Mechanotransducing ion channels in astrocytes

Ion channels present on the soma of neonatal rat astrocytes in primary cell culture were studied using the single channel recording technique. Ion channels were activated by changing the pressure in the back of the pipette. The morphological structure of the patch membrane was examined while recording channel activity. One class of channel was activated by increasing the pipette pressure (curvature-sensitive or CS channels). CS channels were observed in 150 mM KCl, 150 mM NaCl, or 150 mM sodium gluconate. At constant pressure the closed times decreased with depolarization. CS channels had a conductance of 50 pS in 150 mM NaCl, and displayed an inwardly rectifying current-voltage relationship. CS channel activity was found only in cell-attached patches, and were active only when the patch membrane curved towards the soma. The other class of channel was found to be activated by both suction and pressure (stretch-activated or SA channels). Four SA conductance levels were found: 360, 230, 144, and 70 pS in 150 mM KCl. Each conductance displayed a linear current-voltage relationship. At negative membrane potentials SA channels were inhibited by Cs+, Ba2+ or Na+. The relationship between average mechanosensory current and pressure was biphasic for SA channels and monophasic for CS channels. Combinations of SA and CS channels could be observed in the same patch. We propose that CS channels are non-specific cation channels which sense membrane tension only when the patch membrane is in a specific, permissive curvature. SA channels appear to be K(+)-selective channels that sense membrane tension independent of the direction of curvature.