ISSN 0006-3509, Biophysics, 2006, Vol. 51, No. 6, pp. 892–895. © Pleiades Publishing, Inc., 2006. Published in Russian in Biofizika, 2006, Vol. 51, No. 6, pp. 1014–1018. 892 INTRODUCTION The phenomenon of mechanosensitivity, and molec- ular mechanisms underlaying it have been drawing attention of researchers for over a decade (for a recent reviews, see [1, 2]). One example of this phenomenon is presented by the ion channel formed by the antibiotic gramicidin A [3, 4]. Modulation of biophysical proper- ties of this channel due to a local curvature stress in microscopic membranes has been attributed to the ten- sion-induced mismatch between the length of the chan- nel and the thickness of the bilayer [4, 5]. In the present paper we report the observation of mechanoelectrical transduction in gramicidin A channels in bulged macro- scopic Mueller–Rudin bilayer lipid membranes (BLM, [6]), demonstrating that mechanosensitivity can be induced by changes in membrane curvature alone with- out membrane stretching. Namely, we found that bulg- ing the gramicidin-modified macroscopic BLM did not change membrane tension, but produced a marked supralinear increase in membrane current which was proportional to the square of membrane curvature. METHODS Membranes were formed using the Mueller–Rudin technique [6] by passing a bubble from a pipette tip, prewetted with a membrane-forming solution, over an aperture of 100 or 200 μm in diameter in a partition sep- arating two compartments (4 ml in volume) of a teflon chamber. Membrane-forming solution was composed of a 1 : 1 mixture (wt : wt, 25 mg of lipid per ml) of diphytanoylphosphatidyl-choline and diphytanoylphos- phatidylethanolamine (Avanti Polar Lipids, Alabaster, AL) in n-octane. Bathing solutions contained 100 mM NaCl, 10 mM MOPS–Tris (pH 7.4). Currents through the membrane were measured using a conventional cur- rent-to-voltage converter based on an OPA-101 (Burr- Brown, Tucson, AZ) operational amplifier with a 1 GΩ feedback resistor. The current-to-voltage converter was connected to the trans side of the bilayer chamber using a Ag/AgCl electrode and 3M KCl/3% agar bridges. The cis compartment was connected to a voltage source using a Ag/AgCl electrode and 3 M KCl/3% agar bridge. All chemicals were reagent grade, and all solu- tions were made with distilled water and filter sterilized before use (Sterivex-GS, 0.22-μm filter, Millipore Corp., Bedford, MA). Membrane formation was moni- tored by increase in capacitive current to triangle volt- age pulses (10 V/s) from a function generator. The membrane capacitance was measured by comparison with a known capacitor. The specific capacitance of non-bulged membranes was 0.47 ± 0.7 μF/cm 2 , and remained stable for more than three hours. Gramicidin A modified membranes were formed in the presence of 0.5 pM of antibiotic in both compartments of the bilayer cham- ber. Upon formation, the membranes were allowed to reach steady-state conditions for 20–30 min before Mechanosensitivity of Gramicidin A Channels in Bulged Bilayer Membranes at Constant Tension ¶ V. S. Markin a , *, V. Gh. Shlyonsky b, c , S. A. Simon d , D. J. Benos b , and I. I. Ismailov e , * a Department of Anesthesiology, University of Texas, Southwestern Medical Center, Dallas, TX 75235, USA; e-Mail: Vladislav.Markin@UTSouthwestern.edu b Department of Physiology and Biophysics, University of Alabama at Birmingham, Birmingham, AL 35294, USA c Department of Physiology, Free University of Brussels, 1070, Brussels, Belgium d Department of Neurobiology, Duke University Medical Center, Durham, NC 27710, USA e Department of Neuroscience, Baylor College of Medicine, Houston TX 77030, USA; e-Mail: Ismailov@bcm.tmc.edu Received February 1, 2006 Abstract—Mechanoelectrical transduction in gramicidin A channels was studied in macroscopic planar lipid bilayer membranes bulged at constant tension. We found a supralinear increase in the single channel activity that was proportional to the square of membrane radius, but could not be accounted for by the increase in mem- brane surface area, or by recruitment of new channels. Extrapolated to biological membranes, these observa- tions may suggest that the permeability of ion channels can be influenced simply by changing shape of the membrane, with or without stretching. Key words: bilayer lipid membranes, ion channels, gramicidin A DOI: 10.1134/S0006350906060078 CELL BIOPHYSICS * Corresponding authors. ¶ The text was submitted by the authors in English.