suggesting possible changes in other steps required for glucose metabolism.

Effect of thyroxine administration on the IGF/IGF binding protein system in neonatal and adult thyroidectomized rats.

The effects of different doses of thyroxine (T(4)) delivered by injection or s.c. pellet implantation on alterations of the IGF/IGF binding protein (IGFBP) system were studied in neonatal and adult thyroidectomized (Tx) rats. Body weight, blood glucose, plasma insulin, TSH and GH and pituitary GH content, as well as serum IGF-I, IGF-II, IGFBP-1, -2 and -3 and their liver mRNA expression were assayed. Pellet implantation with the smaller dose of T(4) (1.5 microg/100 g body weight (b.w.) per day) in Tx neonatal rats decreased serum IGF-I, -II and the 30 kDa complex of IGFBPs (IGFBP-1 and -2), and increased serum IGFBP-3. Only the larger dose of T(4) (3 microg/100 g b.w. per day) recovered liver mRNA expression of IGF-I and ensured euthyroid status as shown by the normalized levels of plasma TSH. The rapid increase of body weight and serum GH after T(4) administration indicated a high sensitivity to T(4) during the neonatal period. Serum and liver mRNA expression of IGFs and plasma insulin and GH recovered in adult Tx rats after pellet implantation of 1.75 microg/100 g b.w. per day throughout 10 days. The continuous replacement of T(4) by pellet seems to be the most suitable method for thyroid rehabilitation. A very good correlation was found between insulin and IGF-II in Tx neonates treated with T(4) but not between insulin and IGF-I in Tx adults. IGFBP-2 seems to be up-regulated by T(4) deprivation in neonatal and adult rats. Finally, a good correlation as well as a partial correlation were found between IGFs and thyroid hormones in both neonatal and adult Tx populations, suggesting a direct effect in vivo of T(4) on the hepatic secretion of IGFs, as previously suggested in vitro.

Influence of type II 5' deiodinase on TSH content in diabetic rats.

The influence of hypothalamic and pituitary type II 5'deiodinase (5'D-II) activities and T3 content on pituitary TSH content was investigated in streptozotocin (STZ)-induced diabetic rats (D). The results show, first, that hypothalamic and pituitary 5'D-II activities were lower in neonatal D rats versus control (C) rats, and the normal developmental pattern was altered. Secondly, when D and C rats were thyroidectomized (Tx) at 25 days of age (D+Tx, C+Tx), pituitary and hypothalamic 5'D-II activities increased ten days later in both populations vs. intact rats, but the percentage of increase was smaller in D+Tx than in C+Tx. The hypothalamic T3 to T4 ratios were also decreased in D+Tx animals (0.38) as compared to C+Tx rats (1.64). The hypothalamic T3 content was reduced by 30% in D as compared to C rats and by 80% in D+Tx as compared to C+Tx rats, showing a defect in hypothalamic T4 deiodination. Pituitary TSH content increased after Tx in D+Tx, but not in C+Tx. These results in diabetic rats indicate that the hypothalamic and pituitary 5'D-II activity and hypothalamic T3 content are affected by diabetes and play a role in the regulation of pituitary TSH content.

Effects of experimental diabetes on renal IGF/IGFBP system during neonatal period in the rat.

Changes in the renal synthesis and concentration of insulin-like growth factors (IGFs) and their serum-binding proteins (IGFBPs) reported in insulin-deficient diabetes mellitus may be implicated in the alterations of the kidney function and morphology accompanying this disease. Most research on this subject has been carried out in adult animals, as well as in peripubertal rats, when the regulation of the IGF system is fully dependent on serum growth hormone (GH). However, relevant differences in the regulatory pathways of IGFs between adult and neonatal periods have been described. To examine the response of the IGF/IGFBP system of neonatal kidney to diabetes, renal IGF-I and -II and IGFBP-1, -2, and -3 concentration and mRNA expression were determined in streptozotocin-induced diabetic rat neonates. Diabetic neonates exhibited a kidney weight-to-body weight ratio higher than that of control rats, together with decreased IGF-I and increased IGF-II renal concentration. Because kidney mRNA expression of both IGFs decreased, the elevated renal IGF-II might result from increased uptake from circulation. Insulin treatment recovered the altered IGFs to control values, indicating the insulin-dependent regulation of IGFs in the neonatal kidney. Elevated levels of the IGFBP-1 and -2 in the kidney of diabetic neonates did not result from changes in their kidney mRNA transcript expression, suggesting also a possible uptake from circulation.

