Measuring Hydraulic Conductance and Hydration Potential of Brain Extracellular Matrix by Osmotic Stress

Abstract

Neuronal function depends on exquisite local regulation of a great variety of molecules and their interactions in the interstitial spaces. The timing and magnitude of interactions in the brain must depend on the hydraulic properties governing water and solute transfer across its matrix, but quantitative information on these properties has proved notoriously difficult to obtain, and uncertainties remain. Here, we adapt osmotic stress techniques to explore pressure-volume relationships and fluid-transfer parameters in brain tissue. Within 15-30 minutes of euthanasia, sets of brain tissue samples (12/brain, ∼1g each) were obtained from pigs (n =11) and immersed in baths adjusted to colloidosmotic pressures of 3-219 mmHg with polyethyleneglycol Mw 8000. The samples’ water influx/efflux was measured gravimetrically as a function of time, normalized by weight. Initial flow rates were calculated from the first derivative, at time = 0, of second-degree polynomials fitted to the progress curves. Rates were linearly proportional to the bath's pressure (r2 > 0.9). The hydraulic conductance calculated from the slope of fitted lines was 0.029 ± 0.008µl/min/g/mmHg, and the hydration potential, calculated from the pressure at initial rate = 0, was 64 ± 27 mmHg (means ± SD, n = 11).The hydraulic conductance, but not the hydration potential, decreased significantly when the bath temperature was reduced from 37 to 4°C. Compared to heart and skin, brain conductance value was higher. Further, the changes in hydraulic parameters with temperature were distinct for each organ. These results illustrate a simple, ex vivo, quantitative approach to probe specific water-transfer parameters and their changes in the brain. They indicate organ-dependent differences in flow regulation and the hydraulic properties of extracellular matrices.