The fetal and maternal catecholamine response to insulin-induced hypoglycemia in the rat

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.

Developmental programming in response to intrauterine growth restriction impairs myoblast function and skeletal muscle metabolism

Fetal adaptations to placental insufficiency alter postnatal metabolic homeostasis in skeletal muscle by reducing glucose oxidation rates, impairing insulin action, and lowering the proportion of oxidative fibers. In animal models of intrauterine growth restriction (IUGR), skeletal muscle fibers have less myonuclei at birth. This means that myoblasts, the sole source for myonuclei accumulation in fibers, are compromised. Fetal hypoglycemia and hypoxemia are complications that result from placental insufficiency. Hypoxemia elevates circulating catecholamines, and chronic hypercatecholaminemia has been shown to reduce fetal muscle development and growth. We have found evidence for adaptations in adrenergic receptor expression profiles in myoblasts and skeletal muscle of IUGR sheep fetuses with placental insufficiency. The relationship of β-adrenergic receptors shifts in IUGR fetuses because Adrβ2 expression levels decline and Adrβ1 expression levels are unaffected in myofibers and increased in myoblasts. This adaptive response would suppress insulin signaling, myoblast incorporation, fiber hypertrophy, and glucose oxidation. Furthermore, this β-adrenergic receptor expression profile persists for at least the first month in IUGR lambs and lowers their fatty acid mobilization. Developmental programming of skeletal muscle adrenergic receptors partially explains metabolic and endocrine differences in IUGR offspring, and the impact on metabolism may result in differential nutrient utilization.

Low glucose sensitivity and polymodal chemosensing in neonatal rat adrenomedullary chromaffin cells

Glucose is the primary metabolic fuel in mammalian fetuses, yet mammals are incapable of endogenous glucose production until several hours after birth. Thus, when the maternal supply of glucose ceases at birth there is a transient hypoglycemia that elicits a counterregulatory surge in circulating catecholamines. Because the innervation of adrenomedullary chromaffin cells (AMCs) is immature at birth, we hypothesized that neonatal AMCs act as direct glucosensors, a property that could complement their previously established roles as hypoxia and acid hypercapnia sensors. During perforated-patch, whole cell recordings, low glucose depolarized and/or excited a subpopulation of neonatal AMCs; in addition, aglycemia (0 mM glucose) caused inhibition of outward K+ current, blunted by the simultaneous activation of glibenclamide-sensitive KATP channels. Some cells were excited by each of the three metabolic stimuli, i.e., aglycemia, hypoxia (Po2 ∼30 mmHg), and isohydric hypercapnia (10% CO2; pH = 7.4). Using carbon fiber amperometry, aglycemia and hypoglycemia (3 mM glucose) induced robust catecholamine secretion that was sensitive to nickel (50 μM and 2 mM) and the L-type Ca2+ channel blocker nifedipine (10 μM), suggesting involvement of both T-type and L-type voltage-gated Ca2+ channels. Fura-2 measurements of intracellular Ca2+ ([Ca2+] i) revealed that ∼42% of neonatal AMCs responded to aglycemia with a significant rise in [Ca2+] i. Approximately 40% of these cells responded to hypoxia, whereas ∼25% cells responded to both aglycemia and hypoxia. These data suggest that together with hypoxia and acid hypercapnia, low glucose is another important metabolic stimulus that contributes to the vital asphyxia-induced catecholamine surge from AMCs at birth.

