Placental Insufficiency-induced Intrauterine Growth Restriction Research Articles

Elevated Norepinephrine Stimulates Adipocyte Hyperplasia in Ovine Fetuses With Placental Insufficiency and IUGR.

Prevailing hypoxemia and hypoglycemia in near-term fetuses with placental insufficiency-induced intrauterine growth restriction (IUGR) chronically increases norepinephrine concentrations, which lower adrenergic sensitivity and lipid mobilization postnatally, indicating a predisposition for adiposity. To determine adrenergic-induced responses, we examined the perirenal adipose tissue transcriptome from IUGR fetuses with or without hypercatecholaminemia. IUGR was induced in sheep with maternal hyperthermia, and hypercatecholaminemia in IUGR was prevented with bilateral adrenal demedullation. Adipose tissue was collected from sham-operated control (CON) and IUGR fetuses and adrenal-demedullated control (CAD) and IUGR (IAD) fetuses. Norepinephrine concentrations were lower in IAD fetuses than in IUGR fetuses despite both being hypoxemic and hypoglycemic. In IUGR fetuses, perirenal adipose tissue mass relative to body mass was greater compared with the CON, adrenal-demedullated control, and IAD groups. Transcriptomic analysis identified 581 differentially expressed genes (DEGs) in CON vs IUGR adipose tissue and 193 DEGs in IUGR vs IAD adipose tissue. Integrated functional analysis of these 2 comparisons showed enrichment for proliferator-activated receptor signaling and metabolic pathways and identified adrenergic responsive genes. Within the adrenergic-regulated DEGs, we identified transcripts that regulate adipocyte proliferation and differentiation: adipogenesis regulatory factor, C/CCAAT/enhancer binding protein α, and sterol carrier protein 2. DEGs associated with the metabolic pathway included pyruvate dehydrogenase kinase 4, 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 4, IGF-binding proteins (IGFBP-5 and IGFBP-7). Sex-specific expression differences were also found for adipogenesis regulatory factor, pyruvate dehydrogenase kinase 4, IGFBP5, and IGFBP7. These findings indicate that sustained adrenergic stimulation during IUGR leads to adipocyte hyperplasia with alterations in metabolism, proliferation, and preadipocyte differentiation pathways.

Adaptive responses in uteroplacental metabolism and fetoplacental nutrient shuttling and sensing during placental insufficiency.

Glucose, lactate, and amino acids are major fetal nutrients. During placental insufficiency-induced intrauterine growth restriction (PI-IUGR), uteroplacental weight-specific oxygen consumption rates are maintained, yet fetal glucose and amino acid supply is decreased and fetal lactate concentrations are increased. We hypothesized that uteroplacental metabolism adapts to PI-IUGR by altering nutrient allocation to maintain oxidative metabolism. Here, we measured nutrient flux rates, with a focus on nutrients shuttled between the placenta and fetus (lactate-pyruvate, glutamine-glutamate, and glycine-serine) in a sheep model of PI-IUGR. PI-IUGR fetuses weighed 40% less and had decreased oxygen, glucose, and amino acid concentrations and increased lactate and pyruvate versus control (CON) fetuses. Uteroplacental weight-specific rates of oxygen, glucose, lactate, and pyruvate uptake were similar. In PI-IUGR, fetal glucose uptake was decreased and pyruvate output was increased. In PI-IUGR placental tissue, pyruvate dehydrogenase (PDH) phosphorylation was decreased and PDH activity was increased. Uteroplacental glutamine output to the fetus and expression of genes regulating glutamine-glutamate metabolism were lower in PI-IUGR. Fetal glycine uptake was lower in PI-IUGR, with no differences in uteroplacental glycine or serine flux. These results suggest increased placental utilization of pyruvate from the fetus, without higher maternal glucose utilization, and lower fetoplacental amino acid shuttling during PI-IUGR. Mechanistically, AMP-activated protein kinase (AMPK) activation was higher and associated with thiobarbituric acid-reactive substances (TBARS) content, a marker of oxidative stress, and PDH activity in the PI-IUGR placenta, supporting a potential link between oxidative stress, AMPK, and pyruvate utilization. These differences in fetoplacental nutrient sensing and shuttling may represent adaptive strategies enabling the placenta to maintain oxidative metabolism.NEW & NOTEWORTHY These results suggest increased placental utilization of pyruvate from the fetus, without higher maternal glucose uptake, and lower amino acid shuttling in the placental insufficiency-induced intrauterine growth restriction (PI-IUGR) placenta. AMPK activation was associated with oxidative stress and PDH activity, supporting a putative link between oxidative stress, AMPK, and pyruvate utilization. These differences in fetoplacental nutrient sensing and shuttling may represent adaptive strategies enabling the placenta to maintain oxidative metabolism at the expense of fetal growth.

