The effects of low glucose on catecholamine secretion from neonatal adrenomedullary chromaffin cells

Abstract

<p>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 in the first few hours of life. This hypoglycemia places the newborn at increased risk of brain damage, especially in newborns of diabetic mothers, premature infants, and newborns small-for-gestational-age. In newborns hypoglycemia evokes a surge in circulating catecholamines and glucagon and a fall in insulin, which triggers the breakdown of glycogen in liver hepatocytes. While this counterregulatory hormonal surge occurs in adult mammals the mechanisms of glucose-sensing differ. In adults, neurons in the ventromedial hypothalamus sense glucose and initiate the sympathoadrenal release of catecholamines; however the adrenal gland is not functionally mature until several weeks after birth. Moreover, while glucose acts directly on adult pancreatic a cells to elicit glucagon secretion, these cells are glucose-insensitive until several weeks after birth. Thus, I proposed that neonatal adrenomedullary chromaffin cells (AMC) are direct glucose-sensors which secrete catecholamines in response to low glucose in the perinatal period.</p> <p>I used carbon fiber amperometry to measure quantal catecholamine release in response to low glucose (aglycemia and hypoglycemia). Low glucose led to a robust secretory response in a subset of AMCs, however a significant number of AMCs were glucose-insensitive. Despite this fact, these glucose-insensitive AMCs were healthy as evidenced by their robust high K⁺-evoked secretion, which was not significantly different than the high K⁺-evoked response in glucose-sensitive AMCs. Aglycemia increased the quantal frequency and mean quantal size leading to an increase in catecholamine secretion. On the other hand, hypoglycemia (3 mM glucose) increased quantal frequency and had no effect on quantal size.</p> <p>I tested for the requirement of voltage-gated Ca²⁺ channel (VGCC) activation in the aglycemia response. The general VGCC blocker Ni²⁺ (2 mM) abolished the aglycemia-evoked secretory response, and moreover, low concentrations of Ni²⁺ (50 μM) blocked the aglycemia-evoked secretory response indicating the involvement of T-type VGCCs. The specific L-type VGCC blocker nifedipine (10 μM) also inhibited the aglycemia-evoked response, indicating that both the L-type and T-type VGCC are necessary for the secretory response during aglycemia.</p> <p>I developed a fresh thin slice adrenal gland preparation to study the glucose-sensitivity of adrenal glands in slices. This limits phenotypic changes which occur in culture and maintains the integrity of the tissue. In preliminary studies using neonatal adrenal thin slices aglycemia-evoked catecholamine secretion was detected. Aglycemia increased the event frequency but had no effect on quantal size.</p>