PROTEIN ACCUMULATION IN CEREBROSPINAL-FLUID DURING -90-DEGREES HEAD-DOWN TILT IN RABBIT

Plasma proteins are only somewhat larger than the intercellular spaces of the cerebral microvessels that constitute the blood-brain barrier or of the choroid plexus villi that elaborate cerebrospinal fluid (CSF ). We hypothesized that the integrity of these barriers in anesthetize d rabbits might be compromised during head-down tilt (HDT). Plasma pro tein and osmolality, hematocrit, and CSF protein concentration were co mpared in rabbits exposed to 1 h of HDT (n = 20) and prone rabbits (n = 10). In addition, the concentration of trypan blue dye, injected int ravenously at the end of HDT or the prone position, was measured in br ain homogenate. Finally, arterial blood pressure was measured via a ca theterized carotid artery. HDT disrupted the barrier between blood and CSF, as indicated by a significantly (P < 0.01) greater brain trypan blue concentration in the HDT rabbits [172.2 +/- 14.4 (SD) mu g/g dry wt] than in the prone rabbits (29.8 +/- 4.4 mu g/g dry wt). Moreover, CSF protein 5 min after HDT onset was significantly increased compared with control in HDT rabbits (54.6 +/- 1.9 vs. 81.4 +/- 5.2 mg/dl; n = 8) but not in prone rabbits (55.6 +/- 2.7 vs. 57.2 +/- 5.0 mg/dl; n = 6). Changes in the plasma protein-to-hematocrit ratio in the HDT anim als, but not in the prone animals, were also compatible with a loss of fluid from the vascular compartment. Because arterial blood pressure was elevated in the HDT compared with the prone rabbits, it appears li kely that the intercellular spaces of the cerebral microvessels and/or choroid plexus widened as a result of the acute hypertension, permitt ing the leakage of protein from blood to brain.