Maturation of Glucose-Stimulated Insulin Secretion in Fetal Sheep

Insulin and insulin-like growth factor-1 (IGF-1) are fetal hormones critical to establishing normal fetal growth. Experimentally elevated IGF-1 concentrations during late gestation increase fetal weight but lower fetal plasma insulin concentrations. We therefore hypothesized that infusion of an IGF-1 analog for one week into late gestation fetal sheep would attenuate fetal glucose-stimulated insulin secretion (GSIS) and insulin secretion in islets isolated from these fetuses. Late gestation fetal sheep received infusions with IGF-1 LR3 (IGF-1, n=8), an analogue of IGF-1 with low affinity for the IGF binding proteins and high affinity for the IGF-1 receptor, or vehicle control (CON, n=9). Fetal GSIS was measured with a hyperglycemic clamp (IGF-1, n=8; CON, n=7). Fetal islets were isolated, and insulin secretion was assayed in static incubations (IGF-1, n=8; CON, n=7). Plasma insulin and glucose concentrations in IGF-1 fetuses were lower compared to CON (P=0.0135 and P=0.0012, respectively). During the GSIS study, IGF-1 fetuses had lower insulin secretion compared to CON (P=0.0453). In vitro, glucose-stimulated insulin secretion remained lower in islets isolated from IGF-1 fetuses (P=0.0447). In summary, IGF-1 LR3 infusion for one week into fetal sheep lowers insulin concentrations and reduces fetal GSIS. Impaired insulin secretion persists in isolated fetal islets indicating an intrinsic islet defect in insulin release when exposed to IGF-1 LR3 infusion for one week. We speculate this alteration in the insulin/IGF-1 axis contributes to the long-term reduction in β-cell function in neonates born with elevated IGF-1 concentrations following pregnancies complicated by diabetes or other conditions associated with fetal overgrowth.

Download Full-text

Decreased nutrient-stimulated insulin secretion in chronically hypoglycemic late-gestation fetal sheep is due to an intrinsic islet defect

AJP Endocrinology and Metabolism ◽

10.1152/ajpendo.00643.2005 ◽

2006 ◽

Vol 291 (2) ◽

pp. E404-E411 ◽

Cited By ~ 25

Author(s):

Paul J. Rozance ◽

Sean W. Limesand ◽

William W. Hay

Keyword(s):

Insulin Secretion ◽

Late Gestation ◽

Human Infants ◽

Fetal Sheep ◽

Isolated Pancreatic Islets ◽

Acute Hypoglycemia ◽

Arterial Plasma ◽

Glucose Stimulated Insulin Secretion

We measured in vivo and in vitro nutrient-stimulated insulin secretion in late gestation fetal sheep to determine whether an intrinsic islet defect is responsible for decreased glucose-stimulated insulin secretion (GSIS) in response to chronic hypoglycemia. Control fetuses responded to both leucine and lysine infusions with increased arterial plasma insulin concentrations (average increase: 0.13 ± 0.05 ng/ml leucine; 0.99 ± 0.26 ng/ml lysine). In vivo lysine-stimulated insulin secretion was decreased by chronic (0.37 ± 0.18 ng/ml) and acute (0.27 ± 0.19 ng/ml) hypoglycemia. Leucine did not stimulate insulin secretion following acute hypoglycemia but was preserved with chronic hypoglycemia (0.12 ± 0.09 ng/ml). Isolated pancreatic islets from chronically hypoglycemic fetuses had normal insulin and DNA content but decreased fractional insulin release when stimulated with glucose, leucine, arginine, or lysine. Isolated islets from control fetuses responded to all nutrients. Therefore, chronic late gestation hypoglycemia causes defective in vitro nutrient-regulated insulin secretion that is at least partly responsible for diminished in vivo GSIS. Chronic hypoglycemia is a feature of human intrauterine growth restriction (IUGR) and might lead to an islet defect that is responsible for the decreased insulin secretion patterns seen in human IUGR fetuses and low-birth-weight human infants.