Augmented glucose production is not contingent on high catecholamines in fetal sheep with IUGR.

Fetuses with intrauterine growth restriction (IUGR) have high concentrations of catecholamines, which lowers the insulin secretion and glucose uptake. Here, we studied the effect of hypercatecholaminemia on glucose metabolism in sheep fetuses with placental insufficiency-induced IUGR. Norepinephrine concentrations are elevated throughout late gestation in IUGR fetuses but not in IUGR fetuses with a bilateral adrenal demedullation (IAD) at 0.65 of gestation. Euglycemic (EC) and hyperinsulinemic-euglycemic (HEC) clamps were performed in control, intact-IUGR, and IAD fetuses at 0.87 of gestation. Compared to controls, basal oxygen, glucose, and insulin concentrations were lower in IUGR groups. Norepinephrine concentrations were five-fold higher in IUGR fetuses than in IAD fetuses. During the EC, rates of glucose entry (GER, umbilical + exogenous), glucose utilization (GUR), and glucose oxidation (GOR) were greater in IUGR groups than in controls. In IUGR and IAD fetuses with euglycemia and euinsulinemia, glucose production rates (GPR) remained elevated. During the HEC, GER and GOR were not different among groups. In IUGR and IAD fetuses, GURs were 40% greater than in controls, which paralleled the sustained GPR despite hyperinsulinemia. Glucose-stimulated insulin concentrations were augmented in IAD fetuses compared to IUGR fetuses. Fetal weights were not different between IUGR groups but were less than controls. Regardless of norepinephrine concentrations, IUGR fetuses not only develop greater peripheral insulin sensitivity for glucose utilization but also develop hepatic insulin resistance because GPR was maintained and unaffected by euglycemia or hyperinsulinemia. These findings show that adaptation in glucose metabolism of IUGR fetuses are independent of catecholamines, which implicate that hypoxemia and hypoglycemia cause the metabolic responses.

Deficits in growth, muscle mass, and body composition following placental insufficiency-induced intrauterine growth restriction persisted in lambs at 60 d of age but were improved by daily clenbuterol supplementation.

Deficits in growth, muscle mass, and body composition following placental insufficiency-induced intrauterine growth restriction persisted in lambs at 60 d of age but were improved by daily clenbuterol supplementation.

Lower oxygen consumption and Complex I activity in mitochondria isolated from skeletal muscle of fetal sheep with intrauterine growth restriction.

