Cerebral metabolism in hypoglycemic and hyperglycemic fetal lambs

To investigate the mechanism whereby glucose affects fetal breathing movements (FBM), cerebral metabolism was studied in 12 unanesthetized fetal lambs, during fasting induced hypoglycemia and after a subsequent fetal infusion of glucose. Preductal arterial and sagittal sinus blood samples were analyzed for glucose and oxygen concentrations and for blood gases and pH. Measurements of regional brain blood flow were made with radioactive microspheres. Maternal fasting of 24- to 36-h duration resulted in a decrease in fetal blood glucose from 1.061 ± 0.085 mmol·l^-1 to 0.664 ± 0.053 (P < 0.001). Although cerebral glucose and O₂ uptake remained unchanged, sagittal sinus CO₂ partial pressure (PCO₂) and [H⁺] were significantly decreased and may have contributed to the observed decrease in FBM. A 2-h infusion of glucose to the fetuses of fasted animals resulted in an increase in blood glucose to 2.452 ± 0.173 mmol·l^-1. Cerebral O₂ consumption was again unchanged; however, cerebral glucose consumption was significantly increased as were sagittal sinus PCO₂ and [H⁺], which may have contributed to the observed increase in FBM. These findings are consistent with the hypothesis that glucose affects the incidence of FBM in part by altering the environment of central chemoreceptors.