Download Full-text

Chronic Fetal Leucine Infusion Does Not Potentiate Glucose-Stimulated Insulin Secretion or Affect Pancreatic Islet Development in Late-Gestation Growth-Restricted Fetal Sheep

Journal of Nutrition ◽

10.1093/jn/nxaa357 ◽

2020 ◽

Author(s):

Brit H Boehmer ◽

Stephanie R Wesolowski ◽

Laura D Brown ◽

Paul J Rozance

Keyword(s):

Endothelial Cells ◽

Insulin Secretion ◽

Mrna Expression ◽

Pancreatic Islet ◽

Late Gestation ◽

Β Cells ◽

Fetal Sheep ◽

Islet Size ◽

Intrauterine Fetal Growth Restriction ◽

Glucose Stimulated Insulin Secretion

ABSTRACT Background Growth-restricted fetuses have attenuated glucose-stimulated insulin secretion (GSIS), smaller pancreatic islets, less pancreatic β-cells, and less pancreatic vascularization compared with normally growing fetuses. Infusion of leucine into normal late-gestation fetal sheep potentiates GSIS, as well as increases pancreatic islet size, the proportion of the pancreas and islet comprising β-cells, and pancreatic and islet vascularity. In addition, leucine stimulates hepatocyte growth factor (HGF ) mRNA expression in islet endothelial cells isolated from normal fetal sheep. Objective We hypothesized that a 9-d leucine infusion would potentiate GSIS and increase pancreatic islet size, β-cells, and vascularity in intrauterine fetal growth restriction (IUGR) fetal sheep. We also hypothesized that leucine would stimulate HGF mRNA in islet endothelial cells isolated from IUGR fetal sheep. Methods Late-gestation Columbia-Rambouillet IUGR fetal sheep (singleton or twin) underwent surgeries to place vascular sampling and infusion catheters. Fetuses were randomly allocated to receive a 9-d leucine infusion to achieve a 50–100% increase in leucine concentrations or a control saline infusion. GSIS was measured and pancreas tissue was processed for histologic analysis. Pancreatic islet endothelial cells were isolated from IUGR fetal sheep and incubated with supplemental leucine. Data were analyzed by mixed-models ANOVA; Student, Mann-Whitney, or a paired t test; or a test of equality of proportions. Results Chronic leucine infusion in IUGR fetuses did not affect GSIS, islet size, the proportion of the pancreas comprising β-cells, or pancreatic or pancreatic islet vascularity. In isolated islet endothelial cells from IUGR fetuses, HGF mRNA expression was not affected by supplemental leucine. Conclusions IUGR fetal sheep islets are not responsive to a 9-d leucine infusion with respect to insulin secretion or any histologic features measured. This is in contrast to the response in normally growing fetuses. These results are important when considering nutritional strategies to prevent the adverse islet and β-cell consequences in IUGR fetuses.

Download Full-text

Leucine acutely potentiates glucose-stimulated insulin secretion in fetal sheep

Journal of Endocrinology ◽

10.1530/joe-20-0243 ◽

2020 ◽

Vol 247 (1) ◽

pp. 115-126 ◽

Cited By ~ 1

Author(s):

Brit H Boehmer ◽

Peter R Baker ◽

Laura D Brown ◽

Stephanie R Wesolowski ◽

Paul J Rozance

Keyword(s):

Amino Acids ◽

Insulin Secretion ◽

Metabolic Pathways ◽

Structural Changes ◽

Leucine Supplementation ◽

Fetal Sheep ◽

Leucine Oxidation ◽

Glucose Stimulated Insulin Secretion ◽

Fetal Islets

A 9-day infusion of leucine into fetal sheep potentiates fetal glucose-stimulated insulin secretion (GSIS). However, there were accompanying pancreatic structural changes that included a larger proportion of β-cells and increased vascularity. Whether leucine can acutely potentiate fetal GSIS in vivo before these structural changes develop is unknown. The mechanisms by which leucine acutely potentiates GSIS in adult islets and insulin-secreting cell lines are well known. These mechanisms involve leucine metabolism, including leucine oxidation. However, it is not clear if leucine-stimulated metabolic pathways are active in fetal islets. We hypothesized that leucine would acutely potentiate GSIS in fetal sheep and that isolated fetal islets are capable of oxidizing leucine. We also hypothesized that leucine would stimulate other metabolic pathways associated with insulin secretion. In pregnant sheep we tested in vivo GSIS with and without an acute leucine infusion. In isolated fetal sheep islets, we measured leucine oxidation with a [1-14C] l-leucine tracer. We also measured concentrations of other amino acids, glucose, and analytes associated with cellular metabolism following incubation of fetal islets with leucine. In vivo, a leucine infusion resulted in glucose-stimulated insulin concentrations that were over 50% higher than controls (P < 0.05). Isolated fetal islets oxidized leucine. Leucine supplementation of isolated fetal islets also resulted in significant activation of metabolic pathways involving leucine and other amino acids. In summary, acute leucine supplementation potentiates fetal GSIS in vivo, likely through pathways related to the oxidation of leucine and catabolism of other amino acids.