Fetal sheep with placental insufficiency-induced intrauterine growth restriction (IUGR) have lower hindlimb oxygen consumption rates (OCRs), indicating depressed mitochondrial oxidative phosphorylation capacity in their skeletal muscle. We hypothesized that OCRs are lower in skeletal muscle mitochondria from IUGR fetuses, due to reduced electron transport chain (ETC) activity and lower abundances of tricarboxylic acid (TCA) cycle enzymes. IUGR sheep fetuses (n = 12) were created with mid-gestation maternal hyperthermia and compared with control fetuses (n = 12). At 132 ± 1 days of gestation, biceps femoris muscles were collected, and the mitochondria were isolated. Mitochondria from IUGR muscle have 47% lower State 3 (Complex I-dependent) OCRs than controls, whereas State 4 (proton leak) OCRs were not different between groups. Furthermore, Complex I, but not Complex II or IV, enzymatic activity was lower in IUGR fetuses compared with controls. Proteomic analysis (n = 6/group) identified 160 differentially expressed proteins between groups, with 107 upregulated and 53 downregulated mitochondria proteins in IUGR fetuses compared with controls. Although no differences were identified in ETC subunit protein abundances, abundances of key TCA cycle enzymes [isocitrate dehydrogenase (NAD+) 3 noncatalytic subunit β (IDH3B), succinate-CoA ligase ADP-forming subunit-β (SUCLA2), and oxoglutarate dehydrogenase (OGDH)] were lower in IUGR mitochondria. IUGR mitochondria had a greater abundance of a hypoxia-inducible protein, NADH dehydrogenase 1α subcomplex 4-like 2, which is known to incorporate into Complex I and lower Complex I-mediated NADH oxidation. Our findings show that mitochondria from IUGR skeletal muscle adapt to hypoxemia and hypoglycemia by lowering Complex I activity and TCA cycle enzyme concentrations, which together, act to lower OCR and NADH production/oxidation in IUGR skeletal muscle.

Intermittent maternofetal O2 supplementation during late gestation rescues placental insufficiency-induced intrauterine growth restriction and metabolic pathologies in the neonatal lamb.

Intermittent maternofetal O2 supplementation during late gestation rescues placental insufficiency-induced intrauterine growth restriction and metabolic pathologies in the neonatal lamb.

Increased pyruvate dehydrogenase activity in skeletal muscle of growth-restricted ovine fetuses.

Fetal sheep with placental insufficiency-induced intrauterine growth restriction (IUGR) have lower fractional rates of glucose oxidation and greater gluconeogenesis, indicating lactate shuttling between skeletal muscle and liver. Suppression of pyruvate dehydrogenase (PDH) activity was proposed because of greater pyruvate dehydrogenase kinase (PDK) 4 and PDK1 mRNA concentrations in IUGR muscle. Although PDK1 and PDK4 inhibit PDH activity to reduce pyruvate metabolism, PDH protein concentrations and activity have not been examined in skeletal muscle from IUGR fetuses. Therefore, we evaluated the protein concentrations and activity of PDH and the kinases and phosphatases that regulate PDH phosphorylation status in the semitendinosus muscle from placenta insufficiency-induced IUGR sheep fetuses and control fetuses. Immunoblots were performed for PDH, phosphorylated PDH (E1α), PDK1, PDK4, and pyruvate dehydrogenase phosphatase 1 and 2 (PDP1 and PDP2, respectively). Additionally, the PDH, lactate dehydrogenase (LDH), and citrate synthase (CS) enzymatic activities were measured. Phosphorylated PDH concentrations were 28% lower (P < 0.01) and PDH activity was 67% greater (P < 0.01) in IUGR fetal muscle compared with control. PDK1, PDK4, PDP1, PDP2, and PDH concentrations were not different between groups. CS and LDH activities were also unaffected. Contrary to the previous speculation, PDH activity was greater in skeletal muscle from IUGR fetuses, which parallels lower phosphorylated PDH. Therefore, greater expression of PDK1 and PDK4 mRNA did not translate to greater PDK1 or PDK4 protein concentrations or inhibition of PDH as proposed. Instead, these findings show greater PDH activity in IUGR fetal muscle, which indicates that alternative regulatory mechanisms are responsible for lower pyruvate catabolism.

