EVIDENCE FOR BRAIN DOCOSAHEXAENOATE RECYCLING IN THE FREE-MOVING ADULT-RAT - IMPLICATIONS FOR MEASUREMENT OF PHOSPHOLIPID-SYNTHESIS

The specific activity (SA) of unesterified docosahexaenoic acid (22:6 n-3) in the brain and arterial plasma was measured after constant intr avenous infusion of [H-3] 22:6 n-3 in the free-moving rat. Within 40-1 05 min, an apparent steady state of labeled unesterified 22:6 n-3 in p lasma and in brain was reached. However, the values of the brain to pl asma 22:6 n-3 SA ratios ranged from 0.03 to 0.05, indicating that an i sotopic equilibrium between brain and plasma was not attained. This su ggests that a considerable endogenous source of unesterified 22:6 n-3 (95-97%) (likely derived from lipid metabolism) dilutes the SA of the tracer coming from plasma. Using the SA of 22:6 in plasma instead of b rain would thus lead to a gross underestimation of the rate of phospho lipid synthesis.