Differences in rates of incorporation of intravenously injected radiolabeled fatty acids into phospholipids of intracerebrally implanted tumor and brain in awake rats

Abstract

This study investigates the incorporation of three intravenously administered radiolabeled fatty acids, [9,10-3H]palmitate (3H-PAM), [1-14C]arachidonate (14C-ACH) and [1-14C]docosahexaenoate (14C-DHA), into lipids of intracerebrally implanted tumor and contralateral brain cortex in awake rats. A suspension of Walker 256 carcinosarcoma cells (1 x 10(6) cells) was implanted into the right cerebral hemisphere of an 8- to 9-week-old Fischer-344 rat. Seven days later, the awake rat was infused intravenously for 5 min with 3H-PAM (6.4 mCi/kg), 14C-ACH (170 microCi/kg) or 14C-DHA (100 microCi/kg). Twenty min after the start of infusion, the rat was killed and intracranial tumor mass and brain cortex were removed for lipids analysis. Each radiolabel was incorporated more into tumor than into brain cortex. Ratios of net incorporation rate coefficients (k*) into tumor as compared with brain were 4.5, 3.4 and 1.7 for 3H-PAM, 14C-ACH and 14C-DHA, respectively. Lipid radioactivity comprised more than 80% of total tumor or brain radioactivity for each probe. Phospholipids contained 58%, 89% and 68% of tumor lipid radioactivity, and 58%, 82% and 74% of brain lipid radioactivity, for 3H-PAM, 14C-ACH and 14C-DHA, respectively. Incorporation coefficients (k*i) for a phospholipid class (i)--choline phosphoglycerides (PC), inositol monophosphoglycerides (PI), ethanolamine phosphoglycerides (PE), serine phosphoglycerides (PS), and sphingomyelin (SM)--were greater in tumor than in brain for each fatty acid probe, except that values for k*PE and k*PS using 14C-DHA were equivalent. Differences in k*i between tumor and brain were largest for SM and PC and the change in k*PC accounted for 65-90% of the increase in the net phospholipid incorporation rate for each probe. Differences in k*PI, k*PE and k*PS were smaller than those in were smaller than those in k*PC and k*SM, and varied with the probe. Differences in k*i were related to differences in tumor and brain phospholipid composition and metabolism. The results indicate that suitably radiolabeled fatty acids may be used to image and characterize metabolism of lipid compartments of a brain tumor in vivo using positron emission tomography.