Intrauterine Growth Restriction and Hyperoxia as a Cause of White Matter Injury

Intrauterine growth restriction (IUGR) is estimated to occur in 5% of pregnancies, with placental insufficiency being the most common cause in developed countries. While it is known that white matter injury occurs in premature infants, the extent of IUGR on white matter injury is less defined in term infants. We used a novel murine model that utilizes a thromboxane A₂ (TXA₂) analog (U46619), a potent vasoconstrictor, to induce maternal hypertension and mimic human placental insufficiency-induced IUGR to study the white matter. We also investigated the role of hyperoxia as an additional risk factor for white matter injury, as IUGR infants are at increased risk of respiratory comorbidities leading to increased oxygen exposure. We found that TXA₂ analog-induced IUGR results in white matter injury as demonstrated by altered myelin structure and changes in the oligodendroglial cell/oligodendrocyte population. In addition, our study demonstrates that hyperoxia exposure independently results in white matter perturbation. To our knowledge, this is the first study to report single and combined effects of IUGR with hyperoxia impacting the white matter and motor function. These results draw attention to the need for close monitoring of motor development in IUGR babies following hospital discharge as well as highlighting the importance of limiting, as clinically feasible, the degree of oxygen overexposure to potentially improve motor outcomes in this population of infants.

Enhanced insulin secretion and insulin sensitivity in young lambs with placental insufficiency-induced intrauterine growth restriction.

Intrauterine growth restriction (IUGR) is associated with persistent metabolic complications, but information is limited for IUGR infants. We determined glucose-stimulated insulin secretion (GSIS) and insulin sensitivity in young lambs with placental insufficiency-induced IUGR. Lambs with hyperthermia-induced IUGR (n = 7) were compared with control lambs (n = 8). GSIS was measured at 8 ± 1 days of age, and at 15 ± 1 days, body weight-specific glucose utilization rates were measured with radiolabeled d-glucose during a hyperinsulinemic-euglycemic clamp (HEC). IUGR lambs weighed 23% less (P < 0.05) than controls at birth. Fasting plasma glucose and insulin concentrations were not different between IUGR and controls for either study. First-phase insulin secretion was enhanced 2.3-fold in IUGR lambs compared with controls. However, second-phase insulin concentrations, glucose-potentiated arginine-stimulated insulin secretion, and β-cell mass were not different, indicating that IUGR β-cells have an intrinsic enhancement in acute GSIS. Compared with controls, IUGR lambs had higher body weight-specific glucose utilization rates and greater insulin sensitivity at fasting (1.6-fold) and hyperinsulinemic periods (2.4-fold). Improved insulin sensitivity for glucose utilization was not due to differences in skeletal muscle insulin receptor and glucose transporters 1 and 4 concentrations. Plasma lactate concentrations during HEC were elevated in IUGR lambs compared with controls, but no differences were found for glycogen content or citrate synthase activity in liver and muscle. Greater insulin sensitivity for glucose utilization and enhanced acute GSIS in young lambs are predicted from fetal studies but may promote conditions that exaggerate glucose disposal and lead to episodes of hypoglycemia in IUGR infants.

Myoblast replication is reduced in the IUGR fetus despite maintained proliferative capacity in vitro.

Adults who were affected by intrauterine growth restriction (IUGR) suffer from reductions in muscle mass and insulin resistance, suggesting muscle growth may be restricted by molecular events that occur during fetal development. To explore the basis of restricted fetal muscle growth, we used a sheep model of progressive placental insufficiency-induced IUGR to assess myoblast proliferation within intact skeletal muscle in vivo and isolated myoblasts stimulated with insulin in vitro Gastrocnemius and soleus muscle weights were reduced by 25% in IUGR fetuses compared to those in controls (CON). The ratio of PAX7+ nuclei (a marker of myoblasts) to total nuclei was maintained in IUGR muscle compared to CON, but the fraction of PAX7+ myoblasts that also expressed Ki-67 (a marker of cellular proliferation) was reduced by 23%. Despite reduced proliferation in vivo, fetal myoblasts isolated from IUGR biceps femoris and cultured in enriched media in vitro responded robustly to insulin in a dose- and time-dependent manner to increase proliferation. Similarly, insulin stimulation of IUGR myoblasts upregulated key cell cycle genes and DNA replication. There were no differences in the expression of myogenic regulatory transcription factors that drive commitment to muscle differentiation between CON and IUGR groups. These results demonstrate that the molecular machinery necessary for transcriptional control of proliferation remains intact in IUGR fetal myoblasts, indicating that in vivo factors such as reduced insulin and IGF1, hypoxia and/or elevated counter-regulatory hormones may be inhibiting muscle growth in IUGR fetuses.

