Dependence of Oxygen Delivery on Blood Flow in Rat Brain: A 7 Tesla Nuclear Magnetic Resonance Study

Abstract

Magnetic resonance imaging (MRI) and spectroscopy (MRS) were used at a magnetic field strength of 7 T to measure CBF and CMRO₂ in the sensorimotor cortex of mature rats at different levels of cortical activity. In rats maintained on morphine anesthesia, transitions to lower activity and higher activity states were produced by administration of pentobarbital and nicotine, respectively. Under basal conditions of morphine sulfate anesthesia, CBF was 0.75 ± 0.09 mL · g⁻¹ · min⁻¹ and CMRO₂ was 3.15 ± 0.18 μmol · g⁻¹ · min⁻¹. Administration of sodium pentobarbital reduced CBF and CMRO₂ by 66% ± 16% and 61% ± 6%, respectively (i.e., “deactivation”). In contrast, administration of nicotine hydrogen tartrate increased CBF and CMRO₂ by 41% ± 5% and 30% ± 3%, respectively (i.e., “activation”). The resting values of CBF and CMRO₂ for α-chloralose anesthetized rats were 0.40 ± 0.09 mL · g⁻¹ · min⁻¹ and 1.51 ± 0.06 μmol · g⁻¹ · min⁻¹, respectively. Upon forepaw stimulation, CBF and CMRO₂ were focally increased by 34% ± 10% and 26% ± 12%, respectively, above the resting nonanesthetized values (i.e., “activation”). Incremental changes in CBF and CMRO₂, when expressed as a percentage change for “deactivation” and “activation” from the respective control conditions, were linear (R² = 0.997) over the entire range examined with the global and local perturbations. This tight correlation for cerebral oxygen delivery in vivo is supported by a recent model where the consequence of a changing effective diffusivity of the capillary bed for oxygen, D, has been hypothetically shown to be linked to alterations in CMRO₂ and CBF. This assumed functional characteristic of the capillary bed can be theoretically assessed by the ratio of fractional changes in D with respect to changes in CBF, signified by Ω. A value 0.81 ± 0.23 was calculated for Ω with the in vivo data presented here, which in turn corresponds to a supposition that the effective oxygen diffusivity of the capillary bed is not constant but presumably varies to meet local requirements in oxygen demand in a similar manner with both “deactivation” and “activation.”