Influence of metabolic control of pregnant diabetics on fetal lung maturity

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.

Comparative effects of chronic exposure to glucose or sodium butyrate on surfactant development in fetal rabbits

BACKGROUND

Infants of diabetic mothers (IDM) often have delayed lung development and are thus at an increased risk of Respiratory Distress Syndrome (RDS). Both hyperglycemia and/or hyperinsulinemia have been implicated in this delay but the precise mechanism has not been clarified. Another metabolite, sodium butyrate, which is increased in IDM has been shown to decrease surfactant production in vitro but its effects on the development of the fetal lung surfactant system in vivo have not been studied.

AIM

To investigate the in vivo effects of high glucose and sodium butyrate treatment on maternal and fetal glucose and insulin levels and on fetal lung surfactant maturation using timed-pregnant New Zealand White rabbits.

METHODS

On the 24th day of gestation the doe was implanted s.c. with time release pellets containing either glucose (300 mg), sodium butyrate (200 mg) or matching placebo. On the 27th or 30th day maternal (ear vein) and fetal (cardiac puncture) blood samples were drawn for glucose and insulin determinations. Fetal surfactant pools (both intra- and extracellular) were quantitatively harvested using differential and density gradient centrifugation and their phospholipid profiles determined. Data were statistically compared with ANOVA and Duncan's Multiple Range Test.

RESULTS

Neither glucose nor sodium butyrate affected maternal plasma glucose or insulin. Both metabolites significantly increased fetal plasma insulin, decreased fetal plasma glucose but did not delay any of the parameters of surfactant maturation examined.

CONCLUSIONS

Fetal hyperinsulinemia, whether attained by prolonged exposure to elevated glucose or sodium butyrate in vivo does not appear to be the causative agent for delayed lung maturity which frequently occurs in infants of diabetic mothers.

Maternal hypoglycemia during pregnancy in type 1 diabetes: maternal and fetal consequences

There is strong evidence that the avoidance of hyperglycemia is essential inoptimizing pregnancy outcome in type 1 diabetes. The price to pay is a striking increase in severe hypoglycemia (SH), defined as episodes requiring help from another person. During type 1 diabetic pregnancy, occurrence rates of SH up to 15 times higher as in the intensively treated group of the Diabetes Control and Complications Trial (DCCT) are reported. Blood glucose (BG) treatment targets differ considerably between clinics; some authors advocate lower limits as low as 3.3 mmol/l. Improved glycemic control and/or recurrent hypoglycemia (i.e. BG <3.9 mmol/l) may result in impairment of glucose counterregulatory responses. Also, glucose counterregulation may be altered by pregnancy itself. Short-acting insulin analogs may help reduce hypoglycemia with preservation of good glycemic control, but their use during pregnancy has yet to be proven safe.Several clinical studies did not establish an association between maternal hypoglycemia and diabetic embryopathy. However, animal studies clearly indicate that hypoglycemia is potentially teratogenic during organogenesis. Increased rates of macrosomia continue to be observed despite near normal HbA(1c) levels. This may, at least in part, be the result of rebound hyperglycemia elicited by hypoglycemia. Exposure to hypoglycemia in utero may have long-term effects on offspring including neuropsychological defects. It is yet unclear to what extent the benefits of tight glycemic control balance with the increased risk of (severe) hypoglycemia during type 1 diabetic pregnancy. Efforts must be made to avoid low BG, i.e. <3.9 mmol/l, when tightening glycemic control.