Non-invasive measurement of solute permeability of rat pial microvessels

To quantify the permeability of rat pial microvessels, we measured the apparent microvessel permeability (P) to various sized solutes. Rats (SD, 250-300 g) were anesthetized with sodium pentobarbital given subcutaneously and kept warm on a heating pad. The pial microcirculation was observed by a high numerical aperture objective lens through a section of frontoparietal bones thinned with a high-speed micro-grinder (revised method from Easton et al., 1997). During the measurement, the surface of the brain was constantly superfused with 37 degrees Celsius artificial cerebrospinal fluid. Sodium fluorescein (MW 376, Stokes radius 0.45 nm) with concentration 0.5 mg/ml or FITC-dextran (MW 4000, Stokes radius 1.8 nm) with concentration 1 mg/ml in 1% BSA mammalian Ringer, was introduced into the cerebral circulation via the ipsilateral carotid artery by a syringe pump at a constant rate of 3 ml/min. P was determined using a quantitative fluorescence imaging and analyzing method. The mean P to sodium fluorescein for 10 vessels was 1.31 (plusmn0.36 SD) times 10^-6 cm/s. This value is about one-twentieth of that found in rat mesenteric microvessels, 2.6 times 10^-5 cm/s (Fu and Shen, 2004). The mean P to FITC-Dextran for 5 vessels was 0.41 (plusmn 0.29 SD) times 10^-6 cm/s.