Maternal and fetal metabolic effects of prolonged ritodrine infusion.

To investigate metabolic and endocrine changes in the fetus during prolonged maternal tocolysis with beta sympathomimetic drugs, ritodrine hydrochloride (2.1 micrograms/kg per minute) was infused into pregnant sheep near term. Confirming earlier studies, maternal plasma metabolite and hormone levels changed greatly during the first six to eight hours of infusion. Changes in the fetus paralleled these closely: glucose, lactate, and insulin increased sharply, but glucagon and alpha-amino acid nitrogen decreased. After this, most maternal and fetal plasma metabolite and hormone levels returned to the normal range and were unchanged by infusion for 72 to 96 hours. Fetal lactate levels, however, remained elevated. Similar changes occurred during interrupted maternal infusions of ritodrine. Prolonged infusion of ritodrine leads to diminished responsiveness in beta-adrenergic mechanisms regulating maternal plasma metabolite and hormone levels. Comparable unresponsiveness of fetal beta-adrenergic mechanisms, though less certain, could increase hazards during delivery and adaptation to postnatal life.