EFFECTS OF RADIOCHEMICAL IMPURITIES ON MEASUREMENTS OF TRANSFER CONSTANTS FOR [C-14] SUCROSE PERMEATION OF NORMAL AND INJURED BLOOD-BRAIN-BARRIER OF RATS

Radiolabeled sucrose is often used to assess blood-brain barrier (BBB) injury in the rat, but published transfer constants (K(i)s) for sucro se permeation of the intact BBB (control K(i)s) are highly discrepant. A potential problem with the commonly used tracer, [C-14(U)]sucrose, is radiolytic generation, preuse, of radiocontaminants that might read ily penetrate the BBB. How such contaminants might affect measurements of sucrose K(i)s was examined for both the intact and the ischemicall y injured BBB. Three stocks of [C-14(U)]sucrose were studied: newly pu rchased (''new''), 4-year-old, and 7-year-old. A high purity (99.9%) ' 'new'' and a 2-year-old stock of [H-3(fructose-l)]sucrose were also te sted. Pentobarbital-anesthetized male Sprague-Dawley rats were injecte d IV with each tracer separately (six to eight rats) and K(i)s in five brain regions were measured by the multiple-time graphical method. Th e ''new'' C-14-, ''new'' H-3-, and 2-year-old H-3-sucrose yielded comp arable K(i)s, ranging from 1.2 +/- 0.1 to 2.4 +/- 0.3 nl.g(-1).s(-1) ( mean +/- SE) across the regions. The two old stocks of C-14-sucrose yi elded significantly higher regional K(i)s: 5.1-6.3 (4-year-old) and 8. 4-9.7 (7-year-old). Thin-layer chromatography of the three C-14-tracer s revealed that each contained radioimpurities (ca, 2% in both the ''n ew'' and 4-year-old, and 9% in the 7-year-old), but that the old stock s contained larger amounts of relatively mobile (more lipophilic) impu rities, which can be suspected as the main cause of the elevated K(i)s obtained. Additional rats were subjected to 10 min of cerebral ischem ia, which effects a delayed BBB injury, and 6 h later the ''new'' H-3- and the 4-year-old C-14-sucrose were injected together. The K(i)s for both tracers were elevated by like, absolute amounts (Delta K(i)s), b ut by very different percentages, over their disparate baseline values in uninjured rats (for striatum and hippocampus, the most injured reg ions, Delta K(i)s were 3.9 to 4.4 nl.g(-1).s(-1)). It is concluded tha t radiolysis of [C-14(U)]sucrose yields certain labeled products that readily cross the BBB and that can seriously distort baseline K(i)s, e ven if present only in very small amounts. While this appears not to c ompromise assessment of BBB injury, definition of the authentic range of baseline, sucrose K(i)s for the rat BBB would appear to remain a ch allenge. (C) 1998 Elsevier Science Inc.