EVIDENCE FOR PORE-LIKE OPENING OF THE BLOOD-BRAIN-BARRIER FOLLOWING FOREBRAIN ISCHEMIA IN RATS

The nature of the delayed blood-brain barrier (BBB) opening that occur s in rats subjected to forebrain ischemia by the technique of two-vess el (carotid) occlusion plus hypovolemic hypotension (2VO ischemia) was probed by examining the simultaneous, trans-barrier movement of two h ydrophilic, normally poorly permeative solutes of markedly different m olecular size: sucrose (MW = 342) and inulin (MW approximate to 5000). Pentobarbital-anesthetized, male, Sprague-Dawley rats (342-374 g) wer e subjected to 10 min of 2VO ischemia (tympanic temperature, 37.5-38.0 degrees C); 6 h later they were reanesthetized and, along with non-is chemic controls, injected i.v. with [C-14]sucrose and [H-3]inulin. Tra nsfer constants (K(i)s) for blood-to-brain movement of the tracers and V(i)s (apparent initial volumes of tracer distribution) were determin ed for six brain regions by the multiple-time, graphical method(tracer circulation times from 3 to 30 min). V(i)s differed little or insigni ficantly between the two tracers, or between control and post-ischemic rats: the values did not suggest appreciable endothelial binding of e ither tracer that might lead to its uptake by adsorptive-phase endocyt osis. In the controls, regional K(i)s +/-: S.E.M. (nl g(-1) s(-1)) for inulin ranged from 0.18 +/- 0.04 to 0.31 +/- 0.09 and were significan tly lower(P < 0.01) than K(i)s for sucrose (1.53 +/- 0.16-1.91 +/- 0.2 9). The K-i ratio (sucrose/inulin) across brain regions (mean, 6.6; S. E.M., 0.6) was much lower than would be expected according to the conc ept that movement of most organic non-electrolytes across the intact B BB occurs by dissolution in and diffusion through endothelial cell pla sma membranes, at a rate proportional to the lipid solubility and diff usivity of the solute. This finding is interpreted as indicating that a portion of the transfer of sucrose and inulin occurred by a mechanis m other than dissolution-diffusion (e.g., via pores or vesicles). In t he post-ischemic rats, K(i)s for bath tracers were elevated significan tly (P < 0.01) in parietal cortex, striatum, hippocampus, and midbrain . The post-ischemic increases (Delta K(i)s) in these regions were grea ter for sucrose (1.90-3.31 nl g(-1) s(-1)) than for inulin (0.80-1.33) . Across brain regions the ratio between sucrose Delta K-i and inulin Delta K-i averaged 2.9 (S.E.M., 0.2), a value significantly greater th an the ratio of I that would be expected were the BBB opening due to a n enhancement of micropinocytosis and vesicular transport. The corresp ondence of the mean Delta K-i ratio with the ratio of the free diffusi on coefficients of the tracers (D-f,D-suc/D-f,D-inu = 2.9; water, 38 d egrees C) suggests that the delayed opening of the BBB following 2VO i schemia involves the formation of trans- or paracellular, aqueous pore s or channels.