The permeability of mouse cerebral capillaries to sodium

An attempt was made to determine the sodium flux between the fast compartment of brain sodium - assumed to represent the pericapillary sodium - and plasma. Concentration of Na22 in plasma and brain of nephrectomized mice was measured from 40 sec. to 15 min. after intravenous isotope injection. Using a mathematical model of brain sodium compartments an expected course of Na22 concentration in brain corresponding to the course of plasma Na22 concentration was computed. The computed course was fitted to the actual values of brain Na22 concentration by changing the properties of the fast compartment in the model. An approximate value of sodium flux between the fast compartment and plasma determined in this way is 10.48 μEq/min. per 1 g of wet tissue. From this value the permeability coefficient of brain capillaries PNa = 0.50 × 10−5 cm/sec. was estimated.