Fig. 3
Metabolic water production in activated neurons. Given the rapid fluctuations in neuronal activity, it is not feasible to rely on a timely increase in oxygen delivery with each sudden surge. Therefore, the energetic needs are met by rapid rises in glycolysis. Glycolysis of 1 glucose (Glc) molecule produces 4 ATP, 2 of which replace the ATP used to produce glucose-6P (Glc-6P) and fructose-1,6-bisphosphate (Fru-1,6P2). The remaining 2 ATP and the 2 glycolytic H2O are assigned to cation pumping indicated in the upper part of the figure, along with ATP/H2O from oxidative phosphorylation (OxPhos) in mitochondria (Mito) shown in the lower part of the figure. The net 8 H2O from the electron transport chain (ETC) and TCA cycle can be secreted. About half of the pyruvate is converted to lactate and released from the cell (CMRglc-nonox), and the other half is oxidized (CMRglc-ox). Rates of water production and secretion are calculated as the product of the molecules of H2O per pathway times the respective metabolic rate as shown in the right part of the figure (also see Table 1)