Ž . Bioelectrochemistry 52 2000 197–201 www.elsevier.comrlocaterbioelechem Adiabatic compressibility of red blood cell membrane: influence of skeleton Tibor Hianik a, ) , Peter Rybar a , Ingolf Bernhardt b ´ a Department of Biophysics and Chemical Physics, Faculty of Mathematics and Physics, Comenius UniÕersity, Mlynska dolina F1, ´ SK-842 48 BratislaÕa, SloÕak Republic b Laboratory of Biophysics, Faculty of Natural and Technical Sciences III, UniÕersity of the Saarland, P.O. Box 151150, D-66041 Saarbrucken, Germany ¨ Received 23 February 2000; received in revised form 24 July 2000; accepted 10 August 2000 Abstract Measurements of ultrasound velocity and density were used for determination of the adiabatic compressibility of red blood cells Ž . RBC during detachment of the membrane skeleton. Skeleton detachment was induced by addition of nystatin into a low ionic strength Ž . wx RBC suspension resulting in an increase 10% of the ultrasound velocity concentration increment, u , while the specific volume of cells, w , did not change significantly. Changes of the concentration increment had rather long kinetics and were not completed even after 60 V wx min. Both u and w values were used for calculation of the specific apparent adiabatic compressibility of RBC, w rb . The value of V K 0 the specific apparent compressibility decreases following addition of nystatin. This corresponds to an increase in the volume elastic modulus of RBC membranes during detachment of the membrane skeleton. Control experiments with large unilamellar liposomes at wx conditions similar to that performed with the RBC did not reveal significant changes of u after the addition of nystatin. Our results show that the role of the membrane skeleton probably consists in maintaining higher compressibility of the RBC membranes. This may partly provide conditions for conformational changes of RBC membrane proteins. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Red blood cells; Skeleton; Liposomes; Ultrasound velocity; Density; Adiabatic compressibility 1. Introduction The lipid bilayer with embedded or attached proteins, as well as the membrane skeleton, are the main and principal Ž . constituents of red blood cell RBC membranes. The Ž membrane skeleton is formed by a network of proteins the . main components are spectrin and actin and is attached to the lipid bilayer due to the ankyrin–band 3 protein interac- w x tions 1–3 . Under physiological conditions, the membrane skeleton participates in the stability of the erythrocyte wx membrane 4 . At certain conditions, such as elevation of wx intracellular pH above 8.5 3 , or decrease of intracellular wx ionic strength 5 , a detachment of the skeleton from the membrane occurs. The detachment of the skeleton leads ) Corresponding author. Tel.: q 421-7-65426774; fax: q 421-7- 65425882. Ž . E-mail address: hianik@fmph.uniba.sk T. Hianik . also to a solubilisation of the integral membrane proteins wx 6 . The detachment of membrane skeleton might, there- fore, influence the physical properties of the RBC, as well as their shape. This has been demonstrated particularly by wx Mohandas and Evans 7 , who showed that the area exten- sion modulus of the skeleton is much lower than that of the membrane lipid bilayer, thus the detachment should change the compressibility of the RBC. A similar conclu- wx sion follows from theoretical work by Iglic et al. 8 , who ˘ showed a decrease of the attraction forces between the skeleton and the membrane lipid bilayer during partial detachment of the skeleton. In the present work using ultrasound velocimetry and densitometry, we studied how the detachment of skeleton, induced by nystatin in a low ionic strength solution, af- fected the adiabatic compressibility of the RBC. In order to check whether ion efflux induced by nystatin influences the sound velocity, we performed experiments with large unilamellar liposomes at conditions similar to that with RBC. 0302-4598r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. Ž . PII: S0302-4598 00 00102-1