Effects of radiochemical impurities on measurements of transfer constants for [14C]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 sucrose permeation of the intact BBB (control K[i]s) are highly discrepant. A potential problem with the commonly used tracer, [14C(U)]sucrose, is radiolytic generation, preuse, of radiocontaminants that might readily penetrate the BBB. How such contaminants might affect measurements of sucrose K(i)s was examined for both the intact and the ischemically injured BBB. Three stocks of [14C(U)]sucrose were studied: newly purchased ("new"), 4-year-old, and 7-year-old. A high purity (99.9%) "new" and a 2-year-old stock of [3H(fructose-1)]sucrose were also tested. Pentobarbital-anesthetized male Sprague-Dawley rats were injected i.v. with each tracer separately (six to eight rats) and K(i)s in five brain regions were measured by the multiple-time graphical method. The "new" 14C-, "new" 3H-, and 2-year-old 3H-sucrose yielded comparable K(i)s , ranging from 1.2 +/- 0.1 to 2.4 +/- 0.3 nl x g(-1) x s(-1) (mean +/- SE) across the regions. The two old stocks of 14C-sucrose yielded 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 14C-tracers revealed that each contained radioimpurities (ca. 2% in both the "new" and 4-year-old, and 9% in the 7-year-old), but that the old stocks contained larger amounts of relatively mobile (more lipophilic) impurities, which can be suspected as the main cause of the elevated K(i)s obtained. Additional rats were subjected to 10 min of cerebral ischemia, which effects a delayed BBB injury, and 6 h later the "new" 3H- and the 4-year-old 14C-sucrose were injected together. The K(i)s for both tracers were elevated by like, absolute amounts (deltaK[i]s), but by very different percentages, over their disparate baseline values in uninjured rats (for striatum and hippocampus, the most injured regions, deltaK(i)s were 3.9 to 4.4 nl x g[-1] x s[-1]). It is concluded that radiolysis of [14C(U)]sucrose yields certain labeled products that readily cross the BBB and that can seriously distort baseline K(i)s , even if present only in very small amounts. While this appears not to compromise assessment of BBB injury, definition of the authentic range of baseline, sucrose K(i)s for the rat BBB would appear to remain a challenge.