Intrauterine growth-restricted sheep fetuses exhibit smaller hindlimb muscle fibers and lower proportions of insulin-sensitive Type I fibers near term.

Intrauterine growth restriction (IUGR) reduces muscle mass and insulin sensitivity in offspring. Insulin sensitivity varies among muscle fiber types, with Type I fibers being most sensitive. Differences in fiber-type ratios are associated with insulin resistance in adults, and thus we hypothesized that near-term IUGR sheep fetuses exhibit reduced size and proportions of Type I fibers. Placental insufficiency-induced IUGR fetuses were ∼54% smaller (P < 0.05) than controls and exhibited hypoxemia and hypoglycemia, which contributed to 6.9-fold greater (P < 0.05) plasma norepinephrine and ∼53% lower (P < 0.05) plasma insulin concentrations. IUGR semitendinosus muscles contained less (P < 0.05) myosin heavy chain-I protein (MyHC-I) and proportionally fewer (P < 0.05) Type I and Type I/IIa fibers than controls, but MyHC-II protein concentrations, Type II fibers, and Type IIx fibers were not different. IUGR biceps femoris muscles exhibited similar albeit less dramatic differences in fiber type proportions. Type I and IIa fibers are more responsive to adrenergic and insulin regulation than Type IIx and may be more profoundly impaired by the high catecholamines and low insulin in our IUGR fetuses, leading to their proportional reduction. In both muscles, fibers of each type were uniformly smaller (P < 0.05) in IUGR fetuses than controls, which indicates that fiber hypertrophy is not dependent on type but rather on other factors such as myoblast differentiation or protein synthesis. Together, our findings show that IUGR fetal muscles develop smaller fibers and have proportionally fewer Type I fibers, which is indicative of developmental adaptations that may help explain the link between IUGR and adulthood insulin resistance.

Adrenal Demedullation and Oxygen Supplementation Independently Increase Glucose-Stimulated Insulin Concentrations in Fetal Sheep With Intrauterine Growth Restriction

In pregnancies complicated by placental insufficiency and intrauterine growth restriction (IUGR), fetal glucose and oxygen concentrations are reduced, whereas plasma norepinephrine and epinephrine concentrations are elevated throughout the final third of gestation. Here we study the effects of chronic hypoxemia and hypercatecholaminemia on β-cell function in fetal sheep with placental insufficiency-induced IUGR that is produced by maternal hyperthermia. IUGR and control fetuses underwent a sham (intact) or bilateral adrenal demedullation (AD) surgical procedure at 0.65 gestation. As expected, AD-IUGR fetuses had lower norepinephrine concentrations than intact-IUGR fetuses despite being hypoxemic and hypoglycemic. Placental insufficiency reduced fetal weights, but the severity of IUGR was less with AD. Although basal plasma insulin concentrations were lower in intact-IUGR and AD-IUGR fetuses compared with intact-controls, glucose-stimulated insulin concentrations were greater in AD-IUGR fetuses compared with intact-IUGR fetuses. Interestingly, AD-controls had lower glucose- and arginine-stimulated insulin concentrations than intact-controls, but AD-IUGR and AD-control insulin responses were not different. To investigate chronic hypoxemia in the IUGR fetus, arterial oxygen tension was increased to normal levels by increasing the maternal inspired oxygen fraction. Oxygenation of IUGR fetuses enhanced glucose-stimulated insulin concentrations 3.3-fold in intact-IUGR and 1.7-fold in AD-IUGR fetuses but did not lower norepinephrine and epinephrine concentrations. Together these findings show that chronic hypoxemia and hypercatecholaminemia have distinct but complementary roles in the suppression of β-cell responsiveness in IUGR fetuses.

