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Independence of extracellular tortuosity and volume fraction during osmotic challenge in rat neocortex
June Kume-Kick, TomásMazel*, Ivan Vorísek*, Sabina Hrabetová, Lian Tao and Charles Nicholson
Department of Physiology and Neuroscience, NYU School of Medicine, 550 First Avenue, New York, NY 10016, USA, *Department of Neuroscience, 2nd Medical Faculty, Charles University and Institute of Experimental Medicine AS CR, Vídenská 1083, 142 20 Prague, Czech Republic
The structural properties of brain extracellular space (ECS) are summarised
by the tortuosity (lambda) and the volume fraction (alpha). To determine
if these two parameters were independent, we varied the size of the ECS
by changing the NaCl content to alter osmolality of bathing media for rat
cortical slices. Values of alpha and lambda were extracted from diffusion
measurements using the real-time ionophoretic method with tetramethylammonium
(TMA+). In normal medium (305 mosmol kg-1), the average value of lambda
was 1.69 and of alpha was 0.24. Reducing osmolality to 150 mosmol kg-1,
lambda increased to 1.86 and alpha decreased to 0.12. Increasing
osmolality to 350 mosmol kg-1, lambda reduced to about 1.67 where
it remained unchanged even when osmolality increased further to 500 mosmol
kg-1. In contrast, alpha increased steadily to 0.42 as osmolality increased.
Comparison with previously published experiments employing 3000 Mr dextran
to measure lambda, showed the same behaviour as for TMA+, including the
same constant lambda in hypertonic media but with a steeper slope in the
hypotonic solutions. These data show that alpha and lambda behave differently
as the ECS geometry varies. When alpha decreases, lambda increases but
when alpha increases, lambda rapidly attains a constant value. A previous
model allowing cellular shape to alter during osmotic challenge can account
qualitatively for the plateau behaviour of lambda.
