| Regional Biophysics Meeting 2005, March 16-20, Zreče, Slovenia | [CellBiophys] |
Astrocytes release glutamate under various physiological and pathological conditions. Despite intense research, however, the mechanism of glutamate release, whether exocytotic or non-exocytotic is still actively debated. Recently, the release of glutamate during hypotonically-induced astrocyte swelling has been reported, but the exact mechanism is not entirely clear. To examine the possible involvement of regulated exocytosis in hypotonic swelling we monitored membrane capacitance by the whole-cell patch-clamp technique while the osmolarity of the bathing medium was reduced. Membrane capacitance is a parameter proportional to the membrane surface area, which increases when exocytosis, the fusion of vesicle with the plasma membrane, is activated. To test whether a hypotonic shock induces a shape and/or volume change in astrocytes, the image cross-section area of the patched astrocytes was measured. After exposure to the hypotonic solution of 210 mOsm for 200 s the astrocytes swelled up by an average of 40 %. The simultaneous capacitance measurements showed however no increase of membrane area during cell swelling. In contrast to the lack of association between the cell swelling and membrane capacitance changes, hypotonically-induced membrane currents were clearly associated with astrocyte volume changes. We conclude that cell swelling activates membrane conductance but not the exocytotic fusion of vesicles with the plasma membrane. Therefore the hypotonicity-induced glutamate release reported by other researchers is likely mediated by a non-exocytotic mechanism.
Email: tina.pangrsic@mf.uni-lj.si
Address: Inštitut za patološko fiziologijo, Medicinska fakulteta, Zaloška 4, 1000 Ljubljana