Download Full-text

Islet adaptations in fetal sheep persist following chronic exposure to high norepinephrine

Journal of Endocrinology ◽

10.1530/joe-16-0445 ◽

2017 ◽

Vol 232 (2) ◽

pp. 285-295 ◽

Cited By ~ 13

Author(s):

Xiaochuan Chen ◽

Amy C Kelly ◽

Dustin T Yates ◽

Antoni R Macko ◽

Ronald M Lynch ◽

...

Keyword(s):

Insulin Secretion ◽

Pancreatic Islets ◽

Insulin Infusion ◽

Inhibitory Concentration ◽

Initial Assessment ◽

Fetal Sheep ◽

Fetal Plasma ◽

Intracellular Calcium Signaling ◽

Glucose Stimulated Insulin Secretion ◽

Lower Expression

Complications in pregnancy elevate fetal norepinephrine (NE) concentrations. Previous studies in NE-infused sheep fetuses revealed that sustained exposure to high NE resulted in lower expression of α2-adrenergic receptors in islets and increased insulin secretion responsiveness after acutely terminating the NE infusion. In this study, we determined if the compensatory increase in insulin secretion after chronic elevation of NE is independent of hyperglycemia in sheep fetuses and whether it is persistent in conjunction with islet desensitization to NE. After an initial assessment of glucose-stimulated insulin secretion (GSIS) at 129 ± 1 days of gestation, fetuses were continuously infused for seven days with NE and maintained at euglycemia with a maternal insulin infusion. Fetal GSIS studies were performed again on days 8 and 12. Adrenergic sensitivity was determined in pancreatic islets collected at day 12. NE infusion increased (P < 0.01) fetal plasma NE concentrations and lowered (P < 0.01) basal insulin concentrations compared to vehicle-infused controls. GSIS was 1.8-fold greater (P < 0.05) in NE-infused fetuses compared to controls at both one and five days after discontinuing the infusion. Glucose-potentiated arginine-induced insulin secretion was also enhanced (P < 0.01) in NE-infused fetuses. Maximum GSIS in islets isolated from NE-infused fetuses was 1.6-fold greater (P < 0.05) than controls, but islet insulin content and intracellular calcium signaling were not different between treatments. The half-maximal inhibitory concentration for NE was 2.6-fold greater (P < 0.05) in NE-infused islets compared to controls. These findings show that chronic NE exposure and not hyperglycemia produce persistent adaptations in pancreatic islets that augment β-cell responsiveness in part through decreased adrenergic sensitivity.

Download Full-text

Chronic anemic hypoxemia attenuates glucose-stimulated insulin secretion in fetal sheep

AJP Regulatory Integrative and Comparative Physiology ◽

10.1152/ajpregu.00484.2016 ◽

2017 ◽

Vol 312 (4) ◽

pp. R492-R500 ◽

Cited By ~ 14

Author(s):

Joshua S. Benjamin ◽

Christine B. Culpepper ◽

Laura D. Brown ◽

Stephanie R. Wesolowski ◽

Sonnet S. Jonker ◽

...

Keyword(s):

Insulin Secretion ◽

Cell Mass ◽

Plasma Norepinephrine ◽

Low Oxygen ◽

Fetal Sheep ◽

Islet Size ◽

Glucose Stimulated Insulin Secretion ◽

Fetal Islets ◽

The Relationship

Fetal insulin secretion is inhibited by acute hypoxemia. The relationship between prolonged hypoxemia and insulin secretion, however, is less well defined. To test the hypothesis that prolonged fetal hypoxemia impairs insulin secretion, studies were performed in sheep fetuses that were bled to anemic conditions for 9 ± 0 days (anemic, n = 19) and compared with control fetuses ( n = 15). Arterial hematocrit and oxygen content were 34% and 52% lower, respectively, in anemic vs. control fetuses ( P < 0.0001). Plasma glucose concentrations were 21% higher in the anemic group ( P < 0.05). Plasma norepinephrine and cortisol concentrations increased 70% in the anemic group ( P < 0.05). Glucose-, arginine-, and leucine-stimulated insulin secretion all were lower ( P < 0.05) in anemic fetuses. No differences in pancreatic islet size or β-cell mass were found. In vitro, isolated islets from anemic fetuses secreted insulin in response to glucose and leucine as well as control fetal islets. These findings indicate a functional islet defect in anemic fetuses, which likely involves direct effects of low oxygen and/or increased norepinephrine on insulin release. In pregnancies complicated by chronic fetal hypoxemia, increasing fetal oxygen concentrations may improve insulin secretion.

Download Full-text

Increased amino acid supply potentiates glucose-stimulated insulin secretion but does not increase β-cell mass in fetal sheep

AJP Endocrinology and Metabolism ◽

10.1152/ajpendo.00377.2012 ◽

2013 ◽

Vol 304 (4) ◽

pp. E352-E362 ◽

Cited By ~ 15

Author(s):

Monika M. Gadhia ◽

Anne M. Maliszewski ◽

Meghan C. O'Meara ◽

Stephanie R. Thorn ◽

Jinny R. Lavezzi ◽

...