Fetal adrenal demedullation lowers circulating norepinephrine and attenuates growth restriction but not reduction of endocrine cell mass in an ovine model of intrauterine growth restriction.

Placental insufficiency is associated with fetal hypoglycemia, hypoxemia, and elevated plasma norepinephrine (NE) that become increasingly pronounced throughout the third trimester and contribute to intrauterine growth restriction (IUGR). This study evaluated the effect of fetal adrenal demedullation (AD) on growth and pancreatic endocrine cell mass. Placental insufficiency-induced IUGR was created by exposing pregnant ewes to elevated ambient temperatures during mid-gestation. Treatment groups consisted of control and IUGR fetuses with either surgical sham or AD at 98 days gestational age (dGA; term = 147 dGA), a time-point that precedes IUGR. Samples were collected at 134 dGA. IUGR-sham fetuses were hypoxemic, hypoglycemic, and hypoinsulinemic, and values were similar in IUGR-AD fetuses. Plasma NE concentrations were ~5-fold greater in IUGR-sham compared to control-sham, control-AD, and IUGR-AD fetuses. IUGR-sham and IUGR-AD fetuses weighed less than controls. Compared to IUGR-sham fetuses, IUGR-AD fetuses weighed more and asymmetrical organ growth was absent. Pancreatic β-cell mass and α-cell mass were lower in both IUGR-sham and IUGR-AD fetuses compared to controls, however, pancreatic endocrine cell mass relative to fetal mass was lower in IUGR-AD fetuses. These findings indicate that NE, independently of hypoxemia, hypoglycemia and hypoinsulinemia, influence growth and asymmetry of growth but not pancreatic endocrine cell mass in IUGR fetuses.

Increased arterial stiffness and extracellular matrix reorganization in intrauterine growth–restricted fetal sheep

Fetal intrauterine growth restriction (IUGR) results in increased placental resistance to blood flow, fetal hypertension, and increased pulsatility stresses shown to lead to vascular remodeling. We tested our hypothesis that IUGR causes decreased compliance in the carotid and umbilical arteries due to altered extracellular matrix (ECM) composition and structure. A sheep model of placental insufficiency-induced IUGR (PI-IUGR) was created by exposure of the pregnant ewe to elevated ambient temperatures. Umbilical and carotid arteries from near-term fetuses were tested with pressure-diameter measurements to compare passive compliance in control and PI-IUGR tissues. ECM composition was measured via biochemical assay, and the organization was determined by using histology and second-harmonic generation imaging. We found that PI-IUGR increased arterial stiffness with increased collagen engagement, or transition stretch. PI-IUGR carotid arteries exhibited increased collagen and elastin quantity, and PI-IUGR umbilical arteries exhibited increased sulfated glycosaminoglycans. Histomorphology showed altered collagen-to-elastin ratios with altered cellular proliferation. Increased stiffness indicates altered collagen-to-elastin ratios with less elastin contribution leading to increased collagen engagement. Because vessel stiffness is a significant predictor in the development of hypertension, disrupted ECM deposition in IUGR provides a potential link between IUGR and adult hypertension.

