Fig. 5
Metabolic water production and secretion from glucose utilization in rat and human brain during rest and activation. Data are from Table 1. a. Cerebral cortical CMRglc at rest is higher in rats than in humans, and in both species CMRglc increases during activation. b. Total metabolic water generated by glucose metabolism is lower during rises in activity than during rest due to lower oxidation rates (Table 1) and higher activity of cation pumps that use water for hydrolysis of ATP. c. As the increase in CMRglc is mainly due to glycolysis, the amount of water secreted from glucose utilization decreases during activation. d. Metabolic water production during rest and activation in rat and human brains for neurons and astrocytes. For both cell types the main amount of metabolic water is produced in mitochondria. The amount of secreted water is reduced by about half during stimulation because less glucose is oxidized compared with rest and more water is consumed by ATP hydrolysis during cation pumping (Table 1, Figs. 3, 4). The percentage distributions of total and secreted metabolic water are presented as the same in rat and human brain during rest (Fig. 5d), because the rat values are based on resting human data due to a limited number of MRS studies in the awake rat. Of the reports tabulated in reference [46] in its Supplemental Information (SI) Tables SI-4 and SI-5, nine of 10 MRS studies used anesthesia that alters metabolism. The whole brain CMRglc (0.91µmol/g/min, Table 1) in the single awake rat study [142] was ~ 30% higher than that determined by arteriovenous difference or weighted average of brain regions in [14C]deoxyglucose assays (mean, 0.68 µmol/g/min, Table 1). In reference [142] the whole brain CMRglc and the calculated cellular proportions (36% astrocyte, 63% neuron) of glucose oxidation rates are questioned because the rats were decapitated and heads frozen in liquid nitrogen, thereby causing postmortem ischemia that disrupts metabolism [143] (also see Table 1 footnotes). For this reason the astrocyte-neuron partitioning obtained in resting human brain (mean of 19% astrocyte, 81% neuron in 7 studies; Tables SI-6, 7, 9 in reference [46]) was used for calculations and figures in the resting state (Table 1, Fig. 5). Although the percent distributions in resting rat and human brain are the same, the absolute values differ (Table 1)