The uptake of octanoate in rat brain and its metabolism were investigated by means of intravenously injecting [1-11C] or [1-14C]octanoate as a tracer. The radioactivity in the cerebrum was increased by an injection of [1-11C]octanoate, and reached its peak level (0.33% ID/g) in about 2 to 5 min, and then decreased slowly. The cerebrum-to-blood ratio of the radioactivity increased with time over a period of 30 min. At 30 sec, [1-11C]octanoate that remained unchanged in the cerebrum accounted for only 8% of the total radioactivity, in spite of there being about 90% in the blood. By means of an injection of [1-14C]octanoate, more than 70% of the total radioactivity in the cerebrum was found to be attributable to radiolabeled glutamate and glutamine at each time point measured between 30 sec and 30 min. The results show that [1-11C]octanoate enters rat brain easily and is trapped in the cerebrum, probably in the form of glutamate and glutamine, and the usefulness of [1-11C]octanoate as a radiopharmaceutical for studying brain fatty acid metabolism by positron emission tomography is therefore suggested.

Octanoate is taken up into the brain and is converted in astrocytes to glutamine through the TCA cycle after beta-oxidation. We speculate that [1-11C]octanoate may be used as a tracer for astroglial functions and/or fatty acid metabolism in the brain and may be useful for studying cerebral ischemia. In the present study we investigated brain distribution of [1-11C]octanoate and compared it with cerebral blood flow (CBF) by using rat and canine models of middle cerebral artery (MCA) occlusion and a high resolution PET. In rats brain distribution of [15O]H2O measured 1-2 h and 5-6 h after insult was compared with that of [1-11C]octanoate measured 3-4 h after insult. Radioactivity ratios of lesioned to normal hemispheres determined with [15O]H2O were lower than those determined with [1-11C]octanoate. These results were confirmed by a study on a canine model of MCA-occlusion. Twenty-four hours after insult, CBF decreased in the MCA-territory of the occluded hemisphere, whereas normal or higher accumulation of [1-11C]octanoate was observed in the ischemic regions. The uptake of [1-11C]octanoate-derived radioactivity therefore increased relative to CBF in the ischemic regions, indicating that [1-11C]octanoate provides functional information different from CBF. In conclusion, we found that [1-11C]octanoate is a potential radiopharmaceutical for studying the pathophysiology of cerebral ischemia.

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We studied the chemical kinetics of the Grignard reaction between heptylmagnesium bromide and carbon dioxide with a new experimental method utilizing positron emitter C-11 labeled CO 2 . A series of experiments was carried out in the presence of a large excess of heptylmagnesium bromide (C 7 H 15 MgBr ) ca. 1.0 × 10 -4 mol) relative to carbon dioxide (CO 2 ) ca. 1.2 × 10 -8 mol). An empirical scheme was assumed based on the results obtained in experiments at the reaction temperature of 20°C. Differential equations were determined by the scheme. It was possible to calculate rate constant values at each temperature of the reaction (at 10, 20, and 30°C) by fitting the equations to experimental measurements. We obtained the value 1.76 × 10 4 J/mol as the activation energy of the main reaction between C 7 H 15 MgBr and CO 2 from the Arrhenius plot (r 2 ) 0.9986) of the rate constants. The reaction simulated with the equations and the activation energy value were in good agreement with the ones obtained experimentally. The obtained value for the activation energy of the main reaction would be applicable to the more common type of Grignard carboxylation with a large excess of CO 2 .