Dynamics of cerebral metabolism during moderate hypercapnia.

The effects of hypercapnia on the kinetics of cerebral energy metabolism were evaluated in adult rats by the closed system method of LOWRY et al. (1964). Moderate hypercapnia with a Pa_co2 of 61 torr sustained for 20 min resulted in intracellular brain acidosis (7.07-6.97). During hypercapnia the tissue content of glucose increased whereas phosphocreatine, ADP, pyruvate and lactate contents, and the lactate/pyruvate ratio decreased. The ATP/ADP ratio increased from 7.7 to 9.0; the cytoplasmic NADH/NAD + ratio decreased from 2.06 × 10^-3 to 1.49 × 10^-3. There was no change in Energy Charge. Turnover rate of phosphocreatine increased from 3.84 to 4.62 mmol/kg/min, but the turnover rates of ATP, glucose and glycogen were reduced (from 1.98 to 1.86, 6.24 to 4.80, and 3.96 to 2.94 mmol/kg/min, respectively). The utilization rate of total high energy phosphate decreased from 30.6 to 25.4 mmol/kg/min while the post-decapitation EEG during hypercapnia persisted longer than during normocapnia. These results indicate that moderate hypercapnia reduces the overall kinetic activity of cerebral energy metabolism. The steady Energy Charge suggests that the reduction in the rate of high energy phosphate use is proportionally balanced by a lowered production rate of ATP.