EFFECT OF INTRAVENOUS GLUCOSE AND LIPID ON PROTEOLYSIS AND GLUCOSE-PRODUCTION IN NORMAL NEWBORNS

To determine whether nonprotein substrate can suppress proteolysis in normal newborns and to assess the effect of this substrate on glucose production, the rates of appear ance (R(a)) of leucine (reflecting proteolysis) and glucose were measured in healthy 2-day-old full-term newborns during fasting, an intravenous glucose infusion (5.5 mg . kg(-1). min(-1)), an intravenous lipid infusion (2.5 mg . kg(-1). min(-1)), and a combined glucose plus lipid infusion (5.5 mg . kg(-1). min(-1) glucose + 2.5 mg . kg(-1). min(-1) lipid). Leucine R, was not reduced from fasting values during any of the substrate infusions. Intravenous lipid infusion alone neither suppressed nor increased glucose production. In contrast, glucose production was nearly completely suppressed (similar to 90%) during intravenous infusions of glucose provided either alone or in combination with lipid; this suppression was achieved at glucose concentrations of similar to 90 mg/dl and insulin concentrations of similar to 6 mu U/ml. Thus normal newborns respond to intravenous glucose with sustained nearly complete suppression of glucose production, even at moderate levels of glycemia and at low insulin concentrations; however, nonprotein substrate infusion does not result in suppression of proteolysis. It remains unclear to what extent any potential regulator can suppress proteolysis in this population.