Developmental disorders of glucose metabolism in infants

Glucagon, cyclic AMP, and hepatic glucose mobilization: A half‐century of uncertainty

For at least 50 years, the prevailing view has been that the adenylate cyclase (AC)/cyclic AMP (cAMP)/protein kinase A pathway is the predominant signal mediating the hepatic glucose‐mobilizing actions of glucagon. A wealth of evidence, however, supports the alternative, that the operative signal most of the time is the phospholipase C (PLC)/inositol‐phosphate (IP3)/calcium/calmodulin pathway. The evidence can be summarized as follows: (1) The consensus threshold glucagon concentration for activating AC ex vivo is 100 pM, but the statistical hepatic portal plasma glucagon concentration range, measured by RIA, is between 28 and 60 pM; (2) Within that physiological concentration range, glucagon stimulates the PLC/IP3 pathway and robustly increases glucose output without affecting the AC/cAMP pathway; (3) Activation of a latent, amplified AC/cAMP pathway at concentrations below 60 pM is very unlikely; and (4) Activation of the PLC/IP3 pathway at physiological concentrations produces intracellular effects that are similar to those produced by activation of the AC/cAMP pathway at concentrations above 100 pM, including elevated intracellular calcium and altered activities and expressions of key enzymes involved in glycogenolysis, gluconeogenesis, and glycogen synthesis. Under metabolically stressful conditions, as in the early neonate or exercising adult, plasma glucagon concentrations often exceed 100 pM, recruiting the AC/cAMP pathway and enhancing the activation of PLC/IP3 pathway to boost glucose output, adaptively meeting the elevated systemic glucose demand. Whether the AC/cAMP pathway is consistently activated in starvation or diabetes is not clear. Because the importance of glucagon in the pathogenesis of diabetes is becoming increasingly evident, it is even more urgent now to resolve lingering uncertainties and definitively establish glucagon’s true mechanism of glycemia regulation in health and disease.

Glucose production in response to glucagon is comparable in preterm AGA and SGA infants

BACKGROUND AND AIMS

Low plasma glucose concentrations are more often detected in small-for-gestational-age (SGA) than in appropriate-for-gestational-age (AGA) infants. This is ascribed to impaired glucose production due to presumed lower liver glycogen stores in SGA infants. The change in glucose production induced by glucagon is considered to be an indicator of liver glycogen content. We compared the effect of glucagon on glucose kinetics in preterm AGA and SGA infants.

METHODS

In 5 AGA and 5 SGA preterm infants (postnatal age: 3-6 days) glucose production and gluconeogenesis were measured using stable isotopes immediately before and for 1 h after a bolus of glucagon.

RESULTS

After glucagon the plasma glucose concentration and glucose production increased significantly over time (P<0.05 and P<0.0001, respectively). The changes were comparable between AGA and SGA infants. Glycogenolysis contributed 75-80% to the increase in glucose production.

CONCLUSION

The increase in glucose production after glucagon was similar in preterm AGA and SGA infants, and mainly due to an increase in glycogenolysis. Based on the assumption that glycogenolysis is an indicator of liver glycogen content, our data do not support the hypothesis that liver glycogen content is lower in preterm SGA compared to AGA infants after the first postnatal day.

Differential diagnosis and management of neonatal hypoglycemia

Persistent hypoglycemia in the neonate is most often caused by hyperinsulinemia. Recent discoveries in the molecular and biochemical regulation of insulin secretion have increased dramatically our understanding of disorders responsible for syndromes of hyperinsulinemic hypoglycemia. This article focuses on defects and disorders of the KATP channel, activating mutation of glucokinase and glutamate dehydrogenase, and other disorders that may be associated with specific phenotypes to permit appropriate targeted therapies. It is essential to evaluate these entities carefully because of the emerging evidence that at least half, if not more, have focal disease, which can be cured by local excision rather than diffuse disease, which may not be cured even after near total pancreatectomy with risk for future diabetes. Delay in diagnosis may be associated with developmental delay. The mechanisms of hypoglycemia remain incompletely understood.