Effect of early dietary restriction on insulin action and secretion in the GK rat, a spontaneous model of NIDDM.

The availability of the Goto-Kakisaki (GK) rat model of non-insulin-dependent diabetes mellitus prompted us to test the effect of a limited period of undernutrition in previously diabetic young rats on their insulin secretion and insulin action during adult age. Four-week-old female GK rats were either food restricted (35% restriction, 15% protein diet) or protein and energy restricted (35% restriction, 5% protein diet) for 4 wk. Food restriction in the young GK rat lowered weight gain but did not aggravate basal hyperglycemia or glucose intolerance, despite a decrease in basal plasma insulin level. Furthermore, the insulin-mediated glucose uptake by peripheral tissues in the GK rat was clearly improved. We also found that food restriction, when it is coupled to overt protein deficiency in the young GK rat, altered weight gain more severely and slightly decreased basal hyperglycemia but conversely aggravated glucose tolerance. Improvement of basal hyperglycemia was related to repression of basal hepatic glucose hyperproduction, despite profound attenuation of basal plasma insulin level. Deterioration of tolerance to glucose was related to severe blunting of the residual glucose-induced insulin secretion. It is, however, likely that the important enhancement of the insulin-mediated glucose uptake helped to limit the deterioration of glucose tolerance.

Regulation of insulin-like growth factor-I and -II by glucose in primary cultures of fetal rat hepatocytes.

A selective primary culture of fetal rat hepatocytes was established in our laboratory in order to elucidate the molecular mechanisms of action of different factors and conditions on insulin-like growth factor (IGF)-I and -II gene expression during the perinatal period of the rat. In this model we report that, in a serum-free condition and the presence of non-stimulatory doses of insulin, 5-20 mM glucose evoked an increase of IGF-I and -II mRNA abundance. Glucose regulated in a parallel manner IGF peptide secretion, and an excellent correlation was observed between IGF-I and -II mRNA and IGF-I and -II peptide levels in the conditioned media in response to the carbohydrate. The experiment with 2-deoxyglucose suggests that glucose 6-phosphate, but not its further metabolism, is necessary for the induction of IGF transcript abundance in cultured fetal hepatocytes. Finally, the glucose-induced rise in IGF-II mRNA, the main IGF in fetal stages, was mediated by stimulation of gene transcription and increased transcript stability. The results support the idea that IGFs belong to a family of genes that are positively regulated by glucose.

Liver mRNA expression of IGF-I and IGFBPs in adult undernourished diabetic rats.

To investigate the role played by factors other than GH, such as nutrients and insulin, on IGF-I secretion, adult male rats of 200 g.b.w. were food-restricted for 7 days and then made diabetic by streptozotocin administration (UD). Different groups of UD rats were submitted to the following four day treatments: left untreated (UD), refed (UD+R), treated with insulin (UD+I), or a combination of both refeeding and insulin (UD+R+I). Serum concentration of IGF-I and liver mRNA expression of IGF-I, IGF-binding proteins and GH receptor were measured. Insulin treatment alone partially recovered liver IGF-I and IGFBPs mRNA expression, while refeeding alone had no effect. Only a combination of both insulin and refeeding recovered both parameters. Contrary to the results obtained with a longer period of recovery, these experiments show that serum and mRNA expression of IGF-I and IGFBPs in adult undernourished diabetic rats can be restored by insulin and nutrients administration with no prior restoration of serum and pituitary GH to control values and no compensatory changes in GH receptor gene expression.

Mechanism of hypothyroidism action on insulin-like growth factor-I and -II from neonatal to adult rats: insulin mediates thyroid hormone effects in the neonatal period.

