Table 2. Glucose buffering capacity and ATP yields from up-regulation of glycolysis and oxidation.
| ATP yield* (μmol·g−1·min−1) | |||||
|---|---|---|---|---|---|
| Physiological state* | CMRglc* (μmol·g−1·min−1) | Glucose consumed† in 30 s (μmol·g−1) | CMRO2* (μmol·g−1·min−1) | Glycolysis | Oxidation |
| Rest | 1 | 0.5 | 6 | 2 | 192 |
| Activation: 5% | 1.05 | 0.525 | 6.3 | 2.1 | 201.6‡ |
| 10% | 1.1 | 0.55 | 6.6 | 2.2 | 211.2 |
| 30% | 1.3 | 0.65 | 7.8 | 2.6 | 249.6 |
| 50% | 1.5 | 0.75 | 9 | 3 | 288 |
| 100% | 2 | 1† | 12 | 4‡ | 384 |
The resting rate of glucose utilization rate (CMRglc) of 1 μmol·g−1·min−1 approximates the mean value for the entire rat cerebral cortex. For the purpose of illustration, this resting rate is associated with a ratio of oxygen/glucose consumption (CMRO2/CMRglc) set equal to 6, the maximal theoretical molar ratio (1 glucose+6O2 → 6CO2+6H2O). ATP yields were calculated for rest and different hypothetical levels of metabolic activation using 2ATP per glucose for the glycolytic pathway and 32 ATP per glucose for the oxidative pathway.
Normal rat brain glucose concentration is approximately 20–25% of that in arterial plasma, and is generally within the range of 2–3 μmol/g in non-fasted adult animals (Dienel et al., 1991; Holden et al., 1991). At steady state during rest, net glucose delivery matches glucose utilization rate and at rest would be equal to 1 μmol·g−1·min−1. Note that even if glucose delivery does not rise to match increased demand for glucose during activation to evoke a 100% increase in CMRglc there is enough unmetabolized glucose in brain tissue to support this high metabolic rate for 1 min and still maintain the brain glucose level above 1 μmol/g and saturate hexokinase (Km∼0.05 mmol/l; Grossbard and Schimke, 1966). Thus, if brain glucose level is 2 μmol/g and 1 μmol/g is delivered in 1 min, consumption of 2 μmol/g in 1 min due to a 100% increase in CMRglc would still leave 1 μmol/g unmetabolized glucose in brain.
Note that the net ATP yield from a 5% increase in CMRO2 (201.6–192 = 9.6) is 4.8 times greater than from a 100% rise in glycolysis (4−2 = 2) in astrocytes and in neurons, and this oxygen-glucose metabolic mismatch would be associated with an overall fall in the CMRO2/CMRglc ratio from 6 to 3.15 (i.e., 6.3 divided by 2).