Hypoglycemia in newborn infants: Features associated with adverse outcomes

The purpose of this review article is to document from the literature values of blood/plasma glucose concentration and associated clinical signs and conditions in newborn infants (both term and preterm) that indicate a reasonable clinical probability that hypoglycemia is a proximate cause of acute and/or sustained neurological injury and to review the physiological and pathophysiological responses to hypoglycemia that may influence the ultimate outcome of newborns with low blood glucose. Our overall conclusion is that there is inadequate information in the literature to define any one value of glucose below which irreparable hypoglycemic injury to the central nervous system occurs, at any one time or for any defined period of time, in a population of infants or in any given infant. Clinical signs of prolonged and severe neurological disturbance (coma, seizures), extremely and persistently low plasma/blood glucose concentrations (0 to <1.0 mmol/l [0 to <18-20 mg/dl] for more than 1-2 h), and the absence of other obvious central nervous system (CNS) pathology (hypoxia-ischemia, intracranial hemorrhage, infection, etc.) are important for the diagnosis of injury due to glucose deficiency. Specific conditions, such as persistent hyperinsulinemia with severe hypoglycemic episodes that include seizures, also contribute to the diagnosis of hypoglycemic injury. Such lack of definitive measures of injury specific to glucose deficiency indicates that clinicians should be on the alert for infants at risk of hypoglycemia and for clinical signs and conditions that might herald severe hypoglycemia; they should have a low threshold for investigating and diagnosing 'hypoglycemia' with frequent measurements of plasma/blood glucose concentration; and they should treat low glucose concentrations promptly and maintain them in a safe range. Because there is no conclusive evidence or consensus in the literature that defines an absolute value or duration of 'hypoglycemia' that must occur, with our without related clinical complications, to produce neurological injury, clinicians should consider the information currently available, determine a 'target' plasma or blood glucose concentration that is acceptable, and treat infants with glucose concentrations below this value accordingly. Our intent in this review article is to highlight the studies relevant to this issue and help clinicians formulate a safe and, hopefully, effective strategy for the diagnosis and treatment of hypoglycemia.

Determination of monoamines and indoles in amniotic fluid by high-performance liquid chromatography-electrochemical detection

A technique is presented for the separation and detection in amniotic fluid of various substances associated with catecholamine metabolism. Monoamines and their metabolites were separated using reversed-phase ion-pair high-performance liquid chromatography. Detection and quantification were performed electrochemically. The retention times of 28 standards associated with the monoamines and their precursors and metabolites were evaluated with 14 different eluents. On the basis of the retention times of each standard and the modification of the retention times of the various peaks detected in amniotic fluid, the following substances were identified in this biological fluid: 4-hydroxy-3-methoxyphenylacetic acid, 5-hydroxyindoleacetic acid, 3,4-dihydroxyphenylacetic acid, 4-hydroxy-3-methoxyphenylglycol, epinephrine, 4-hydroxy-3-methoxymandelic acid, octopamine, tyrosine and tryptophan.

Effects of insulin infusion on plasma catecholamine concentration in fetal sheep

To evaluate the response of the sympathoadrenal system in fetal sheep receiving exogenous insulin infusion, we measured plasma catecholamine levels in 14 chronically catheterized fetal sheep before and during an infusion of insulin for 2 days. Catecholamine values were measured in fetal arterial plasma by an electrochemical detection method. Fetal plasma norepinephrine and epinephrine concentrations increased significantly during insulin infusion. Significant inverse correlations were observed between the log norepinephrine concentration and fetal arterial oxygen content and glucose values. A significant direct correlation between the log norepinephrine concentration and fetal arterial carbon dioxide concentration was also observed. The log epinephrine concentration correlated inversely with plasma glucose concentration. Increases in fetal heart rate during both the noninfused and insulin-infused states correlated significantly with increases in norepinephrine concentration. We conclude that the sympathoadrenal system is activated during fetal insulin infusion, potentially supporting some of the fetal cardiovascular responses to insulin infusion.

Effects of hypoglycaemia on early embryogenesis in rat embryo organ culture

As congenital malformations may be caused by perturbations of glycolytic flux on early embryogenesis [16], effects of hypoglycaemia were investigated by using rat embryo organ culture. Nine and one-half day old rat embryos were grown in vitro for 48 h (day 9 1/2 to 11 1/2) in the presence of hypoglycaemic serum for different hours during the culture period. Hypoglycaemic serum was obtained from rats given insulin intraperitoneally. On exposure to hypoglycaemic serum during the first 24 h of culture (day 9 1/2 to 10 1/2), embryos showed marked growth retardation and had increased frequencies of neural lesions (42.7% versus 0%, p less than 0.01), in contrast to hypoglycaemic exposure during the second 24 h of culture (day 10 1/2 to 11 1/2), where only minor growth retardation and low frequencies of neural lesions (2.4% versus 0%, NS) were seen. Even exposure to hypoglycaemic serum for a relatively short period (8 h) during the first 24 h of culture resulted in neural lesions at the frequency of 9.3-13.3%. The embryos exposed to hypoglycaemia demonstrated decreased glucose uptake and lactic acid formation, indicating decreased energy production via glycolysis that constitutes the principal energy pathway at this stage of embryonic development. These results suggest that hypoglycaemia during critical periods of embryogenesis has adverse effects on the development of the embryo and these effects might be mediated through metabolic interruption of embryogenesis.