The effects of thyroid hormone deficiency on serum levels and liver messenger RNA (mRNA) expression of insulin-like growth factors (IGFs) were studied in neonatal (until 20 days of life), weaned (22-37 days), and adult (72-87 days) rats, short periods (5, 10, and 15 days) after thyroidectomy (T). Serum levels and liver mRNA expression of IGF-I, plasma and pituitary GH, plasma insulin, and glycemia were measured in all populations; and serum levels and liver mRNA expression of IGF-II were measured only in the neonatal populations. Surprisingly, plasma insulin and GH and serum and liver mRNA expression of IGF-I were found elevated in T neonatal rats, and they decreased in weaned and adult rats and in neonatal rats rendered hypothyroid by mercapto-1-methylimidazole (MMI) treatment (MMI-hypothyroid). T and MMI-treatment of neonatal rats disturbed the normal pattern of progressive decrease of IGF-II with age. A positive correlation between insulin and IGF-I and a poor correlation between GH and IGF-I were found in both hypothyroid neonates (T and MMI-hypothyroid). On the contrary, a positive correlation between GH and IGF-I and a poor correlation between insulin and IGF-I were found for control and T adult rats. Because plasma insulin and GH changed in the same direction in all groups, insulin secretion in T neonatal was suppressed by streptozotocin, and insulin was given to T adult rats. The combined results of these experiments support the idea that the effects of thyroid hormones on IGF-I secretion are age-dependent, and they are mediated mainly by insulin during the neonatal period and by GH during adulthood.

Contrasted impact of maternal rat food restriction on the fetal endocrine pancreas.

The effects of food restriction of the mother (65% restriction of ad libitum food intake) on fetal and maternal insulin secretion and islet function were studied at 21 days gestation in three different rat populations: 1) undernourished from 0-7 days gestation, 2) undernourished from 7-14 days gestation, and 3) undernourished from 14-21 days gestation. The body weights of mothers were decreased in groups 2 and 3 vs. those in control fed pregnant animals, and no changes in basal parameters were found in any group. A glucose tolerance test in mothers from group 3 showed a mild intolerance to glucose and a decreased islet insulin content, although islet stimulation in vitro with glucose alone or plus arginine showed a normal insulin secretory response. Body weight was decreased in fetuses from the three groups (P < 0.01), and pancreas weight was reduced only in group 3. Insulinemia was increased in groups 2 and 3, and pancreatic insulin content increased only in group 3. However, fetuses from mothers of group 3 showed increased islet insulin content, increased response of insulin in vitro to glucose or glucose plus arginine, and hypertrophy of beta-cell mass. These results indicate, first, that the development of the fetal pancreas depends on a balanced maternal glucose homeostasis and, second, that adaptive maternal changes to undernutrition seem to induce alterations on the fetal endocrine pancreas, especially when food restriction is applied during the last week of gestation.

Insulin secretion in adult rats that had experienced different underfeeding patterns during their development.

Four populations of 70-day-old undernourished rats were studied. All rats were submitted to a 65% restriction of food intake as follows: 1) undernutrition of the mother from day 14 of gestation until birth only (fetal period, group F), 2) undernourished during fetal and suckling periods only (group FS), 3) undernourished from day 14 of gestation until day 70 of life (group FSA), and 4) undernourished from day 28 of life until day 70 (group A). Body weight, plasma glucose, glucose tolerance, basal plasma insulin, in vivo glucose-induced insulin secretion, pancreatic insulin content, and in vitro response of isolated islets to glucose and amino acids were investigated. Islet insulin content decreased in all four groups studied, and islet mass was found reduced in group FSA. Glucose tolerance was decreased in group A, and in vitro response of islets decreased in both groups FSA and A. We conclude that two different adaptive processes should be considered: the possible long-term consequences of impaired pancreas development due to inadequate perinatal nutrition and the functional alteration of insulin secretion to undernutrition in adulthood.

In vivo insulin-dependent glucose uptake of specific tissues is decreased during aging of mature Wistar rats.

