Measurement of unidirectional P_ito ATP flux in human visual cortex at 7 T by usingin vivo³¹P magnetic resonance spectroscopy

Abstract

Taking advantage of the high NMR detection sensitivity and the large chemical shift dispersion offered by ultra-high field strength of 7 T, the effect of magnetization transfer on inorganic phosphate (Pi) resonance during saturation of ␥-ATP resonance, mediated by the ATP synthesis reaction, was observed noninvasively in the human primary visual cortex by using in vivo 31 P magnetic resonance spectroscopy. The unidirectional flux from Pi to ATP was measured by using progressive saturation transfer experiments. The cerebral ATP synthesis rate in the human primary visual cortex measured by 31 P magnetic resonance spectroscopy in this study was 12.1 ؎ 2.8 mol ATP͞g per min, which agreed well with the value that was calculated indirectly from the cerebral metabolic rate of glucose consumption reported previously. E nergy requirements in tissues are met predominantly through hydrolysis of the high-energy molecule ATP. In the brain, this molecule is synthesized almost exclusively by mitochondrial oxidative phosphorylation, with only a small contribution coming from nonoxidative glycolysis (1). Despite the critical role of ATP in cellular functions, the kinetics of ATP synthesis and hydrolysis is often difficult to measure directly in intact cells and even more so in intact animals and humans. Cerebral ATP synthesis rate has been calculated indirectly from experimentally measured cerebral metabolic rates of glucose (CMR glu ) and oxygen (CMRO 2 ) (1, 2), and it has been estimated to be Ϸ14 mol ATP͞g per min (2) in the resting human brain.The magnetization transfer (MT) technique based on magnetic resonance spectroscopy (MRS) has the capability of measuring the reaction kinetics of enzymes noninvasively in situ when the reaction rates involved are relatively fast (e.g., for review see refs. 3 and 4). Using this technique to study the kinetics of ATP synthesis in biological tissues with 31 P MRS was first introduced in the late 1970s in suspensions of Escherichia coli cells (5). Since then, it has been applied to yeast (6), intact perfused hearts (4, 7-11), liver (12), kidney (13), canine myocardium in vivo (14), skeletal muscle (15), and rat brain (16). It was demonstrated that, under appropriate conditions, the MT technique is capable of measuring the net rate of oxidative ATP synthesis catalyzed by mitochondrial ATPase (7-10); because, by definition, this rate is proportional to the oxygen consumption rate by the P͞O ratio, those experiments yielded for the first time the P͞O ratio in an intact organ under functional conditions.Because of the relatively low concentration of P i in normal intact tissues and the low intrinsic detection sensitivity of in vivo 31 P MRS, virtually all previous experiments using the MT methodology to study P i -ATP exchange were performed in excised perfused organ models or in small animals, where the radio frequency coil geometry relative to the organ of interest can be optimized to yield significant gains in signal-to-noise ratio compared with what is possible in humans. In addition, all ...

pH as a Biomarker of Neurodegeneration in Huntington's Disease: A Translational Rodent-Human MRS Study

“…Consequently, pH values measured in our study are likely to represent intracellular pH, under physiological as well as under pathological conditions. Recent in vivo MRS data suggest that a large majority of brain Pi is located in neurons as opposed to glial cells, based on the fact that Pi is in a compartment where oxidative metabolism strongly dominates glycolytic metabolism (Chaumeil et al, 2009;Lei et al, 2003). Altogether, these observations allow us to interpret our results as an increase in neuronal pH in HD.…”

Tightly coupled brain activity and cerebral ATP metabolic rate

“…In contrast, we have recently demonstrated that the unidirectional Pi 3 ATP rate measured by the in vivo 31 P MT approach in the resting human brain was consistent with the oxidative ATP synthesis rate (9). This finding has led us to hypothesize that the in vivo 31 P MT approach could provide a unique and completely noninvasive tool for quantita-tively determining the oxidative ATP synthesis rate in the brain, which is vital for studying the cerebral bioenergetics and brain function (9). In the present study, we test this hypothesis and attempt to establish a noninvasive 31 P MRS approach for directly and quantitatively measuring the rate of oxidative phosphorylation and its change in the brain.…”

“…The brain ATP metabolism is mainly controlled by ATPase and creatine kinase (CK) reactions that are coupled together and constitute a complex chemical exchange system involving ATP, phosphocreatine (PCr), and intracellular inorganic phosphate (Pi) (i.e., a PCr^ATP^Pi chemical exchange system) (7)(8)(9)(10). One vital function of this ATP metabolic network is to maintain a stable cellular ATP concentration by adjusting the reaction rates to ensure a continuous energy supply for sustaining electrophysiological activity and maintaining normal function in the brain.…”

“…Logically, both the kinetics of ATP metabolism and the associated chemical exchange rates should be more sensitive to the brain activity and energy states than the steady-state ATP concentration; they should provide an essential measure of cerebral bioenergetics under different brain activity states. The sole approach able to noninvasively and directly assess the cerebral ATP metabolic rates is in vivo 31 P magnetic resonance spectroscopy (MRS) combined with the magnetization transfer (MT) method (9)(10)(11)(12)(13)(14)(15)(16).…”

The Oxygen Paradox of Neurovascular Coupling

“…of arterial blood 9,000 nmoL/mL Glucose conc. of arterial blood 5,000-6,000 nmoL/mL 53 Oxygen extraction fraction 30%-55% 3,5,6,27,75,93 Glucose extraction fraction 8%-15% ATP concentration of brain tissue 1,000-3,000 nmoL/mL 124,137,138 Total ADP concentration of brain tissue 300 nmoL/mL 124,138 Free ADP concentration of brain tissue 30-35 nmoL/mL [121][122][123] PCr concentration of brain tissue 4,000-5,000 nmoL/mL 123,124,138 Diffusion constant for O2 in brain 78 Assuming mitochondrial oxygen tension close to zero (meaning so low that any further reduction would limit oxidative metabolism), the driving force for oxygen delivery, the gradient between the capillary and mitochondria, can only be increased by an increase in capillary oxygen tension, which in turn requires a decrease of the oxygen extraction fraction. Later, the observation of constant CMRO 2 during pharmacological reductions of CBF led to a modification of the model by incorporating potential changes of cytochrome c oxidase affinity to oxygen during increases in neuronal activity.…”