Dopamine binding to the alpha receptor in pregnant rabbit myometrium

This study evaluated in vitro binding of dopamine ligands to myometrial alpha adrenoceptors. With cell membranes from pregnant rabbits, receptor radioligand binding studies utilizing [3H] dihydroergocryptine +/- dopamine demonstrated receptor affinity (KD) = 0.75 +/- 0.10 nM (+/- SEM) and density (Bmax) = 533.2 +/- 45.2 fM/mg protein. Similar studies utilizing phentolamine or apomorphine gave essentially identical results. Competition binding studies demonstrated steriospecific butaclamol binding, along with significant binding of haloperidol, spiperone, apomorphine, and bromoergocryptine. These observations provide a mechanism for the observed uterotonic effects of dopamine.

Embryotoxic effects of brief maternal insulin-hypoglycemia during organogenesis in the rat

To test whether maternal hypoglycemia can impair organogenesis, we induced brief glucopenia with insulin in conscious pregnant rats during either the headfold stage or the early neural tube closure stage of embryogenesis. At each time, 10 pairs of animals received identical insulin infusions for 1 h. Half the animals were maintained at euglycemia during the infusions, while the others were allowed to become hypoglycemic. Euglycemia was maintained or restored in all animals immediately after the insulin was stopped. Spontaneous activity was diminished during the hypoglycemia but consciousness was preserved. Embryos were removed from mothers and examined 2 d later. This examination revealed that embryos from the hypoglycemic mothers were growth-retarded and displayed a small but significant incidence of gross developmental anomalies compared with embryos from the insulin-infused euglycemic mothers. Thus, brief, mild maternal hypoglycemia during early organogenesis can disrupt normal embryo development in the rat. The effect is due to the hypoglycemia per se rather than to the insulin employed for its induction.

Subhourly variability of circulating norepinephrine and epinephrine in the pregnant ewe and fetal and newborn lamb

Circulating norepinephrine and epinephrine were determined nine times over a 1-hour period in chronically instrumented ewes (n = 4), their fetuses (n = 4), and lambs (n = 4). In an apparent resting state, assessed by subjective and biophysical parameters, marked fluctuations were demonstrated in the mean norepinephrine and epinephrine values between animals and in the range of the concentrations of each individual animal. The significance of the fluctuations in resting plasma norepinephrine and epinephrine concentrations is discussed.

Fetal catecholamines

Experimental data from fetal human and animal research suggest that the fetal sympathoadrenal system, composed of the adrenal medulla, sympathetic neurons, and extra-adrenal chromaffin tissue functions from early fetal life to maintain fetal homeostasis. The extra-adrenal chromaffin tissue undergoes maturation at 9 to 11 weeks of gestation, whereas the adrenal medulla and sympathetic nervous system mature later in fetal life. The fetal catecholamine response to hypoxia, mediated predominantly by norepinephrine, is an important component of the fetal cardiovascular response to hypoxia, i.e., through alpha-receptor stimulation, fetal cardiac output redistribution occurs. Fetal catecholamine secretion in response to substrate availability, through alpha- and beta-receptor stimulation, provide a mechanism by which the fetus can utilize its own substrate stores. Pulmonary beta-receptor stimulation by catecholamines has been demonstrated to increase lecithin synthesis, increase surfactant secretion, and decrease lung fluid production near term. beta-Receptor stimulation has also been demonstrated to have trophic effects on the development of thermogenic brown adipose tissue. Although the exact stimulus for the initiation of parturition in the primate is unknown, fetal catecholamines, through direct myometrial alpha-adrenergic or dopaminergic receptor stimulation and/or through the stimulation of prostaglandin production, have the potential of facilitating the onset of parturition.

Catecholamines in human amniotic fluid

This study was undertaken to determine the amniotic fluid levels of epinephrine (E), norepinephrine (N), and dopamine (D) at different gestational ages in the human. Amniotic fluid obtained from 32 pregnant women undergoing amniocentesis for medical indications was analyzed with a radioenzyme assay for these catecholamines. The gestational ages ranged from 17 to 39 weeks. No patient was in active labor at the time of amniocentesis. The data obtained in this study demonstrated a rise of all three catecholamines at the end of the third trimester: N = 362.7 +/- 61.2 pg/ml (+/- SEM), E = 179.6 +/- 45.4 pg/ml (p < 0.05), and D = 2717.0 +/- 836.6 pg/ml (p < 0.01). The physiologic role for the increasing amounts of catecholamines, especially D, in amniotic fluid is known; however, these amines could be the stimulus for the intrauterine synthesis of prostaglandins as parturition approaches.