The Q10 ratio for basal cerebral metabolic rate for oxygen in rats.

Previously the authors showed that hypothermia exerts a greater effect on the cerebral metabolic rate for oxygen (CMRO2) that is associated with the maintenance of cellular viability, or "basal" CMRO2, than on electroencephalogram (EEG)-associated CMRO2 or "functional" CMRO2. On the basis of their findings, the authors hypothesized that the ratio of CMRO2 over a 10 degrees C temperature range (Q10) for basal CMRO2 was greater than that for functional and total CMRO2. They tested their hypothesis by determining the Q10 for basal CMRO2 from 38 degrees C to 28 degrees C. They measured whole-brain cerebral blood flow (CBF) and CMRO2 in six rats during progressive hypothermia at a brain temperature of 38 degrees C and, after induction of an isoelectric EEG signal (50 microV/cm) with thiopental sodium, they repeated the measurements at 38 degrees C, 34 degrees C, 30 degrees C, and 28 degrees C. In a control group (five rats), six sequential measurements of CBF and CMRO2 were made while the animals were anesthetized by 0.5% isoflurane/70% N2O/30% O2 at a brain temperature of 38 degrees C over a time span equivalent to the hypothermic group, that is, approximately 3 hours. The Q10 for basal CMRO2 calculated over 38 degrees C to 28 degrees C was 5.2 +/- 0.92. However, the decrease in basal CMRO2 between 38 degrees C and 28 degrees C was nonlinear on a log plot, revealing a two-component response: a high temperature sensitivity component between 38 degrees C and 30 degrees C with a Q10 of 12.1, and a lower temperature sensitivity component between 30 degrees C and 28 degrees C with a Q10 of 2.8. The combined overall Q10 for basal CMRO2 between 38 degrees and 28 degrees C was 5.2. The energy-requiring processes associated with these high and low temperature sensitivity components of basal CMRO2 have yet to be identified.