Aging has been associated with peripheral insulin resistance in both humans and rats. However, the specific tissues that become insensitive to insulin before glucose homeostasis is altered remain to be elucidated. In the present work we studied the glucose metabolic index of a number of tissues known to be insulin sensitive in 3- and 24-month-old Wistar rats by measuring 2-deoxy-D-[1-3H]glucose uptake both under euglycemic-hyperinsulinemic conditions and in the basal state. Analysis of the glucose infusion rate to maintain normoglycemia during the clamp confirmed that the old rats show overall insulin resistance at both saturating and subsaturating insulin concentrations. The maximal response of glucose uptake to insulin as well as insulin sensitivity in red and white quadriceps were unaltered in old rats. In contrast, glucose uptake by soleus and diaphragm was poorly stimulated in old animals, and a marked decrease in insulin sensitivity was observed in both tissues. In heart, only the sensitivity to the hormone, not the maximal response, was impaired in old rats. In white adipose tissue, no significant stimulation was detected. We conclude that during aging in Wistar rats and before fasting plasma insulin and glucose levels become altered, specific tissues develop insulin resistance, whereas other remain insulin sensitive. We postulate that fat tissue plays a qualitative important role in eliciting the insulin resistance in old animals. Due to the metabolic characteristics of the aged Wistar rat, the changes reported might reflect what occurs in nonobese elderly humans, nongenetically committed to develop type 2 diabetes.

Effects of refeeding of undernourished and insulin treatment of diabetic neonatal rats on IGF and IGFBP.

The effect of refeeding and insulin treatment of undernourished and diabetic neonatal rats, respectively, on the regulation of insulin-like growth factor (IGF) and insulin-like growth factor binding protein (IGFBP) was investigated. The changes in body weight, insulinemia, glycemia, serum IGF-I, and growth hormone (GH) as well as the increase of the 30-kDa IGFBP in undernourished and diabetic neonatal rats previously shown elsewhere were reversed by refeeding and insulin treatment, respectively. Also, changes in liver mRNA expression of IGF-I and-II and IGFBP-1 and -2 were restored in refed undernourished and IGF-I and IGFBP-1 levels recovered in insulin-treated diabetic rats. However, serum GH was still below normal after rehabilitation in both situations. Thus the present results support the idea of a GH-independent IGF/ IGFBP regulation mediated by a balance of insulin and nutrients as has already been suggested in previous neonatal studies.

Changes in insulin action and insulin secretion in the rat after dietary restriction early in life: influence of food restriction versus low-protein food restriction.

The effect of a limited period of undernutrition in young rats on insulin secretion and insulin action during adulthood has been studied. Four-week-old female rats were either food-restricted (35% restriction, 15% protein diet) or protein-calorie-restricted (35% restriction, 5% protein diet) for 4 weeks. Food-restricted rats gained weight at a lower rate than control rats. In the protein-calorie-restricted group, the alteration of weight gain was more severe. Basal plasma insulin was reduced only in protein-calorie-restricted rats. Glucose-stimulated insulin secretion (delta I) obtained in vivo after an intravenous glucose load was only moderately decreased in food-restricted group, whereas it was severely blunted in the protein-calorie-restricted group. In this last group, impairment of the insulin secretory response to glucose was related to an intrinsic impairment of beta-cell secretory capacity, since the insulin secretory response to glucose or arginine was decreased when tested in vitro (perfused pancreas). In food-restricted rats, basal plasma glucose level was kept normal, while a mild deterioration of glucose tolerance was detectable. This was related, of course, to the decrease of delta I as identified in vivo. However, data obtained under basal or euglycemic-hyperinsulinemic conditions provided direct evidence that insulin-mediated total glucose uptake (weight-related expression) was paradoxically enhanced. A similar conclusion was reached in protein-calorie-restricted rats; the increase of overall insulin-mediated glucose uptake was even more important. Such an adaptation, which was operating in both types of restriction, may help limit the deterioration of glucose tolerance in the face of impaired insulin release. In the basal postabsorptive state, the higher glucose utilization rate in both models originated from increased hepatic glucose production rates. During hyperinsulinemia, endogenous glucose production in food-restricted rats was normally blunted, but not in protein-calorie--restricted rats, thus indicating resistance of the hepatic glucose production pathway to insulin action in this group.