The Q(10) 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 (CMRO(2)) that is associated with the maintenance of cellular viability, or ''basal'' CMRO(2), than on electroencephalogram (EEG)-associated CMRO(2) or ''functional'' CMRO(2). On the basis of their findings, the authors hypothesized that the ratio of CMRO(2) over a 10 degrees C temperature range (Q(10)) for basal CMRO(2) was greater than that for functional and total CMRO(2). They tested their hypothesis by determining the Q(10) for basal CMRO(2) from 38 degrees C to 28 degrees C. They measured whole-brain cerebral blood flow (CBF) and CMRO(2) in six rats during progressive hypothermia at a brain temperature of 38 degrees C and, after induction of an isoelectric EEG signal (50 mu V/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 CMRO(2) were made while the animals were anesthetized by 0.5% isoflurane/70% N2O/30% O-2 at a brain temperature of 38 degrees C over a time span equivalent to the hypothermic group, that is, approximately 3 hours. The Q(10) for basal CMRO(2) calculated over 38 degrees C to 28 degrees C was 5.2 +/- 0.92. However, the decrease in basal CMRO(2) 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 Q(10) of 12.1, and a lower temperature sensitivity component between 30 degrees C and 28 degrees C with a Q(10) of 2.8. The combined overall Q(10) for basal CMRO 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 CMRO(2) have yet to be identified.