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Table 1 Calculated rates of cellular metabolic water production during rest and activation

From: A budget for brain metabolic water production by glucose catabolism during rest, rises in activity and sleep

Species, brain region, pathway, cell type, and reference

CMRglc (µmol/g/min)

Total H2O/Glca (µL/g/min)

Secreted H2O/Glcb (µL/g/min)

Total H2O/Glc (ml/brain/day)

Secreted H2O/Glc (ml/brain/day)

Rat

Restc

Stim.d

Rest

Stim

Rest

Stim

Rest

Rest

Whole brain

 CBF x AVglc [146, 147]

0.67

 

0.46

 

0.12

 

1.31

0.35

 Weighted average [14C]DG [148]

0.69

 

0.47

 

0.12

 

1.35

0.36

 [13C]Glc MRS e [142]

0.91

 

0.62

 

0.16

 

1.78

0.47

  CMRglc-ox-a

0.33

 

0.22

 

0.06

 

0.65

0.17

  CMRglc-ox-n

0.57

 

0.39

 

0.10

 

1.12

0.30

Cerebral cortex, [6-14C]Glc f [111]

0.74

0.93

0.50

 

0.13

 

0.73

0.19

  CMRglc-ox-a

0.14

0

0.09

 

0.02

0

0.14

0.04

  CMRglc-ox-n

0.60

0.47

0.41

0.32

0.11

0.07

0.59

0.15

  CMRglc-nonox-n

0

0.47

 

0.02

 

0

  

  CMRglycogen-nonox

0

0.54

0

0.02

0

0

  

Human

Whole brain

        

 CBF x AVglc [32]

0.23

 

0.16

 

0.04

 

315

84

 [18F]FDG PET [149]

0.23

 

0.16

 

0.04

 

315

84

 [18F]FDG PET-gray matter [150]

0.28

 

0.19

 

0.05

 

384

102

Cerebral cortex [13C]Glc MRS g [46]

0.37

0.49

0.25

 

0.07

 

380

101

  CMRglc-ox-a

0.07

0

0.05

 

0.01

0

72

19

  CMRglc-nonox-a

0

       

  CMRglc-ox-n

0.30

0.25

0.20

0.17

0.06

0.03

308

82

  CMRglc-nonox-n

0

0.25

 

0.01

0

0

  
  1. CMRglc, rate of glucose utilization; where subscripts denote the pathway and cell type of Glc metabolism: -ox, oxidized; nonox, nonoxidized, -a astrocyte, -n, neuron; Rest denotes the resting (physically inactive), awake, non-stimulated subject; Stim., sensory stimulation to activate metabolism in cortical regions; CBF, cerebral blood flow rate; AVglc, arteriovenous difference for glucose; DG, 2-deoxyglucose; Glc, glucose; MRS, magnetic resonance spectroscopy; FDG, fluorodeoxyglucose; PET, positron emission tomography
  2. aRates of total water production were calculated as the product of metabolic rate in that pathway and the water formed per glucose, then converted to volume: (µmol Glc/g/min)*(X µmol H2O produced per pathway/Glc)*(18 µg H2O/µmol H2O)*(10–3 µL/µg H2O), where X = 38 for oxidation and 2 for glycolysis during rest and activation for both neurons and astrocytes. Total and secreted water per day were calculated based on rat whole brain and cerebral cortex weights of 2 g and 1 g, respectively. The relative size of human cerebral cortex is about 75% of the entire brain mass or volume [151], i.e., 1050 g for a 1400 g brain
  3. bRates of secreted water production were calculated as: (µmol Glc/g/min)*(X µmol H2O secreted per pathway/Glc)*(18 µg H2O/µmol H2O)*(10–3 µL/µg H2O), where X = 10 for oxidation during rest, 8 for oxidation during stimulation, and 0 for glycolysis during stimulation
  4. cIn resting, awake, non-stimulated subjects ~ 90–95% of the glucose is oxidized; for simplicity, nonoxidative metabolism is assigned to zero and CMRglc = CMRglc-ox
  5. dSensory stimulation and physical activity increase brain metabolism with a disproportionate rise in non-oxidative metabolism in rats [110, 146] and humans [32, 33]. Based on the glucose sparing by glycogenolysis model [46], glycogen is the sole fuel for astrocytes during stimulation and CMRglc-a is zero. Accordingly, measured CMRglc is assigned to neurons, with about half oxidized and half nonoxidized
  6. eThe calculated resting value for whole brain CMRglc is ~ 30% higher than other reports, perhaps influenced by postmortem ischemic changes in metabolite levels (due to decapitation followed by freezing the heads in liquid nitrogen) that may affect calculated pathway rates (Also see Fig. 5 legend)
  7. fThe cellular basis of CMRglc at rest was not measured in this study and the proportion oxidized by astrocytes (18%) and neurons (82%) was based on the mean CMRglc-ox-n (0.30) for human brain tabulated in the Supplemental Information (SI) Table SI-9 of reference [46] and the mean CMRglc-ox-a (0.07) reported in the references cited in Table SI-9. During activation measured CMRglc was assigned to neurons and glycogenolysis to astrocytes. Our previous studies [40, 43] demonstrated upregulation of nonoxidative metabolism that during activation to produce labeled and unlabeled lactate that is rapidly released from the brain. Rapid lactate release causes underestimation of CMRglc and the non-oxidized fraction in [14C]DG assays in rats and in [13C]glucose MRS studies in rats and humans (see text)
  8. gThe glucose utilization rates in neurons and astrocytes are mean values of literature reports summarized in reference [46] in their Supplemental Information (SI) Tables SI-6, SI-7, SI-8, and SI-9. Anesthesia alters metabolism, and only values from awake subjects were included in the means
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Fluids and Barriers of the CNS

ISSN: 2045-8118