Correlation between thermodynamic properties of the nicotinic acetylcholine receptor channel expressed in native and heterologous cellular systems and physical state of the membrane in which it is inserted

ZANELLO LAURA; AZTIRIA EUGENIO; ANTOLLINI SILVIA; ROCCAMO ANA MARIA; BARRANTES FANCISCO

San Francisco, CA

Congreso; 39th Annual Meeting of the Biophysical Society; 1995

Biophysical Society

Correlation between thermodynamic properties of the nicotinic acetylcholinereceptor channel expressed in native and heterologous cellular systems andphysical state of the membrane in which it is Inserted. L.P. Zanello, E. Aztiria,S. Antollini, A.M. Roccamo and F.J. Barrantes. Instituto de InvestigacionesBioquimicas, 8000 Bahia Blanca, Argentina.The temperature sensitivity of mouse muscle nicotinic acetylcholine receptor(AChR) channel was studied in a) the native embryonic (y-)AChR in the BC3H-1clonal cell line, and b) embryonic (y-) and adult (e-) forms of the AChRheterologously expressed in CHO cells. Unitary conductance and channel closingrate, a, were measured in inside-out patches over a temperature range from 5° to35°C. Arrhenius plots obtained for the two AChR types enabled calculation of Q10values of 0l.2 and 2.0 for the processes of K+ conductance and channel closurerespectively. Using Eyring transition rate theory both y- and e-AChR were found todisplay similar characteristics in all cell types, with relatively low thermal sensitivityof the ionic conductance. Activation energies (Ea) of 3.0-5.0 kcal.mol-1 suggestthat ions traverse the pore by diffusion. Channel closure showed higher energyrequirements, with activation enthalpies of x13.5 kcal.mol-1 at 200C. Although yandc-AChRs exhibited similar thermodynamic parameters, a slightly higher Eavalue for the conductance and kinetic processes was obtained for the y-AChR inthe heterologous CHO-AR42 cellular system. Fluorescence measurements usingthe probe Laurdan enabled us to establish a correlation between theelectrophysiological and energetic parameters of the AChR proper and thephysicochemical properties of the membrane bilayer in which the protein isembedded. All AChRs heterologously expressed in CHO cells were found to bemore sensitive than the native y-AChR in BC3H-1 cells to thermal changes inmembrane fluidity. Changes in the physical state of the membrane environmentmay thus exert subtle but noticeable effects on AChR activity.