SURFACE-POTENTIALS MEASURE ION CONCENTRATIONS NEAR LIPID BILAYERS DURING RAPID SOLUTION CHANGES

We describe a puffing method for changing solutions near one surface o f lipid bilayers that allows simultaneous measurement of channel activ ity and extent of solution change at the bilayer surface, ion adsorpti on to the lipid headgroups and screening of the bilayer surface charge by mobile ions provided a convenient probe for the ionic composition of the solution at the bilayer surface, Rapid ionic changes induced a shift in bilayer surface potential that generated a capacitive transie nt current under voltage-clamp conditions, This depended on the ion sp ecies and bilayer composition and was accurately described by the Ster n-Gouy-Chapman theory, The time course of solute concentrations during solution changes could also be modeled by an exponential exchange of bath and puffing solutions with time constants ranging from 20 to 110 ms depending on the flow pressure, During changes in [Cs+] and [Ca2+] (applied separately or together) both the mixing model and capacitive currents predicted [Cs+] and [Ca2+] transients consistent with those d etermined experimentally from: 1) the known Cs+-dependent conductance of open ryanodine receptor channels and 2) the Ca2+-dependent gating o f ryanodine receptor Ca2+ channels from cardiac and skeletal muscle.