Keyword(s):

Amino Acids ◽

Insulin Secretion ◽

Amino Acid ◽

Cell Mass ◽

Late Gestation ◽

Insulin Content ◽

Acid Mixture ◽

Β Cell ◽

Fetal Sheep ◽

Glucose Stimulated Insulin Secretion

Amino acids and glucose acutely stimulate fetal insulin secretion. In isolated adult pancreatic islets, amino acids potentiate glucose-stimulated insulin secretion (GSIS), but whether amino acids have this same effect in the fetus is unknown. Therefore, we tested the effects of increased fetal amino acid supply on GSIS and morphology of the pancreas. We hypothesized that increasing fetal amino acid supply would potentiate GSIS. Singleton fetal sheep received a direct intravenous infusion of an amino acid mixture (AA) or saline (CON) for 10–14 days during late gestation to target a 25–50% increase in fetal branched-chain amino acids (BCAA). Early-phase GSIS increased 150% in the AA group ( P < 0.01), and this difference was sustained for the duration of the hyperglycemic clamp (105 min) ( P < 0.05). Glucose-potentiated arginine-stimulated insulin secretion (ASIS), pancreatic insulin content, and pancreatic glucagon content were similar between groups. β-Cell mass and area were unchanged between groups. Baseline and arginine-stimulated glucagon concentrations were increased in the AA group ( P < 0.05). Pancreatic α-cell mass and area were unchanged. Fetal and pancreatic weights were similar. We conclude that a sustained increase of amino acid supply to the normally growing late-gestation fetus potentiated fetal GSIS but did not affect the morphology or insulin content of the pancreas. We speculate that increased β-cell responsiveness (insulin secretion) following increased amino acid supply may be due to increased generation of secondary messengers in the β-cell. This may be enhanced by the paracrine action of glucagon on the β-cell.

Download Full-text

Enhanced insulin secretion responsiveness and islet adrenergic desensitization after chronic norepinephrine suppression is discontinued in fetal sheep

AJP Endocrinology and Metabolism ◽

10.1152/ajpendo.00517.2013 ◽

2014 ◽

Vol 306 (1) ◽

pp. E58-E64 ◽

Cited By ~ 23

Author(s):

Xiaochuan Chen ◽

Alice S. Green ◽

Antoni R. Macko ◽

Dustin T. Yates ◽

Amy C. Kelly ◽

...

Keyword(s):

Insulin Secretion ◽

Pancreatic Islets ◽

Chronic Exposure ◽

Uncoupling Protein ◽

Adrenergic Blockade ◽

Protein Subunit ◽

Β Cell ◽

Fetal Sheep ◽

Isolated Pancreatic Islets ◽

Glucose Stimulated Insulin Secretion

Intrauterine growth-restricted (IUGR) fetuses experience prolonged hypoxemia, hypoglycemia, and elevated norepinephrine (NE) concentrations, resulting in hypoinsulinemia and β-cell dysfunction. Previously, we showed that acute adrenergic blockade revealed enhanced insulin secretion responsiveness in the IUGR fetus. To determine whether chronic exposure to NE alone enhances β-cell responsiveness afterward, we continuously infused NE into fetal sheep for 7 days and, after terminating the infusion, evaluated glucose-stimulated insulin secretion (GSIS) and glucose-potentiated arginine-induced insulin secretion (GPAIS). During treatment, NE-infused fetuses had greater ( P < 0.05) plasma NE concentrations and exhibited hyperglycemia ( P < 0.01) and hypoinsulinemia ( P < 0.01) compared with controls. GSIS during the NE infusion was also reduced ( P < 0.05) compared with pretreatment values. GSIS and GPAIS were approximately fourfold greater ( P < 0.01) in NE fetuses 3 h after the 7 days that NE infusion was discontinued compared with age-matched controls or pretreatment GSIS and GPAIS values of NE fetuses. In isolated pancreatic islets from NE fetuses, mRNA concentrations of adrenergic receptor isoforms (α1D, α2A, α2C, and β1), G protein subunit-αi-2, and uncoupling protein 2 were lower ( P < 0.05) compared with controls, but β-cell regulatory genes were not different. Our findings indicate that chronic exposure to elevated NE persistently suppresses insulin secretion. After removal, NE fetuses demonstrated a compensatory enhancement in insulin secretion that was associated with adrenergic desensitization and greater stimulus-secretion coupling in pancreatic islets.

Download Full-text