The effects of ventricular fluid osmolality on the bulk flow of nascent fluid into the cerebral ventricles of anesthetized cats was measured during ventriculocisternal perfusion. This nascent fluid consists of both cerebrospinal fluid (CSF) and fluid which results from an osmotic gradient between ventricular fluid and the blood and/or brain. Perfusions were carried out with both mock CSF and with solutions containing either sucrose, urea, or NaCl. Differences between the normal bulk flow rate of nascent CSF and bulk flow rate measured during perfusion with anisotonic solutions were linearly related to corresponding differences in osmolality of the effluent fluid from the ventricles. The coefficients of somotic flow using sucrose (0.231 mul/min per mOsm) and NaCl (0.224) were similar, and greater than that using urea (0.156). During perfusion with sucrose when effluent osmolality increased by 200 mOsm (63% of normal), bulk flow rate of nascent fluid increased by 50 mul/min (200% of normal). Flow was undetectable when the effluent osmolality was 190 mOsm (decrease of 135 mOsm), although osmotically active particles continued to enter the ventricular system. Intravenous injection of acetazolamide reduced these coefficients to similar values of 0.0963 for NaCl, and 0.0955 for urea. In all experimental conditions no changes were found in cerebral water content. These results suggest that the increased bulk flow which occurs during perfusion with hypertonic solutions originates from the choroid plexus.
The effects of changes in serum osmolarity on the rate and osmolarity of bulk flow of fluid into the cerebral ventricles and on cortical white and grey matter water content were studied in cats. Bulk flow rates and osmolarities were measured during ventriculocisternal perfusion both before and after intravenous infusion of glucose solutions. Infusions of glucose in concentrations greater than 6% decreased fluid bulk flow rate and its osmolarity. Glucose in concentrations less than 6 percent increased fluid bulk flow rate and decreased its osmolarity. Bulk flow rate and serum osmolarity were found to be linearly related with a coefficient of osmotic flow of minus 0.835 mul/min per mOsm/l. At the extremes of induced serum osmolarities, (290 and 360 mOsm/l) bulk flow rate was either increased by 120 percent or completely inhibited. Effluent osmolarity also increased proportionately to serum osmolarity (0.338 mOsm/l per mOsm/l). When compared to controls, cortical grey and white matter water content increased by 1.9 percent and 2.9 percent, respectively, when the infused glucose concentration was 2.5 percent or less, and decreased by 1.8 percent and 2.9 percent when the concentration was 10 percent or more. The results of these experiments suggest that the increased bulk flow comes from the brain, rather then directly from the blood.
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