β2-Adrenergic receptor desensitization in perirenal adipose tissue in fetuses and lambs with placental insufficiency-induced intrauterine growth restriction

Placental insufficiency-induced intrauterine growth restriction (IUGR) fetuses have chronic hypoxaemia and elevated plasma catecholamine concentrations. In this study, we determined whether adrenergic responsiveness becomes desensitized in the perirenal adipose tissue of IUGR fetuses and lambs by measuring adrenergic receptor (AR) mRNA and protein levels. We also tested the ability of adrenaline to mobilize non-esterified fatty acids (NEFAs) in young lambs. Perirenal adipose tissue was collected from IUGR and control fetuses at 133 days of gestational age (dGA) and lambs at 18 days of age (dA). β(2)-AR mRNA concentrations were 59% and 74% lower (P < 0.05) in IUGR fetuses and lambs compared to controls, respectively, which also resulted in lower protein levels (P < 0.05). No treatment differences were detected for α(1A)-, α(1B)-, α(1D)-, α(2A)-, α(2B)-, α(2C)-, β(1)- and β(3)-AR expression. mRNA concentrations were also determined for hormone sensitive lipase (HSL), perilipin (lipid droplet-associated protein), and two adipokines, leptin and adiponectin. Adiponectin and HSL were not different between treatments at either age. Compared to controls, perilipin and leptin mRNA concentrations were lower (P < 0.05) in IUGR fetuses but not in lambs. Because of the β(2)-AR results, we challenged a second cohort of lambs with exogenous adrenaline at 21 dA. The ability of adrenaline to mobilize NEFA was 55 ± 15% lower (P < 0.05) in IUGRs than controls. Collectively, our findings indicate that elevated catecholamine exposure in utero causes desensitization of adipose tissue by down-regulation of β(2)-AR, and this persists in lambs. This impairment in adrenergic stimulated lipolysis might partially explain early onset obesity in IUGR offspring.

Chronic exposure to elevated norepinephrine suppresses insulin secretion in fetal sheep with placental insufficiency and intrauterine growth restriction

In this study, we examined chronic norepinephrine suppression of insulin secretion in sheep fetuses with placental insufficiency-induced intrauterine growth restriction (IUGR). Glucose-stimulated insulin secretion (GSIS) was measured with a square-wave hyperglycemic clamp in the presence or absence of adrenergic receptor antagonists phentolamine (alpha) and propranolol (beta). IUGR fetuses were hypoglycemic and hypoxemic and had lower GSIS responsiveness (P < or = 0.05) than control fetuses. IUGR fetuses also had elevated plasma norepinephrine (3,264 +/- 614 vs. 570 +/- 86 pg/ml; P < or = 0.05) and epinephrine (164 +/- 32 vs. 60 +/- 12 pg/ml; P < or = 0.05) concentrations. In control fetuses, adrenergic inhibition increased baseline plasma insulin concentrations (1.7-fold, P < or = 0.05), whereas during hyperglycemia insulin was not different. A greater (P < or = 0.05) response to adrenergic inhibition was found in IUGR fetuses, and the average plasma insulin concentrations increased 4.9-fold at baseline and 7.1-fold with hyperglycemia. Unlike controls, basal plasma glucose concentrations fell (P < or = 0.05) with adrenergic antagonists. GSIS responsiveness, measured by the change in insulin, was higher (8.9-fold, P < or = 0.05) in IUGR fetuses with adrenergic inhibition than controls (1.8-fold, not significant), showing that norepinephrine suppresses insulin secretion in IUGR fetuses. Strikingly, in IUGR fetuses, adrenergic inhibition resulted in a greater GSIS responsiveness, because beta-cell mass was 56% lower and the maximal stimulatory insulin response tended (P < 0.1) to be higher than controls. This persistent norepinephrine suppression appears to be partially explained by higher mRNA concentrations of adrenergic receptors alpha(1D), alpha(2A), and alpha(2B) in a cohort of fetuses that were naïve to the antagonists. Therefore, norepinephrine suppression of insulin secretion was maintained, in part, by upregulating adrenergic receptor expression, but the beta-cells also appeared to compensate with enhanced GSIS. These findings may begin to explain why IUGR infants have a propensity for increased glucose requirements if norepinephrine is suddenly decreased after birth.