Cryoscopic osmometer

A microfluidic cell volume sensor was used to study volume changes in astrocytes, as previously described (Ateya et al. 2005 (link)). Astrocytes were grown on a microscope slide that was inverted over the sensor chip thereby creating a flow chamber with a fixed volume of 60 nl. A sinusoidal current of 50 Hz, 1 μA was provided to the two outer electrodes in the chamber and the voltage was measured between the two inner electrodes. An increase in cell volume will increase the chamber resistance and thus an increase in voltage serves as the functional read-out of cell swelling. The solutions containing the different K⁺ concentrations were created from two isosmolar stock solutions: a 0 mM K⁺ stock consisting of (in mM): 129 NaCl, 30 ChCl, 1 MgCl₂, 1.2 CaCl₂, 5 glucose, 20 HEPES, 24.15 mannitol, and a 30 mM K⁺ stock consisting of (in mM): 114 NaCl, 30 KCl, 1 MgCl₂, 1.2 CaCl₂, 5 glucose, 20 HEPES, 36.2 mannitol. The concentrations of NaCl and mannitol in the two solutions were adjusted to obtain identical conductivity and osmolarity (verified with a cryoscopic osmometer (Gonotec, Berlin, Germany) and a conductivity meter (Thermo Scientific)). The flow rate of the solution (~30 μl/min) was controlled by adjusting hydraulic pressure at the inlet.