Neurovascular coupling is optimized to compensate for the increase in proton production from nonoxidative glycolysis and glycogenolysis during brain activation and maintain homeostasis of pH, pCO2, and pO2

Abstract

During transient brain activation cerebral blood flow (CBF) increases substantially more than cerebral metabolic rate of oxygen consumption (CMRO₂) resulting in blood hyperoxygenation, the basis of BOLD-fMRI contrast. Explanations for the high CBF versus CMRO₂ slope, termed neurovascular coupling (NVC) constant, focused on maintenance of tissue oxygenation to support mitochondrial ATP production. However, paradoxically the brain has a 3-fold lower oxygen extraction fraction (OEF) than other organs with high energy requirements, like heart and muscle during exercise. Here, we hypothesize that the NVC constant and the capillary oxygen mass transfer coefficient (which in combination determine OEF) are co-regulated during activation to maintain simultaneous homeostasis of pH and partial pressure of CO₂ and O₂ (pCO₂ and pO₂). To test our hypothesis, we developed an arteriovenous flux balance model for calculating blood and brain pH, pCO₂, and pO₂ as a function of baseline OEF (OEF₀), CBF, CMRO₂, and proton production by nonoxidative metabolism coupled to ATP hydrolysis. Our model was validated against published brain arteriovenous difference studies and then used to calculate pH, pCO₂, and pO₂ in activated human cortex from published calibrated fMRI and PET measurements. In agreement with our hypothesis, calculated pH, pCO₂, and pO₂ remained close to constant independently of CMRO₂ in correspondence to experimental measurements of NVC and OEF₀. We also found that the optimum values of the NVC constant and OEF₀ that ensure simultaneous homeostasis of pH, pCO₂, and pO₂ were remarkably similar to their experimental values. Thus, the high NVC constant is overall determined by proton removal by CBF due to increases in nonoxidative glycolysis and glycogenolysis. These findings resolve the paradox of the brain's high CBF yet low OEF during activation, and may contribute to explaining the vulnerability of brain function to reductions in blood flow and capillary density with aging and neurovascular disease.

Keywords: aerobic glycolysis; brain activation; cerebral lactate; glucose sparing by glycogenolysis; homeostasis; neurovascular coupling.