A novel method for measuring membrane conductance changes by a voltage-sensitive optical probe

This study presents a method whose principles enable using a voltage-sensitive optical probe, to quantitatively measure conductivity changes elicited in membrane vesicles and cells. The procedure is based on the fact that the amplitude of the transmembrane potential difference, established across a membrane by an external electric field, is decreased when membrane conductivity is increased upon incorporation of ionophores into the membrane. The method was applied to osmotically swollen thylakoid membranes whose membrane conductivity was changed by the addition of gramicidin or ionomycin. The electric field induced stimulated luminescence from photosystem I (electrophotoluminescence-EPL) was used as a voltagesensitive optical probe. We calculated the induced conductance changes by using a calibrated EPL vs external electric field response curve and measuring the ionophore-mediated attenuation of the EPL signal. The calculated ionophore-unmodified conductance of the thylakoid membrane yields a value of 171 ± 56 nS cm. The value of the membrane conductance, modified by 10 nM gramicidin was found to be 190 ± 56 nS cm. The modified membrane conductance and the membrane conductance changes induced by 1 μM ionomycin in the presence of CaCl₂ were found to be 186 ± 3 nS cm and 15 ± 3 nS cm, respectively.